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Jeong-Yeol Yoon

Tissue
Engineering
A Primer with Laboratory
Demonstrations
Tissue Engineering
Jeong-Yeol Yoon

Tissue Engineering
A Primer with Laboratory Demonstrations
Jeong-Yeol Yoon
Department of Biomedical Engineering
The University of Arizona
Tucson, AZ, USA

ISBN 978-3-030-83695-5    ISBN 978-3-030-83696-2 (eBook)


https://doi.org/10.1007/978-3-030-83696-2

© Springer Nature Switzerland AG 2022


This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of
the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation,
broadcasting, reproduction on microfilms or in any other physical way, and transmission or information
storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology
now known or hereafter developed.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication
does not imply, even in the absence of a specific statement, that such names are exempt from the relevant
protective laws and regulations and therefore free for general use.
The publisher, the authors and the editors are safe to assume that the advice and information in this book
are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the
editors give a warranty, expressed or implied, with respect to the material contained herein or for any
errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional
claims in published maps and institutional affiliations.

This Springer imprint is published by the registered company Springer Nature Switzerland AG
The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland
Preface

I have been teaching a semester-long upper-division undergraduate college class on


tissue engineering. There are several good textbooks, albeit not many, on tissue
engineering. (This is understandable since tissue engineering is still a new disci-
pline.) However, many textbooks are exceptionally long, close to a thousand pages
or even two thousand pages. They are written by many authors, in a manner of an
edited book, potentially lacking the following: consistency, explanations of basic
terms, example questions, and engineering design/calculation.
In addition, I have been a firm believer that the core bioengineering and related-­
discipline classes are to be delivered together with the hands-on laboratory exer-
cises. I have already demonstrated this concept in my other textbook, Introduction
to Biosensors, whose second edition was also published by Springer in 2016.
With this book, I offer a single-authored primer textbook on tissue engineering,
not in the manner of a lengthy edited book. It covers the fundamental basics of tis-
sue engineering in a concise manner, accompanied by a series of laboratory exer-
cises. While we can certainly use this book for a class with laboratory exercises, we
can also use it without laboratories. A list of questions and discussion topics are
added to each laboratory exercise. The instructor can lead such discussion collab-
oratively using the experimental procedures and results included in the textbook. As
the laboratory results are included with actual images and data for all laboratory
exercises, scientists and engineers not in colleges can also learn the concepts in a
hands-on and visual manner to better understand concepts. It will also lay a founda-
tion to build their experiments towards their research and commercial development.
With this book, I aimed to accomplish the followings:
–– Most up-to-date aspects of tissue engineering are covered in a concise manner
while providing easy-to-understand basic concepts.
–– Step-by-step learning of all necessary concepts is provided.
–– A large number of figures are provided. There are 226 figures through 14 chap-
ters in this book.
–– Simple, low-cost, and easy-to-implement laboratory exercises are included in all
chapters, except for the first and last chapters.

v
vi Preface

–– Photographs of all equipment and results of all laboratories are provided to visu-
alize tissue engineering concepts. These can be used as guides for practical labo-
ratory exercises towards better understanding the concepts or starting points
towards further research and development.
I sincerely thank my graduate teaching assistants at the University of Arizona.
They have helped me create and implement the laboratory exercises in my Cell and
Tissue Engineering class in the spring semesters from 2015 to 2021. I specifically
appreciate the following graduate teaching assistants: Dr. Katherine Klug (currently
at Davids Engineering), Dr. Soohee Cho (currently at Abbott), Dr. Tiffany-Heather
Ulep (currently at Roche), and Dr. Kattika Kaarj (presently a faculty member at
Mahidol University in Thailand), who had laid foundations in these laboratory exer-
cises. The following graduate teaching assistants have also helped establish the
laboratory exercises: Dr. Soo Chung (presently at United States Department of
Agriculture – Agricultural Research Service), Mr. Kenneth Schackart (currently at
the University of Arizona), and Mr. Ryan Zenhausern (currently at Georgia Institute
of Technology). Other graduate teaching assistants, Mr. Christopher Camp and Ms.
Carissa Grijalva, both at the University of Arizona, have also contributed substan-
tially to the laboratory exercises. I also thank all students who took my cell and
tissue engineering class for providing critical feedback and corrections.
I also thank my former and current department heads, Dr. Urs Utzinger and Dr.
Arthur Gmitro in the Biomedical Engineering Department. They have supported my
class by providing personnel, equipment, and laboratory space at the University of
Arizona. Support and suggestions from the editorial office at Springer, especially
Michael McCabe, are greatly appreciated. Finally, I want to sincerely thank my
wife, Dr. Sunhi Choi (Mathematics Department at the University of Arizona), for
her continuous inspiration and support during the preparation of this textbook.

Tucson, AZ, USA Jeong-Yeol Yoon


Contents

1 Introduction����������������������������������������������������������������������������������������������    1
1.1 Narrow Definition of Tissue Engineering ����������������������������������������    1
1.2 Early Attempt in Scaffold Development: Decellularized Matrix������    2
1.3 Simple TE Transplant Example: Skin����������������������������������������������    4
1.4 Simple TE Transplant Example: Pancreas����������������������������������������    6
1.5 Expanded Definition of Tissue Engineering: Organ-on-­a-Chip
(OOC) ����������������������������������������������������������������������������������������������    8
1.6 Overview of This Book��������������������������������������������������������������������   10
References��������������������������������������������������������������������������������������������������   12
2 Cell Culture����������������������������������������������������������������������������������������������   13
2.1 What Is Cell Culture?������������������������������������������������������������������������   13
2.2 Cell Physiology: Cell Membrane and Cytoskeleton������������������������   15
2.3 Focal Adhesion����������������������������������������������������������������������������������   18
2.4 Cell Classification: Anchorage-Dependent Versus Anchorage-­
Independent Cells������������������������������������������������������������������������������   19
2.5 Cell Classification: Normal Versus Immortalized Cells�������������������   19
2.6 Cell Classification: Normal Versus Stem Cells��������������������������������   22
2.7 Maintaining Sterile Environment: Biosafety Cabinet����������������������   22
2.8 Maintaining Sterile Environment: Autoclave������������������������������������   23
2.9 Cell Culture: CO2 Incubator��������������������������������������������������������������   24
2.10 Cell Imaging: Fluorescence Microscope������������������������������������������   25
2.11 Laboratory Task 1: Bacterial Cell Culture����������������������������������������   26
2.12 Laboratory Task 2: Fluorescence Microscopic Imaging
of Mammalian Cells��������������������������������������������������������������������������   30
Reference ��������������������������������������������������������������������������������������������������   32
3 Cell Metabolism ��������������������������������������������������������������������������������������   33
3.1 What Is Cell Metabolism?����������������������������������������������������������������   33
3.2 Energy Currency: ATP����������������������������������������������������������������������   34
3.3 Cell Metabolism: Glycolysis and TCA Cycle����������������������������������   35
3.4 Glucose Transport and Uptake����������������������������������������������������������   37

vii
viii Contents

3.5 Cell Metabolism: Glutaminolysis and TCA Cycle ��������������������������   39


3.6 Glutamine Transport and Uptake������������������������������������������������������   39
3.7 Role of Oxygen in Cell Metabolism ������������������������������������������������   40
3.8 Culture Media ����������������������������������������������������������������������������������   42
3.9 Cell Feeding��������������������������������������������������������������������������������������   44
3.10 Cell Passaging����������������������������������������������������������������������������������   44
3.11 Cell Growth Kinetics: First-Order Growth Model����������������������������   45
3.12 Laboratory Task 1: Media Preparation and Cell Feeding ����������������   47
3.13 Laboratory Task 2: Cell Passaging����������������������������������������������������   51
Reference ��������������������������������������������������������������������������������������������������   54
4 Cell Imaging ��������������������������������������������������������������������������������������������   55
4.1 Overview of Fluorescence Microscopy��������������������������������������������   55
4.2 Fluorescence ������������������������������������������������������������������������������������   56
4.3 Fluorescent Dyes������������������������������������������������������������������������������   59
4.4 Bioreceptors��������������������������������������������������������������������������������������   62
4.5 Fluorescence Microscope������������������������������������������������������������������   65
4.6 Photobleaching����������������������������������������������������������������������������������   67
4.7 Smartphone Fluorescence Microscope ��������������������������������������������   68
4.8 Laboratory Task 1: Fluorescence Imaging of Nucleus and
Cytoskeleton ������������������������������������������������������������������������������������   70
4.9 Laboratory Task 2 (Alternative): Fluorescence Imaging
of Nucleus, Cytoskeleton, and Focal Adhesion��������������������������������   78
References��������������������������������������������������������������������������������������������������   80
5 Stem Cells ������������������������������������������������������������������������������������������������   81
5.1 What Are Stem Cells?����������������������������������������������������������������������   81
5.2 Why Do We Need Stem Cells for Tissue Engineering?��������������������   83
5.3 Embryonic Versus Adult Stem Cells ������������������������������������������������   83
5.4 Use of Embryonic Stem Cells for Tissue Engineering Applications  85
5.5 Induced Pluripotent Stem Cells (iPSCs) ������������������������������������������   88
5.6 Isolation of Stem Cells����������������������������������������������������������������������   89
5.7 Culturing Stem Cells������������������������������������������������������������������������   90
5.8 Morphogenetic Factors ��������������������������������������������������������������������   92
5.9 Hazards of Stem Cell Differentiation������������������������������������������������   93
5.10 Laboratory Task 1: Culturing Stem Cells ����������������������������������������   94
5.11 Laboratory Task 2: Embryoid Body Formation��������������������������������   95
References��������������������������������������������������������������������������������������������������   97
6 Biomaterial Surfaces��������������������������������������������������������������������������������   99
6.1 Development of Biomaterial Surfaces for Tissue Engineering��������   99
6.2 Size and Shape Requirements���������������������������������������������������������� 100
6.3 Synthetic Materials �������������������������������������������������������������������������� 102
6.4 Natural Materials������������������������������������������������������������������������������ 103
6.5 Nonspecific Cell–Surface Interactions���������������������������������������������� 106
6.6 Specific Cell–Surface Interactions���������������������������������������������������� 107
Contents ix

6.7 Bone Biomaterials, Apatite, and Bioglass���������������������������������������� 107


6.8 Hydrogels������������������������������������������������������������������������������������������ 110
6.9 Biodegradable Scaffolds ������������������������������������������������������������������ 110
6.10 Encapsulation Scaffolds�������������������������������������������������������������������� 112
6.11 Laboratory Task 1: Preparation of Various Biomaterial Surfaces���� 112
6.12 Laboratory Task 2: Contact Angle Measurements���������������������������� 117
6.13 Laboratory Task 3: Surface Roughness Measurements�������������������� 119
References�������������������������������������������������������������������������������������������������� 121
7 Focal Adhesion����������������������������������������������������������������������������������������� 123
7.1 What Is Focal Adhesion?������������������������������������������������������������������ 123
7.2 Cell Adhesion and Proliferation on Biomaterial Surfaces���������������� 125
7.3 Cell Migration After Focal Adhesion������������������������������������������������ 127
7.4 Morphogenesis���������������������������������������������������������������������������������� 130
7.5 Laboratory Task 1: Fluorescence Staining of Focal Adhesion���������� 131
7.6 Laboratory Task 2: Fluorescence Imaging of Focal Adhesion���������� 132
References�������������������������������������������������������������������������������������������������� 135
8 Contact Guidance and Cell Patterning�������������������������������������������������� 137
8.1 What Is Contact Guidance?�������������������������������������������������������������� 137
8.2 Contact Guidance on Basement Membrane�������������������������������������� 138
8.3 Nanogrooves ������������������������������������������������������������������������������������ 139
8.4 Shear Flow-Resistant Composite Nanosurfaces ������������������������������ 142
8.5 Contact Guidance by Flow���������������������������������������������������������������� 144
8.6 Cell Patterning via Lithographic Protein Patterns���������������������������� 144
8.7 Cell Patterning via Direct Protein Deposition���������������������������������� 145
8.8 Cell Patterning via Microcontact Printing���������������������������������������� 146
8.9 Direct Cell Patterning via Inkjet Printing (Cell Printing) ���������������� 147
8.10 Laboratory Task 1: Contact Guidance on Microgroove�������������������� 149
8.11 Laboratory Task 2: Cell Patterning via Droplet Collagen
Deposition ���������������������������������������������������������������������������������������� 152
References�������������������������������������������������������������������������������������������������� 154
9 3D Scaffold Fabrication�������������������������������������������������������������������������� 155
9.1 3D Scaffolds�������������������������������������������������������������������������������������� 155
9.2 Electrospinning �������������������������������������������������������������������������������� 156
9.3 Materials Selection for Electrospinning Toward Tissue Engineering  158
9.4 Modification of Electrospun Fibers for Tissue Engineering ������������ 159
9.5 Contact Guidance on Electrospun Fibers������������������������������������������ 159
9.6 Controlled Release of Growth Factors and Bioactive Factors from
Electrospun Fibers���������������������������������������������������������������������������� 161
9.7 Core–Shell Structured Electrospun Fibers���������������������������������������� 161
9.8 Addition of GAG-Like Structure to Electrospun Fibers ������������������ 162
9.9 3D Printed Scaffolds ������������������������������������������������������������������������ 163
9.10 Various 3D Printing Methods������������������������������������������������������������ 164
9.11 Hydrogel Bioprinting������������������������������������������������������������������������ 167
x Contents

9.12 Adding Porosity and Nanostructure to 3D Printing�������������������������� 168


9.13 Indirect 3D Printing�������������������������������������������������������������������������� 169
9.14 Laboratory Task 1: Electrospinning�������������������������������������������������� 169
9.15 Laboratory Task 2: 3D Printing�������������������������������������������������������� 171
References�������������������������������������������������������������������������������������������������� 173
10 Design of In Vitro Culture and Bioreactor�������������������������������������������� 175
10.1 Design of In Vitro Culture�������������������������������������������������������������� 175
10.2 Doubling Time (td) ������������������������������������������������������������������������ 176
10.3 Mean Residence Time (tres) ������������������������������������������������������������ 179
10.4 Oxygen Depletion Time (tO2dep)������������������������������������������������������ 181
10.5 Oxygen Diffusion Time (tO2diff) ������������������������������������������������������ 183
10.6 Design of In Vitro Culture Considering Both Oxygen
Depletion Time and Oxygen Diffusion Time���������������������������������� 185
10.7 Design of Tissue-Engineered Device Using Characteristic Time�� 187
10.8 Tissue Engineering Bioreactor�������������������������������������������������������� 187
10.9 Measurements from a Tissue Engineering Bioreactor�������������������� 190
10.10 Laboratory Task 1: Design of In Vitro Culture and Co-culture������ 190
10.11 Laboratory Task 2: Design of Artificial Vasculature Using
Oxygen Depletion and Diffusion Times ���������������������������������������� 191
References�������������������������������������������������������������������������������������������������� 192
11 Organ-on-a-Chip�������������������������������������������������������������������������������������� 193
11.1 2D Versus 3D Cell Culture�������������������������������������������������������������� 193
11.2 Organ-on-a-Chip ���������������������������������������������������������������������������� 194
11.3 How Do You Fabricate Lab-on-a-Chip (LOC)?������������������������������ 195
11.4 OOC Example: Kidney-on-a-Chip ������������������������������������������������ 198
11.5 OOC Example: Liver-on-a-Chip���������������������������������������������������� 200
11.6 OOC Example: Lung-on-a-Chip���������������������������������������������������� 203
11.7 OOC Example: Angiogenesis-on-a-Chip���������������������������������������� 204
11.8 OOC Example: Blood–Brain Barrier (BBB)-on-a-Chip���������������� 205
11.9 Other OOC Examples �������������������������������������������������������������������� 206
11.10 Multiple-Organs-on-a-Chip and Human-on-a-Chip ���������������������� 206
11.11 OOC Application: Drug Testing ���������������������������������������������������� 207
11.12 OOC Application: Disease Model�������������������������������������������������� 208
11.13 Mechanical Stimuli to OOCs���������������������������������������������������������� 209
11.14 Laboratory Task 1: OOC Fabrication���������������������������������������������� 210
11.15 Laboratory Task 2: Drug Testing with OOC���������������������������������� 213
References�������������������������������������������������������������������������������������������������� 216
12 Tissue-Engineered Skin Transplant ������������������������������������������������������ 219
12.1 When Do You Need Skin Transplants? ������������������������������������������ 219
12.2 Basic Anatomy of Skin ������������������������������������������������������������������ 221
12.3 How Can We Culture Keratinocytes In Vitro?�������������������������������� 222
12.4 Langerhans Cells and Immune Response of Skin�������������������������� 222
Contents xi

12.5 Scaffold for Tissue-Engineered Skin Transplant���������������������������� 223


12.6 Use of Stem Cells for TE Skin Transplant�������������������������������������� 226
12.7 Laboratory Task 1: Construction of Skin TE Scaffold�������������������� 227
12.8 Laboratory Task 2: Seeding and Proliferating Keratinocytes
on the TE Skin Scaffold������������������������������������������������������������������ 227
12.9 Laboratory Task 3: Force: Deflection Curve of the TE Skin
Transplant���������������������������������������������������������������������������������������� 228
References�������������������������������������������������������������������������������������������������� 232
13 Vascularization of Tissue Transplants���������������������������������������������������� 235
13.1 Angiogenesis and Vascularization�������������������������������������������������� 235
13.2 Anatomy and Physiology of Blood Vessel�������������������������������������� 236
13.3 Process of Angiogenesis ���������������������������������������������������������������� 238
13.4 Angiogenesis-Stimulating Growth Factors and Angiogenesis
Inhibitors ���������������������������������������������������������������������������������������� 239
13.5 HUVEC������������������������������������������������������������������������������������������ 241
13.6 Formation of Vasculature Network ������������������������������������������������ 241
13.7 Alignment of Vascular Endothelial Cells to Flow�������������������������� 243
13.8 Laboratory Task 1: Vessel Sprouting by VEGF on Paper-Based
Model���������������������������������������������������������������������������������������������� 243
13.9 Laboratory Task 2 (Optional): Vessel Sprouting by
Mechanical Stimuli on Paper Model���������������������������������������������� 245
References�������������������������������������������������������������������������������������������������� 248
14 Advanced Topics�������������������������������������������������������������������������������������� 249
14.1 Cartilage Tissue Engineering���������������������������������������������������������� 249
14.2 Bone Marrow Transplantation�������������������������������������������������������� 252
14.3 Cardiac Patches ������������������������������������������������������������������������������ 253
14.4 Immunoisolated Pancreas �������������������������������������������������������������� 255
14.5 Kidney Tissue Engineering ������������������������������������������������������������ 256
14.6 Other TE Transplants���������������������������������������������������������������������� 258
References�������������������������������������������������������������������������������������������������� 259

Index������������������������������������������������������������������������������������������������������������������ 261
About the Author

Jeong-Yeol Yoon received his B.S., M.S., and Ph.D. degrees in chemical engineer-
ing from Yonsei University, Seoul (South Korea), in 1992, 1994, and 1999, respec-
tively. His dissertation advisor was Dr. Woo-Sik Kim, and his co-advisor was Dr.
Jung-Hyun Kim, while he worked primarily on polymer colloids. Dr. Yoon received
his second Ph.D. degree in biomedical engineering from the University of California,
Los Angeles, in 2004, working on lab-on-a-chip and biomaterials under the guid-
ance of Dr. Robin L. Garrell. He joined faculty at the University of Arizona in
August 2004 and currently holds split home appointments in the Department of
Biomedical Engineering (primary) and Department of Biosystems Engineering
(secondary). Dr. Yoon also holds joint appointments in Chemistry & Biochemistry
and BIO5 Institute. He is also associate head for graduate affairs in the Department
of Biomedical Engineering, starting from July 2018. He is currently directing
Biosensors Lab. Dr. Yoon is a member of IBE, ASABE, SPIE, BMES, and ACS and
was councillor-at-large for IBE for the 2010 and 2011 calendar years. He was the
president of the Institute of Biological Engineering (IBE) in 2015. Dr. Yoon cur-
rently serves as editor-in-chief for the Journal of Biological Engineering (the offi-
cial journal of IBE), associate editor for Biosensors and Bioelectronics (Elsevier),
and editorial board member for Scientific Reports (Springer Nature) and
Micromachines (MDPI). Dr. Yoon is the sole author of another Springer book,
Introduction to Biosensors – From Electric Circuits to Immunosensors, second edi-
tion, published in 2016, written similarly to this book.

xiii
Chapter 1
Introduction

In this chapter, we will learn several different definitions of tissue engineering as


well as its applications.
Inquiry 1. In your own words, define tissue engineering.
Inquiry 2. What can we do with tissue engineering?

1.1 Narrow Definition of Tissue Engineering

When a tissue or an organ starts malfunctioning and drugs cannot resolve such
problems, you may wish to “substitute” the damaged tissue or organ with a new one.
Tissue or organ can be harvested from a human donor, either deceased or alive, and
transplanted to the patient. This procedure is called organ transplantation. While it
has successfully been practiced in the past, you have probably heard about the
extremely long waiting list of organ transplantation, and many patients would die
before receiving it. Because of this difficulty, synthetic materials have been used to
replace the damaged tissue or organ, made from metals, polymers, or ceramics.
They are called implants, which have shown limited successes in organs, such as
bones. However, such implants cannot metabolize nutrients, cannot produce pro-
teins, etc., and subsequently cannot truly replace the functions of tissues or organs.
Therefore, a new cell-based approach is needed to replicate a living tissue’s or
organ's structure and function. Tissue or organ can be damaged from traumatic
injury, degradation from exercise, cancer, aging, and adult-onset deficiency such as
diabetes. As tissue is made from cells and extracellular matrix (ECM), it is impor-
tant to replicate both the cells and ECM. (Implants may replace the ECM but not the
cells.) Both the cells and ECM must be “engineered” to provide structural integrity
and metabolic behavior found in normal tissue. With the narrow definition of tissue
engineering, engineered mimic of the ECM is constructed, made from either a natu-
ral material (e.g., collagen fibers) or a synthetic material (e.g., polymers). This ECM

© Springer Nature Switzerland AG 2022 1


J.-Y. Yoon, Tissue Engineering, https://doi.org/10.1007/978-3-030-83696-2_1
2 1 Introduction

mimic is called tissue-engineered scaffold (TE scaffold) or just scaffold. Cells are
then harvested from a patient or a donor. We can also use stem cells. They are cul-
tured and seeded on the scaffold in a laboratory, that is, in vitro (meaning “in a
representation,” typically representing a dish, flask, beaker, tube, etc.). The antonym
to in vitro is in vivo, meaning “in life,” typically representing a (human) body.
Sometimes the term ex vivo is used as an antonym to in vivo. While in vitro and ex
vivo do not necessarily represent the same, these two terms are sometimes used
interchangeably. Once the cells proliferate and exhibit normal metabolism in a scaf-
fold, it is then transplanted back to the body to replace the tissue or organ. This final
product is called tissue-engineered transplant (TE transplant). This procedure
describes the narrow definition of tissue engineering.
Early successes in tissue-engineered constructs date back to the late 1990s. The
most famous example would be the work by Cao et al., published in 1997, where a
tissue-engineered human ear was constructed on a nude mouse [Cao et al., 1997;
Bear in mind that there were many limitations and issues with this early pioneering
work]. Since then, tissue engineering has become widely known and practiced. In
the early 2000s, it has been established as a discipline. Tissue engineering is still a
new discipline at the time of writing.
Figure 1.1 shows one example of creating a tissue-engineered transplant (TE
transplant), that is, narrow definition of tissue engineering. Cells are removed from
a healthy host (or a healthy tissue from the same patient). They are cultured in a
laboratory, and the best-performing cell line is isolated. If stem cells are used, they
should also be differentiated using proper differentiation factors, physical stimuli,
and other environmental factors. A scaffold is constructed and the chosen cells are
seeded. Once the transplant is complete, it is transplanted to the patient.
Tissue-engineered transplants have numerous benefits over implants. Many
implants can be rejected from a body via inflammatory and immune responses,
where such issue is minimal with TE transplants. Since the best-performing cell line
can be selected and the scaffold design can be optimized, the resulting TE trans-
plants can perform far better than implants, while there is no availability issue com-
mon in organ transplants.

1.2  arly Attempt in Scaffold Development:


E
Decellularized Matrix

Since its conception in the early 2000s, scaffold design has been considered quite
challenging toward fully mimicking the natural ECM. One early attempt was the
use of natural ECM rather than designing and fabricating a whole new scaffold. An
organ from a dead body (cadaver) was harvested and the cells were removed with
surfactant (decellularization). This decellularized matrix was then used as a scaf-
fold. Ott et al. demonstrated this method using the heart from cadaver (published in
2008). After harvesting, they decellularized with surfactant (Fig. 1.2) and added
1.2 Early Attempt in Scaffold Development: Decellularized Matrix 3

Fig. 1.1 Narrow definition of tissue engineering. (Ude et al., 2018. (C) Open access article distrib-
uted under the terms of the Creative Commons Attribution 4.0 International License)

cardiac cells onto it. Cells were proliferated and maintained for up to 28 days in a
bioreactor (in vitro) (Fig. 1.3). The resulting construct generated the pump function
equivalent to about 2% of adult heart function. While 2% seemed very low, it was
the first demonstration of a thriving TE heart with pump function. It showed the
early promise of tissue engineering and has received a lot of media attention.
This method was groundbreaking in many aspects, eliminating many organ
transplantation issues, especially tissue healthiness or organ availability. However,
it still suffered numerous problems, such as (1) availability of cadavers and (2) lack
of engineering control over the scaffold design and structure. Therefore, follow-up
works have been focused on the design and construction of “engineered” scaffolds
that would meet the requirements of:
–– Size (volume) that can “process” the required amounts of nutrient metabolism,
protein production, waste removal, etc.
–– Mechanical properties that can fit within the available space, resist external
bending and shear force, etc.
–– Longevity that can last for a necessary duration of time
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things? Put that paper down at once, and don’t let me see you touch
a newspaper again until you are old enough to know what to read,
and what to leave alone.”
Dulcie blushed.
“Miss Hammond says everybody ought to read the newspaper,” she
began. “It’s very interesting about that man. Won’t you please let
me finish it, Grandma?”
“Certainly not, and don’t argue. Such things are not proper reading
for a child of your age. Your father would be very angry if he ever
heard of your reading such disgusting stories.”
“Would he?” said Dulcie, and she instantly put down the paper.
There was no one in the world whom Dulcie loved as she loved her
father.
“Of course he would,” said Mrs. Winslow. “Remember, you are not to
look at a newspaper again until I give you permission. What are you
rubbing your eyes in that way for, Maud?”
“I’m sleepy,” said Maud. Maud was less afraid of Grandma than any
of the others, and if Mrs. Winslow had a favorite among her
stepson’s children, it was little curly-headed Maud, who was scarcely
more than a baby when the family had arrived from the West five
years ago.
Grandma glanced at the clock.
“Nearly five minutes to eight,” she said; “you may as well all go to
bed.”
Four little girls sprang from their chairs with so much alacrity that, if
Grandma had been a real grandmother, instead of “only a step,” as
Dulcie called her, her feelings might have been hurt. But Mrs.
Winslow had no objection to the children’s evident dislike of her
society. She meant to do her duty to her husband’s grandchildren,
but she never thought of them in any other light than as a
troublesome incumbrance. They each gave her a sedate “duty kiss,”
and murmured a polite “Good-night, Grandma,” and she heaved a
sigh of relief that another day was over. As for Aunt Kate, she frankly
confessed that she hated to be kissed, and the children never
dreamed of troubling her in any such way.
“Oh, it is nice to get up here again, all by ourselves, isn’t it?” cried
Daisy, with a happy little skip, as they entered their own big nursery,
and Dulcie lighted the gas. “I feel sometimes as if I couldn’t breathe
down there with Grandma and Aunt Kate. Let’s hurry to bed, and
then you’ll talk to us about Mamma, won’t you, Dulcie?”
Dulcie nodded rather absently. She was still thinking about the
newspaper story that Grandma had interrupted.
“Hark!” exclaimed Maud, eagerly. “There’s the singing lady.”
They all paused to listen, and, sure enough, from somewhere that
sounded as if it came from within the wall, could be distinctly heard
the notes of a piano, and of a sweet voice singing. The walls in the
old house were rather thin, and by pressing their ears against the
party wall, which divided the Winslows’ from the house next door,
they could even distinguish the words of the song.
“It’s ‘Robin Adair,’” said Molly. “Isn’t it pretty? I think I like it best of
all the songs she sings.”
“I like ‘Darby and Joan’ best,” affirmed Daisy; “it always makes me
think of such nice, comfortable things. I do wish we knew her. I’m
sure she must be nice; she’s got such a lovely voice.”
“Grandma would never let us go to see her,” said Dulcie, with
conviction. “She says it isn’t proper to call on people she doesn’t
know.”
“Perhaps it’s more interesting not to know her,” said cheerful Daisy.
“It’s so exciting to make up stories about her. She must be rather
poor to live away up on the top floor of that boarding-house. I wish
we could see her in the street sometimes.”
“Maybe we do see her,” said Dulcie; “we haven’t any idea what she
looks like. Now, hurry and get undressed, children. It’s pretty cold up
here; I think the furnace must be very low.”
Daisy and Molly began unfastening their dresses, but Maud still
remained with her ear glued to the wall.
“Come, Maud, don’t dawdle,” commanded Dulcie, a little impatiently.
“I’ll help you undress.”
“I want to listen to the singing lady,” objected Maud. “I love music.”
“You can listen in bed just as well, and if you stay up in this cold
room, you may get another sore throat, and you wouldn’t like that,
you know. My goodness! there’s the door-bell. Who can it be at this
time of night?”
Evening visitors were not frequent at the Winslows’, and Molly was
dispatched to peep over the banister.
“Perhaps it’s that minister who comes to see Aunt Kate,” said Dulcie,
and this opinion was rather strengthened when Molly reported
having heard a gentleman’s voice speaking to Mary.
Aunt Kate’s visitors were not interesting to the children, and they
had almost forgotten the incident of the door-bell, when there came
an unexpected tap at the nursery door.
“Children,” called Mary’s voice, rather breathless from the three long
flights of stairs, “your grandma says you’re to come down right
away. Your uncle’s here.”
There was a simultaneous exclamation of astonishment from four
very excited little girls.
“Our uncle! What uncle? Oh, Mary, do tell us quick.” And the door
was flung open, revealing four children in various stages of
undressing.
“His name is Maitland,” said Mary, “and he’s a youngish gentleman. I
never saw him before.”
“It must be Uncle Stephen; Mamma’s brother from California,” said
Dulcie. “I think he’s the only uncle we’ve got. Oh, isn’t it exciting?
Hurry, children, do please hurry!”
“I can’t go down with my boots unbuttoned,” complained Daisy. “O
dear! where’s the shoe buttoner? Fasten your dress, Molly, and take
those curlers off Maud’s hair.”
“I’ll help you,” said Mary, good-naturedly. “I’m glad you’ve got an
uncle to look after you. You’d better tell him a few things before he
goes away again.”
“What sort of things?” inquired Daisy, innocently.
Mary laughed.
“Oh, I guess you know as well as I do,” she said, evasively. “If you
don’t, so much the better.”
“Did our uncle ask for Grandma?” Dulcie wanted to know.
“Oh, yes, and she’s in the parlor with him now. So’s Miss Kate.”
Dulcie’s face fell.
“There isn’t much use in our going down, then,” she said, with a
sigh. “Grandma won’t let us talk. She never does when there’s
company.”
“Perhaps she will this time, because it’s our uncle,” said Daisy, who
was always hoping pleasant things were going to happen. “Anyhow,
it will be lovely to see somebody belonging to Mamma. I remember
Papa told us about Uncle Stephen. He’s lived in California ever since
he was twenty, and none of us has ever seen him. There! my boots
are done. Now I can help Maud, if you’ll button Molly’s dress, Mary.”
Four little hearts were beating rather quickly, as the children hurried
down-stairs to the parlor, from whence the sound of voices could be
heard.
“Grandma’s talking in her ‘company voice,’” whispered Dulcie. “She
must like Uncle Stephen or she wouldn’t sound so polite.”
Grandma and Aunt Kate were both smiling when the children
entered the parlor, and their companion, a tall, broad-shouldered
young man, rose from the sofa, and came forward to meet them.
“So these are Ethel’s little girls,” he said, and Grandma answered,
still in her “company voice”:
“Yes, here they are, all four. Children, this is your Uncle Stephen
from California.”
“I know,” said Dulcie, holding out her hand, with her most grown-up
air; “Papa told us all about you. I think you were very kind to take
the trouble to come to see us. I’m Dulcie, the eldest, and this is
Daisy. Her real name is Margaret, after Grandma Maitland, but
everybody calls her Daisy. These others are Molly and Maud. Molly’s
named for Mamma’s sister, who died, and Maud is just a name
Mamma liked in a book.”
Dulcie paused, rather breathless from her long speech. The three
younger children gazed at her in undisguised admiration. Under no
combination of circumstances could any one of them have dared to
make such a wonderful speech, and in Grandma’s presence, too. The
visitor smiled, and they all thought he had a very pleasant smile
indeed.
“Of course I wanted to come to see you,” he said in a voice that was
as pleasant as his smile. And, instead of taking Dulcie’s outstretched
hand, he bent and kissed her.
That broke the ice, for of course, all the others had to be kissed, too,
and in a very few minutes Maud was perched on Uncle Stephen’s
knee, and the other three were sitting beside him on the sofa. If
Grandma and Aunt Kate were displeased with this state of affairs,
they did not show it. Grandma continued to talk in her “company
voice,” and Aunt Kate smiled as her needles flew.
Mr. Maitland explained that he had come east on a business trip, and
was only spending a few days in New York.
“Indeed, I am starting back to California to-morrow night,” he said,
“but I couldn’t leave without having a glimpse of Ethel’s children. Jim
stopped to see me in San Francisco, on his way to Hong Kong, and I
asked for your address, thinking I might be in this part of the world
sometime.”
“Papa’s coming home next year,” ventured Maud, who suddenly felt
very safe in Grandma’s presence, for was not Uncle Stephen’s kind
arm around her, and had he not said that she had eyes like
Mamma’s? “When he comes home we’re going to have a little house
of our own, and perhaps Lizzie——”
Maud paused, admonished by a warning nudge from Dulcie.
Grandma had forbidden the mention of Lizzie’s name.
“We had a letter from Papa last week,” put in Dulcie, quickly, hoping
that Grandma had not noticed Maud’s slip. “He tells us such funny
things about China. Does he ever write to you, Uncle Stephen?”
“Yes, occasionally. I heard from him about a month ago.”
“Did he tell you about the Chinese people eating rats and mice?”
inquired Molly. “We used to worry for fear Papa might have to eat
them, but he says he doesn’t.”
Uncle Stephen laughed, and even Grandma and Aunt Kate looked
amused, but just then Grandma gave the little warning cough, which
always meant “children should be seen and not heard,” and Molly
instantly relapsed into embarrassed silence.
Altogether, the call was a trifle disappointing. Aunt Kate talked about
missions, but Uncle Stephen didn’t seem particularly interested in
that subject, and in about twenty minutes he took out his watch,
and remarked that he was afraid he must be going.
“I have an engagement with a business friend at nine,” he said, “but
I want to see these little nieces of mine again before I leave New
York. To-morrow is Saturday, and I expect to finish all my business
by noon. My train doesn’t leave till half-past six. May I have these
young people to spend the afternoon with me? I will promise to take
good care of them.”
That was a tremendous moment. Would Grandma consent? That
was the question that four little eager girls were asking themselves.
Daisy ventured to give the old lady a pleading glance. Dulcie and
Molly clasped their hands nervously. There was a moment of
breathless suspense, and then, to everybody’s surprise, Grandma
answered quite pleasantly:
“I am sure they would enjoy it very much, and I see no objection, if
you really want to be troubled with them.”
“I want them very much,” said Uncle Stephen, with his kind,
pleasant smile. “I will call for them at about noon, and we will lunch
at the Fifth Avenue, where I am staying, and do something together
in the afternoon. Now I must be off, as I see it is getting near the
time for my appointment, so good-night, chicks. Be sure to be ready
for me at twelve to-morrow.”
“I never believed she’d let us,” declared Daisy, when they were
talking things over in the nursery, ten minutes later. “My heart just
stood still; I was so sure she was going to say no.”
“Perhaps she didn’t dare,” suggested Molly. “He’s our uncle, you
know. Oh, aren’t uncles lovely? I never had any idea they were so
nice.”
“We didn’t know anything about them,” said Daisy. “We don’t know
much about any relations except fathers. Now let’s hurry to bed, and
get to sleep as quick as we can, so it won’t seem so long till to-
morrow.”
CHAPTER III
A WONDERFUL DAY

“IT’S the most interesting thing that ever happened to us,” declared
Molly. “It’s almost like a book thing.”
“It would be even more exciting if we had thought Uncle Stephen
was dead,” said Dulcie, in a tone of some regret. “You remember
how exciting it was in ‘Kathie’s Three Wishes,’ when her Uncle
Robert came home rich, after everybody had thought he was dead
for years and years. I wonder if Uncle Stephen is rich.”
“I don’t know, I’m sure,” said Daisy. “He must have a good deal of
money to be able to take us all to the Fifth Avenue Hotel to lunch. I
wonder where he’ll take us afterwards. It might be to the Aquarium.
Do you remember the time Papa took us there, Dulcie, and we saw
those wonderful fish, and snakes, and things?”
Maud’s face clouded.
“I don’t like snakes,” she protested; “I hope Uncle Stephen won’t
take us there. I dream about snakes sometimes, and it’s horrid.”
“Don’t be a baby,” began Molly, rather sharply, but Daisy interposed.
“I wouldn’t worry, Maudie, till we know where we really are going.
Perhaps Uncle Stephen doesn’t intend to take us anywhere except to
the hotel. We may just stay there all the afternoon, and watch the
people. That would be very interesting.”
Dulcie glanced at herself in the mirror. It was only half-past eleven,
but they were already dressed, because, as Daisy wisely remarked,
“Uncle Stephen might happen to come ahead of time, and it
wouldn’t be polite to keep a gentleman waiting.”
“I wish I hadn’t let my best hat get rained on that day,” remarked
Dulcie, with a sigh. “It’s so spotted, I don’t think it’s at all the right
thing to wear to a hotel. If Papa were here, I know he would have
bought me a new one, but Grandma doesn’t care how shabby our
things are.”
“Oh, it isn’t so very spotty, and perhaps nobody will notice,” said
Daisy, hopefully. “Don’t let’s think about anything that isn’t pleasant
to-day. Isn’t it fortunate the sun has come out? If it had kept on
raining, Grandma would have made us all wear our old clothes, and
that would have been a great deal worse than just a few spots on
one hat.”
“Yes, but it isn’t your hat,” objected Dulcie. “Yours looks almost as
good as new, and Molly’s and Maud’s are all right, too.”
For a moment Daisy hesitated, and then, with sudden determination,
she took off her own hat, and held it out to Dulcie.
“Let’s change,” she proposed cheerfully. “You’re the eldest, and
ought to look the best, and I really don’t mind a bit.”
Dulcie drew back, blushing.
“As if I would do anything so mean,” she declared, indignantly. “I
believe you’re one of the most unselfish people in the world, Daisy.
It was all my own fault, anyhow. If I had taken an umbrella that day,
as Grandma told me to, I wouldn’t have spoiled my hat. Now,
suppose we go down and wait for Uncle Stephen on the sidewalk.
It’s rather hot up here, with all our things on.”
This suggestion was greeted with favor, and a few minutes later the
front door had closed behind four very happy little girls. Grandma
and Aunt Kate were both out, so there was no one but Mary to see
them start, but Mary happened to be in a good humor that morning,
and greatly comforted Dulcie by the assurance that nobody would
notice the spots on her hat, and that they all looked “just as nice as
could be.”
“We’ll walk up and down,” said Dulcie; “it’s too cold to stand still, but
we mustn’t go far, or we might miss Uncle Stephen. Oh, it is grand
to be going somewhere, isn’t it?”
“Do you suppose there’ll be ice-cream for lunch?” inquired Maud,
anxiously.
“Of course there will be,” said Molly. “You can have anything you
want at a hotel. You just pay a dollar, and they’ll bring you whatever
you ask for. I know, because Papa took me to the Clarendon once,
the time you all had the measles, and mine hadn’t come out yet.”
“Can you even ask for two helpings?” questioned Maud, with
sparkling eyes.
“Yes, I guess so, but perhaps it wouldn’t be polite to take more than
one. Uncle Stephen might think it was piggish.”
“Of course he would,” said Dulcie, who had grown suddenly grave;
“it wouldn’t do at all. And that makes me think of something I want
to say to you all. Give me your hand, Maud, so we can all walk
together. It’s about our loyalty to Grandma. You know what Papa
used to tell us about always being loyal to our family, and never
telling things that happen at home. We mustn’t let Uncle Stephen
think we don’t have ice-cream, and nice things like that every day.
We mustn’t mention Grandma’s being cross, or—or any disagreeable
things at all. Will you all remember?”
“Yes,” promised Daisy, readily, but Molly looked a little doubtful.
“I don’t see why we should have to be so very particular with Uncle
Stephen,” she objected; “he’s our real uncle, and Grandma’s only a
step.”
“But we live with Grandma,” rebuked Dulcie. “Papa said it was very
disloyal to talk about people we live with. Don’t look so solemn,
Maudie. Of course, if Uncle Stephen or the waiter should ask us if
we would like another helping of ice-cream, it would be all right to
say yes.”
Maud’s face brightened.
“I sort of think Uncle Stephen will ask us,” she said. “He seemed so
very kind, and I’m sure he likes me best, because he said I looked
like Mamma. Let’s cross over. If the singing lady should happen to be
at her window, she might like to see how nice we look.”
The others laughed, but complied with the request.
“There isn’t anybody at the windows,” said Molly, glancing up at the
top floor of the boarding-house. “What makes you so much
interested in that lady, Maud? She may not be a bit interesting.”
“I love to hear her sing,” said Maud, “and besides, I’ve got a secret,”
she added, but in so low a tone that the others did not catch the
words. At that moment there was an excited exclamation from Daisy,
of “here he comes; he’s just turned the corner.” And everything else
was forgotten in the joy of running to meet Uncle Stephen.
“Well, well,” laughed Mr. Maitland, kissing them all round, “so here
you are, all four. No danger of being kept waiting, I see.”
“Oh, we wouldn’t do that,” protested Dulcie, quite shocked at the
mere suggestion. “We got ready early, in case you should happen to
come before twelve. Grandma and Aunt Kate have both gone out, so
there isn’t any use of your going in to see them.”
“You are the people I want to see this time,” said Uncle Stephen,
with a rather peculiar smile. “I came a little early on purpose, so as
to have plenty of time for lunch. I have tickets for ‘The Pirates of
Penzance’ this afternoon.”
“‘The Pirates of Penzance,’” repeated Dulcie, with a little gasp. “Why
—why, that’s at a theatre, isn’t it?”
“To be sure it is, and a very charming little operetta it is, too. I hope
you haven’t all seen it already.”
“Oh, no,” said Dulcie, “we never—that is, I mean we don’t often go
to theatres. Daisy and I saw ‘Rip Van Winkle’ once with Papa. It’s
very wonderful—I mean it’s very kind of you to take us.”
And despite all Dulcie’s attempts to maintain what she considered
the proper demeanor of a grown-up young lady, she could not
refrain from a little skip of delight.
As for the other three, they made no attempt whatever to conceal
their delight, and began plying Uncle Stephen with a shower of
questions about “The Pirates of Penzance,” which lasted till they
reached the corner of Fifth Avenue, where he was obliged to
interrupt them, to ask whether they would prefer walking to the
hotel or taking a stage.
“Oh, a stage, please—that is, if you don’t mind,” pleaded Molly. “We
just love riding in the stages. We hardly ever get a ride now, since
Papa and Lizzie went away, because Grandma won’t let us go by
ourselves.”
“Who is Lizzie?” Mr. Maitland asked, as they paused on the corner, to
await an approaching stage.
“She was our nurse,” Dulcie explained, “but she went away last
summer. We really don’t need a nurse any more, we’re getting so
big.”
Mr. Maitland glanced down at the four little figures, as if he did not
consider them “so very big,” after all, but just then the stage came
within hailing distance, and he made no remarks on the subject.
It was only a short distance to the hotel, but the children thoroughly
enjoyed the little ride, especially Maud, who, somewhat to Dulcie’s
disapproval, requested to be permitted to pay the fares. Because, as
she explained, “it made one feel so grand to spend money.” Uncle
Stephen laughed so much, and was so kind and genial, that even
Dulcie forgot to be dignified, and by the time they reached their
destination, they were all the best of friends.
“I am going to leave you in the reception-room for a few moments,”
Mr. Maitland said, leading the way across the marble hall of the big
hotel, “while I look up two ladies who are to lunch with us. They are
friends of mine from San Francisco, who have met your father, and
are anxious to see you all.”
Nobody said anything, but all were conscious of a sensation of
disappointment, which Molly was the first to put into words, the
moment they found themselves alone in the reception-room.
“If there are going to be ladies,” she said, ruefully, “Uncle Stephen
will talk to them all the time, and we won’t have half so much fun.”
“Perhaps they are very nice ladies,” suggested Daisy. “He said they
knew Papa, and wanted to know us. Anyhow, we’re going to a real
theatre, and nothing can spoil that.”
“I’m afraid ladies notice other people’s clothes more than gentlemen
do,” said Dulcie, with a sigh, and a glance in the long mirror. “Do you
think those spots show very much, Daisy?”
“No, not so very much,” answered Daisy, divided between her desire
to speak the truth, and fear of making her sister still more
uncomfortable. “Perhaps the ladies won’t notice the spots at all, if
the light isn’t too bright.”
Dulcie sighed again, but was forced to make the best of the
situation, and in another moment Uncle Stephen returned,
accompanied by such a very pretty young lady that, in their surprise
and admiration, the children quite forgot to worry about their own
shortcomings.
“This is Miss Florence Leslie, children,” said Mr. Maitland. “Her
mother, Mrs. Leslie, will be down in a few moments.”
“You see, I couldn’t wait for Mother,” the young lady explained,
smiling, and showing such fascinating dimples, that Daisy and Molly
both longed to kiss her. “I was so anxious to see you all. Now let me
see if I can guess which is which, from your father’s description. This
tall one must be Dulcie, I am sure, and the little curly-haired one is
Maud. These others are Daisy and Molly.”
“Why, you know all our names,” exclaimed Molly, in astonishment.
“Did you ever see us before?”
“No, but I have heard a great deal about you from your father. We
saw a good deal of him in San Francisco, before he sailed for Hong
Kong, and he and my brother are in business together now. I
wonder if you would each be willing to give me a kiss.”
“Of course we would,” said Dulcie, heartily, and four little faces were
eagerly raised. Miss Leslie kissed them all, “not just duty kisses,”
Molly said afterwards, but as if she really liked doing it, and in less
than five minutes they were chattering away to this new
acquaintance as if they had known her all their lives.
Then Mrs. Leslie appeared, and they all went into the dining-room.
Mrs. Leslie was not as pretty as her daughter, but she had a very
sweet face, and was so kind and motherly that the little girls soon
felt almost as much at home with her as with Miss Florence.
“And now who is going to order the luncheon?” Uncle Stephen
asked, when they had taken their places at one of the round tables
in the big, crowded dining-room. “Will you do it, Mrs. Leslie?”
“Suppose we let Dulcie order,” suggested Miss Florence. “When I
was a little girl, and we went to a hotel, I remember half the fun was
in ordering things to eat.”
Dulcie gasped, as the waiter handed her the long bill of fare.
“I—I don’t think I could,” she faltered; “there are so many things, I
shouldn’t know where to begin. What’s the matter, Maud?”
“It’s about the ice-cream,” whispered Maud. “It doesn’t matter what
else we have.” Maud’s whisper was sufficiently audible to be heard
by the whole party, and all the grown-ups laughed, somewhat to the
little girl’s embarrassment. Then Miss Leslie said, kindly:
“I will help you, if you would like to have me,” and on Dulcie’s
grateful request, she gave the waiter an order, which seemed to the
children almost appallingly large.
What a delicious meal it was, and how they all enjoyed it! Even
Dulcie forgot her intention of taking a light lunch, for fear Uncle
Stephen might think she was hungry, which would reflect
unfavorably on Grandma’s providing. Miss Leslie certainly did not
forget to order ice-cream, and, better still, she took two helpings of
it herself, and advised them all to do likewise. Mr. Maitland and Mrs.
Leslie seemed to have a good deal to say to each other, but Miss
Florence devoted herself almost exclusively to the children, and
before luncheon was over, had succeeded in winning all their hearts.
“I wish you were going to the theatre with us,” Molly remarked,
regretfully, as they were leaving the dining-room, and she gave her
new friend’s hand an affectionate squeeze.
“I am going,” said Miss Leslie, smiling; “your uncle invited me. He
asked Mother, too, but she declined on account of a headache.”
Molly gave vent to her satisfaction by a little squeal of delight, and
Maud—who was nothing if not truthful—remarked in a sudden burst
of confidence:
“We didn’t think we were going to like it when Uncle Stephen said
ladies were coming to lunch, but you’re not a bit like an ordinary
lady.”
“Maud!” cried Dulcie, reprovingly, but Miss Leslie laughed merrily,
and did not seem in the least offended.
That was a wonderful, never-to-be-forgotten afternoon. Long after
their elders had ceased to think of it, the four little girls loved to
recall its delights. The bright little opera, with its charming music,
and amusing dialogue. The funny pirate chief, who frightened Maud
at first, and then fascinated her for the rest of the afternoon. The
pompous major-general, with his numerous family of daughters.
And, last but not least, the gallant policemen, who were as much
afraid of the pirate band as the pirates were afraid of them. It was
all one continuous delight. But even better than the play was the
pleasant companionship. Long before the afternoon was over, they
had all come to the conclusion that, with the exception of Papa, and
possibly the faithful Lizzie, Uncle Stephen and Miss Leslie were “the
two nicest grown-ups” they had ever met.
But everything, even “The Pirates of Penzance,” must come to an
end at last, and all too soon the curtain had fallen on the last
rollicking chorus, and they were making their way out through the
crowd, into the dusk of the winter afternoon.
“Wouldn’t it be lovely if nice things never came to an end?”
remarked Dulcie, as they stood on the cold corner, while Uncle
Stephen went in quest of a cab.
Miss Leslie smiled.
“There wouldn’t be any next time to look forward to, then,” she said.
“But we don’t have any next times,” began Molly, and checked
herself, warned by a reproving glance from Dulcie.
Miss Leslie looked rather surprised, but before she could ask any
questions, Uncle Stephen returned, and they were all packed into a
cab, Mr. Maitland explaining that he and Miss Florence were in a
hurry, and must get home as soon as possible.
“It’s been the loveliest afternoon we ever had in our lives,” declared
Daisy, as the cab drew up before their own door. “Oh, Uncle
Stephen, won’t we see you again—have you really got to go back to
California to-night?”
“I am afraid so,” Uncle Stephen answered, with a kind glance at the
row of sober little faces, “but perhaps I shall come back again before
such a very long time.”
“Don’t forget there’s always a next time to look forward to,” said
Miss Leslie, with her bright smile. “We’ve all had a delightful
afternoon to look back upon. I hope you won’t forget me.”
“Indeed we won’t!” cried Dulcie and Daisy both together, and Molly
added, plaintively:
“Oh, have you got to go back to California, too?”
“Yes, dear, Mother and I are leaving to-night, on the same train with
Mr. Maitland. But I want you to remember me, for I have an idea
that we shall meet again some day, and in the meantime I wonder if
you would write to me occasionally. I love to get letters from little
girls.”
“We’d love to,” said Daisy, blushing with pleasure. “We none of us
write very well except Dulcie, but if you wouldn’t mind a few
mistakes in spelling——”
Miss Leslie said she wouldn’t mind in the least, and by that time
Mary had opened the front door, in answer to Uncle Stephen’s ring,
and the good-byes had to be said.
“I feel just the way I’m sure Cinderella must have felt when she got
back from the ball,” remarked Dulcie, throwing herself wearily on the
nursery sofa. “That’s the only trouble about having good times;
everything seems so dull when they’re over.”
“I don’t mind,” said cheerful Daisy. “Just think what fun we’re going
to have talking it all over. I don’t think we shall ever feel quite so
lonely again, now that we know Uncle Stephen and Miss Leslie.”
“I don’t see what good they can be to us away off in California,”
objected Molly, who was sharing some of Dulcie’s depression.
“But we’ve promised to write to them both,” argued Daisy, “and that
will be very interesting. I wonder how soon it will do to write our
first letter.”
“I think we might write just a short one to Uncle Stephen to-
morrow,” said Molly. “It would be polite to tell him again what a
beautiful time we had, don’t you think so?”
Nobody answered, and there was a short silence, which Maud broke.
“I don’t think I want any dinner,” she remarked, with a long sigh.
“There’s going to be corned beef, there always is on Saturday, and I
hate corned beef. I’d like some more ice-cream, but I don’t want
anything else to eat. My head aches, and I think I’m going to have
another sore throat.”
CHAPTER IV
THE SINGING LADY

MAUD’S sore throats were one of the greatest trials to her sisters.
Not only were they of frequent occurrence, but they were always
regarded by Grandma in the light of an especial grievance to herself,
for which somebody must be held responsible. If Maud had lived in
the present day, some doctor would probably have decided that her
tonsils needed to be removed, but in 1880 people did not think so
much about operations, and the family physician contented himself
with prescribing simple remedies, and the advice that the child
should be kept out of draughts, and not allowed to get her feet wet.
Maud’s prediction on the present occasion proved only too true. In
the middle of the night Daisy was aroused by a feverish demand
from her little sister, for a drink of water, and by morning Maud could
not swallow without considerable difficulty, and the too familiar white
spots had appeared on her throat. Of course Grandma had to be
told, and the consequence was a severe lecture to the other three,
which lasted all through breakfast.
“I might have known what would happen when I let you all go off
yesterday,” grumbled Mrs. Winslow, as she prepared Maud’s gargle
in the nursery after breakfast. “I don’t suppose it ever occurred to
one of you to see that the child did not sit in her warm coat all the
afternoon.”
“Miss Leslie made her take off her coat,” protested Daisy, “and I
don’t really think she got over-heated or anything.”
“Well, she evidently caught cold in some way. At any rate, this has
taught me a lesson. Now remember, Maud, you are to gargle your
throat regularly every two hours, and take one of these powders
every hour. If I hear of your getting out of bed I shall punish you
severely.”
“Who is going to stay with Maud this morning, Grandma?” Daisy
asked, following Mrs. Winslow out into the hall. “I suppose one of us
will have to stay home from church.”
Grandma reflected for a moment. She was very particular about
church-going, but under the present circumstances it was evident
that Maud could not be left alone.
“I think you and Daisy had better come to church with me,” she said.
“Maud doesn’t need anything except her gargle and the powders,
and Molly can attend to them.”
So it was settled, much to Molly’s satisfaction, and at half-past ten
Dulcie and Daisy departed for church, with Grandma and Aunt Kate,
and the two younger children were left to themselves. Maud, who
was feverish and rather cross, was inclined to resent this
arrangement, which deprived her of the society of her two older
sisters.
“I want Dulcie to stay and tell me stories,” she pleaded. “Nobody can
tell stories but Dulcie.”
“I’ll tell you stories this afternoon,” said Dulcie. “I don’t believe
Grandma will make me go to church twice to-day, on account of your
being sick.”
“But I want stories this morning,” fretted Maud; “I want to hear
about Mamma. Ask Grandma to let you stay at home instead of
Molly.”
“It wouldn’t be any use; it would only make her crosser than she is
already. Molly will read to you. There’s a very nice book I got from
the library. It’s called ‘Ministering Children,’ and it’s a regular Sunday
story.”
“I don’t like the way Molly reads,” complained the invalid. “She can’t
pronounce the long words, and she keeps stopping to spell things. I
can read ’most as well as she can myself.”
But whether Maud liked it or not, there was nothing to be done, as
they all knew well. Grandma never changed her mind about things,
and when she had once given an order she expected implicit
obedience.
“I’ll do anything you want me to,” said Molly, good-naturedly, as the
retreating footsteps of the church-goers died away in the distance.
“We can’t play lotto, because it’s Sunday, but perhaps it wouldn’t be
wicked to cut out some paper dolls.”
Maud brightened a little at this suggestion, and for the next half-
hour all went well. Then it was time for Maud’s medicine, and she
began to rebel.
“I don’t like those nasty powders, and I’m not going to take any
more till Grandma comes home.”
“Then we shall both get an awful scolding,” said Molly, desperately.
“Grandma knows just how many powders there are, and she’ll count
to see if you’ve taken them all right. Do swallow this one, like a good
girl, and I’ll give you a drink of water to take away the taste.”
Perhaps Maud realized the force of her sister’s argument. At any
rate, she made no further objection to swallowing the medicine, over
which she made a wry face.
“When I grow up, I’m never going to take medicine,” she
announced, decidedly. “I’m not going to do a single thing I don’t
want to.”
“Maybe you’ll have to,” said Molly. “Grown-up people can’t always do
just as they like. Papa didn’t want to go to China and leave us all,
but he had to, and Lizzie didn’t want to go away. Listen, the lady
next door is beginning to sing.”
Maud’s face brightened.
“I’m glad,” she said. “She always sings hymns on Sunday. I wonder
why she doesn’t go to church. Maybe she’s sick, too.”
For ten minutes the room was very still, while the two children
listened to the music, which reached them distinctly through the
party wall. Then Maud began to show signs of restlessness again.
“I wish she’d sing ‘Only an Armor-Bearer,’” she complained, fretfully.
“‘Only an Armor-Bearer’ is my favorite hymn; it’s got such a nice,
lively tune. She ’most always sings it on Sunday.”
“Perhaps she will in a little while,” said Molly, and again there was
silence. But, contrary to their expectations, the lady next door did
not sing “Only an Armor-Bearer,” and after a few minutes the music
ceased.
“O dear!” cried Maud, “now she’s stopped, and I did want ‘Only an
Armor-Bearer’ so much. Can’t we ask her to sing some more?”
“Why, Maud, how could we? We don’t know her. Oh, Maud, don’t
begin to cry. You’ll be worse if you do.”
“I am worse now,” declared Maud, seizing eagerly upon this new
idea. “I’m much worse. Maybe I’m going to die and go to heaven,
like Mamma. If I do you’ll be sorry you wouldn’t ask the lady to sing
‘Only an Armor-Bearer.’”
“But how can I ask her, Maudie? It would be dreadfully rude to call
through the wall, and I don’t believe she’d understand, anyway. If I
went in next door I should have to ring the bell to get back, and
then Mary would see me, and she’d be sure to tell Grandma.
Besides, I wouldn’t know whom to ask for. We don’t even know the
lady’s name.”
Maud stopped crying, and raised herself on one elbow.
“If you’ll promise never to tell Grandma,” she said, “I’ll tell you
something. It’s my secret; I’ve had it for ever so many days.”
“A secret! What kind of a secret?” Molly was beginning to be
interested.
“It’s a very lovely secret,” said Maud, proudly. “You big ones are
always having secrets, so I got one, too. I won’t tell it, though,
unless you promise not to tell Grandma.”
“Of course I’ll promise. You know I never tell Grandma things, or
Aunt Kate either.”
“I don’t know that we ought to tell Dulcie and Daisy,” said Maud,
doubtfully; “they might think Grandma ought to know. That’s why I
didn’t talk about it. It was so exciting. I peeked in, but I was scared
to go any farther.”
“Peeked in?” repeated Molly; “where did you peek in?”
“Next door. Through the door in the trunk-room, you know.”
“Do you mean the door Grandpa had cut between the houses when
Uncle George lived next door? I thought it was locked up after Uncle
George died, and the boarding-house people came there.”
“It isn’t locked up,” said Maud, triumphantly. “I found out, and that’s
my secret.”
“Maud!” gasped Molly, her eyes round with astonishment. “You mean
you knew such an exciting thing, and never told any one.”
Maud nodded.
“I wanted to have a secret,” she said, “and I was afraid Dulcie or
Daisy would tell Grandma. It was the last time I had a cold, and
Grandma wouldn’t let me go out. I was up here playing all by myself.
I was looking for my littlest china doll. I couldn’t find her, and I
thought perhaps I’d left her in the trunk-room the day we played
Libby Prison in there, so I went to look. I did find her behind one of
the biggest trunks, and then I saw the door. I thought it was locked,
of course, but I shook the handle just for fun, and all of a sudden it
came open, and I looked right in next door.”
“What did you see?” demanded Molly, in a tone of breathless
interest.
“I didn’t see very much,” confessed Maud, reluctantly. “It was just a
big closet, and there were brooms and dust-pans in it, but it really
was next door. First I was going to tell, but then I was afraid if
Grandma knew she’d have the door locked up right away, and then
we could never go to see the singing lady.”
“I’m sure Grandma would have it locked right up,” said Molly, “and
perhaps the lady who keeps the boarding-house would, too, but it’s
very interesting to know it isn’t locked now. Why, it must have been
unlocked all the time since Uncle George died, and nobody ever
found it out before. I don’t believe the people next door know it any
more than we did.”
“Of course they don’t,” said Maud, “that’s what makes it so
interesting. Now you see you can go to see the singing lady just as
easy as anything, and ask her to sing ‘Only an Armor-Bearer.’”
“Oh, Maud, I couldn’t,” protested Molly; “it would be such a very
queer thing to do. The lady might not like it a bit, and Grandma
would make such a fuss. She never lets us talk to people she doesn’t
know.”
“You promised you wouldn’t tell Grandma, and I know the singing
lady wouldn’t be angry. You’ve got to do it, Molly, or else maybe I’ll
die and go to heaven.”
Molly hesitated. It would certainly be a thrilling experience to go
uninvited, and without even ringing the door-bell, into the house
next door, that mysterious boarding-house, upon whose occupants
Grandma and Aunt Kate looked down from their height of social
superiority. Molly loved adventure, and yet—what would Grandma
say? Would even Dulcie and Daisy altogether approve? Maud noticed
the hesitation in her sister’s manner, and was quick to take
advantage of it.
“If you won’t go,” she announced, sitting up in bed, “I’ll get right
straight up and go myself.”
Molly rose irresolutely.
“If I go, will you promise faithfully not to get out of bed for a single
minute till I come back?”
Maud nodded emphatically.
“I’ll promise, cross my heart, and that’s the solemnest promise
anybody can make, and if you break it something awful will happen
to you. Mary told me it would. I’ll lie just as still, as still, and when
you come back you can tell me all about the singing lady.”
“And will you gargle and take your powders all day without making
any more fuss?”
“Yes, and I’ll give you my best paper doll, and all her dresses. Don’t
you think I’m kind?”
Molly moved slowly towards the door.
“It seems an awful thing to do,” she said, “but I’ll only stay a minute,
and I can’t let you get out of bed.”
The door swung open so quietly and easily that she nearly fell
over backward.—Page 63.

Molly’s heart was beating very fast as she crossed the hall to the
dark room, which Grandma used for storing trunks and boxes. There
was no one to see her, for both the servants were in the kitchen,
and she and Maud had the upper part of the house quite to
themselves. The trunk-room was not locked, and she made her way
amid various impediments, to the heavy door, which she had always
known communicated with the adjoining house. Old Dr. Winslow had
had it made in days gone by, when the house next door had
belonged to his only brother, of whom he was very fond. This
brother had died before the children came to New York, and
although the house still belonged to the Winslow family, it had been
rented to a lady, who took boarders, much to the disgust of
Grandma and Aunt Kate, who looked upon a boarding-house as a
blot on the neighborhood. Molly was telling herself that her little
sister must have made a mistake. It did not seem possible that the
communicating door could have been left unfastened all these years,
without the fact having been discovered. With a trembling hand she
turned the knob. The door stuck a little, and she was just about to
turn away, convinced that Maud had dreamed the whole thing, when
suddenly the door swung open, so quietly and easily, that, in her
astonishment, she nearly fell over backward.
There, sure enough, was the closet, just as Maud had described it.
Molly fairly gasped, and in that one moment everything else was
forgotten in the excitement of the wonderful discovery she had
made. She did not shrink back, as Maud had done, but pushing her
way through brooms and brushes, and stumbling over various
articles on the floor, reached another door, which she opened, and
the next moment she had stepped out into a hall, which was exactly
like the hall of their own top floor.
It was very quiet, and there was no one to be seen. Molly closed the
closet door softly, and stood looking about her. There were four
rooms on the floor, and all the doors were closed. The singing lady’s
room was in the front, she knew, and after one moment’s hesitation,
she stepped boldly forward, and knocked.
“Come in,” called a pleasant voice, and there was a sound as of
some piece of furniture being moved rapidly along the floor. Before
Molly could quite make up her mind to turn the handle, the door was
opened from the inside, and a little lady in a wheel-chair suddenly
confronted her.
She was such a tiny lady that for the first moment Molly thought she
must be a child, but when the pleasant voice spoke again, it
sounded oddly familiar.
“Won’t you come in?” she said, and the face that looked at Molly
from the wheel-chair was so very sweet and winning, that half her
embarrassment melted away at once.
“I hope you’ll excuse me for coming,” she faltered, “but—but, you
see, we live next door, and my little sister is sick. We can hear you
sing through the wall, and we all love it. My sister wants me to ask if
you won’t please sing ‘Only an Armor-Bearer,’ because it’s her
favorite hymn.”
“Come right in,” said the lady, hospitably, “and would you mind
closing the door? The halls are rather chilly.”
Molly complied, and found herself in a room exactly like their own
nursery on the other side of the wall. Indeed, the two houses had
been built at the same time, and were alike in every particular. It
was evidently used as both bed and sitting-room, for a piano stood
between the windows, and by the empty fireplace stood a small
mahogany bookcase well filled with rather shabby-looking books.
The room might have been more tidy, for the bed was still unmade,
and on the table was a tray containing the remains of a breakfast,
but the lady herself was as neat as possible, although her blue
wrapper was somewhat faded, and the slippers on the little feet that
hung helplessly over the edge of the wheel-chair had long ago lost
their first freshness.
“You must excuse things being a little upset,” the lady said,
apologetically. “It’s Sunday morning, you know, and the
chambermaid has gone to church. She’s a nice girl, and very kind
and obliging, but I am afraid I give her a good deal of trouble. Take
those bedclothes off that comfortable chair, and sit down. It’s a great
pleasure to have a little girl come to see me. And so your sister likes
my singing. I am very glad. I had no idea any one cared about it.”
“We all like it,” said Molly, who had obeyed her hostess’ instructions,
and seated herself. “You see, our room is just on the other side of
the wall, and we can hear very well indeed. Maud is in bed to-day,
with a sore throat, and she loved the music.”
“Bless her heart!” cried the little lady, fairly beaming with pleasure,
“she shall have all the music I can give her. I love to sing, though I
know I haven’t much of a voice. Would you mind telling me your
name?”
“My name is Molly Winslow,” said Molly, “and my sisters’ names are
Dulcie, Daisy and Maud. It’s Maud who is sick. She’s only seven. I’m
nine, and Dulcie and Daisy are eleven and ten. Our mamma is dead,
and our papa has gone to China. We live next door with Grandma
Winslow.”
“I know who you are now,” said the lady, smiling; “you are old Dr.
Winslow’s grandchildren. I have always admired your grandfather’s
writing so much. I have read a number of his books, and I was so
much interested when I heard his house was next door.”
“Were you?” said Molly. “I’m glad you like Grandpa’s books. I didn’t
know anybody did. Dulcie began one once, but she said it wasn’t
very interesting. I suppose people ought to like their relations’
books.”
The lady laughed such a merry laugh that Molly found herself
laughing, too, though she did not know why.
“I think Dr. Winslow’s books might seem rather dull to a little girl,”
she said. “Perhaps I might have found them dull myself, if I were
able to get about like other people, but when one has to live in a
wheel-chair one is glad of almost anything to read.”
“Do you always have to stay in the chair?” asked Molly,
sympathetically. “I thought perhaps you had just sprained your ankle
or something like that. Papa sprained his ankle once and he had to
keep his foot up for three whole weeks.”
“I haven’t walked a step for nearly three years,” said the lady,
quietly.
“Can’t you even go up and down stairs?”
The lady shook her head.
“I was carried up here the day I left the hospital,” she said, sadly,
“and I have lived in this room ever since. I shall never walk again,
the doctors tell me. But I manage to get on very well,” she added,
brightening at sight of Molly’s distressed face. “You would really be
surprised to know all the things I can do without getting out of my
chair. Then people are very kind to me. Miss Collins, the lady who
keeps this house, was an old friend of my mother’s, and she often
comes to sit with me in the evening. The chambermaid helps me in
many little ways, and with my books, and my dear piano, I really get
on very comfortably indeed.”
Molly was deeply impressed.
“Could you walk when you were a little girl?” she inquired, anxiously.
A shadow crossed the lady’s sweet face.
“Oh yes, indeed,” she said. “I walked just like any one else till three
years ago, when I met with my accident.”
“What sort of an accident was it?” Molly was so much interested that
she quite forgot that some people might have considered her
questions rather impertinent.
“I was run over, crossing Broadway one very slippery day. The
ground was covered with ice, and I fell in the middle of the street.
Before I could get on my feet again, a horse-car came around the
corner, and the driver could not stop his horses in time. It really
wasn’t anybody’s fault.”
Molly rose. She was beginning to feel embarrassed again. There was
something in the sight of the helpless little figure in the wheel-chair
that made her feel all at once as if she wanted to cry.
“I’m afraid I must go,” she said a trifle unsteadily. “I can’t leave
Maud any longer. I’m awfully glad I know you, and the others will be
so interested when I tell them about you.”
“And I am delighted to know you, too,” her new acquaintance said,
heartily. “I have been more interested in my little neighbors than you
might suppose. You see, I can hear your voices through the wall,
just as you hear my singing, and when one spends a good deal of
time alone, one gets interested in all sorts of little things. I hope you
will come to see me again, and bring all your little sisters.”
“We’d love to come,” declared Molly. “Will you please tell me your
name in case we should want to ask for you at the front door?”
“My name is Oliver, Mary Oliver, but everybody calls me Miss Polly,
and I like it much better. My brother Tom always called me Polly. I
am sorry you must go so soon, for it is a great treat to have a visitor,
but I suppose you mustn’t leave your little sister any longer. I hope
you will find things in better order the next time you come. Maggie is
really very good about keeping the room neat, but Sunday morning
——” And Miss Polly glanced regretfully at the unmade bed and the
tray of breakfast dishes.
“Good-bye,” said Molly, holding out her hand.
Miss Polly shook the little hand—her own hand was not much bigger
—and then she looked at her visitor rather anxiously.
“Aren’t you afraid of taking cold without any wrap?” she questioned.
“To be sure it is only next door.”
“Oh, I don’t have to go out in the street at all,” said Molly,
unthinkingly. “I came through the door in the wall.”

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