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Gels
Special Issues
Bio-Inspired Polymeric Gels and Their Applications
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Bio-Inspired Polymeric Gels and Their Applications

A special issue of Gels (ISSN 2310-2861). This special issue belongs to the section "Gel Applications".

Deadline for manuscript submissions: closed (15 March 2023) | Viewed by 12192

Special Issue Editor

Prof. Dr. Chunxin Ma

E-Mail Website
Guest Editor
State Key Laboratory of Marine Resource Utilization in South China Sea, School of Chemistry and Chemical Engineering, Hainan University, Haikou 570228, China
Interests: hydrogels; intelligent materials; biomimetic soft actuators; supramolecular self-assembly; nanomaterials; fluorescent materials and displayers; adsorption and separation; degradable polymers, wastewater treatment and environmental protection
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Bio-inspired polymeric gels have seen rapid development in the last ten years, involving many cutting-edge fields. Through the design of bio-mimetic gel structures with anisotropic, hierarchical layered, directional porous, double-network and self-assembled structures, bio-inspired polymeric gels can provide various specific functions for intelligent complex/powerful self-actuations, high-strength/self-healing artificial cartilages, soft wearable systems, high-efficient adsorption/ separation, and so on. However, compared with biological tissues/structures through hundred million years of natural evolution, existing artificial polymeric gels only have decades of development and can only mimic some simple structures/functions of organisms. Thus, today, it remains a huge challenge for us to explore new polymeric gels with novel bio-inspired structures, which will significantly enhance their functions and extend their applications in the future.

Therefore, this Special Issue on “Bio-Inspired Polymeric Gels and Their Applications” focuses on research on bio-mimetic polymeric gels (hydrogels, aerogels, organic gels, ionic liquid gels and their composites) and their applications. We are pleased to invite you to contribute your recent work to this Special Issue. Your work can be recent research articles, reviews, or prospects about bioinspired polymeric gels, including but not limited to bio-mimetic structures, stimuli-responsive shape/color changes, self-healing, anti-biofouling, high-strength functions, adsorption, wearable sensing, and bio-medicine applications.

Dr. Chunxin Ma
Guest Editor

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Gels is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2100 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • bio-inspired polymeric gels
  • bio-mimetic structures
  • stimuli responsiveness
  • self-healing gels
  • anti-biofouling gels
  • high-strength gels
  • gel adsorbers
  • wearable sensing devices
  • bio-medicine applications

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Published Papers (5 papers)

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Research

15 pages, 3757 KiB  
Article
Anisotropic Bi-Layer Hydrogel Actuator with pH-Responsive Color-Changing and Photothermal-Responsive Shape-Changing Bi-Functional Synergy
by Chao Ma, Shuyi Peng, Lian Chen, Xingyu Cao, Ye Sun, Lin Chen, Lang Yang, Chunming Ma, Qijie Liu, Zhenzhong Liu and Shaohua Jiang
Gels 2023, 9(6), 438; https://doi.org/10.3390/gels9060438 - 25 May 2023
Cited by 7 | Viewed by 1916
Abstract
Stimuli-responsive color-changing and shape-changing hydrogels are promising intelligent materials for visual detections and bio-inspired actuations, respectively. However, it is still an early stage to integrate the color-changing performance and shape-changing performance together to provide bi-functional synergistic biomimetic devices, which are difficult to design [...] Read more.
Stimuli-responsive color-changing and shape-changing hydrogels are promising intelligent materials for visual detections and bio-inspired actuations, respectively. However, it is still an early stage to integrate the color-changing performance and shape-changing performance together to provide bi-functional synergistic biomimetic devices, which are difficult to design but will greatly expand further applications of intelligent hydrogels. Herein, we present an anisotropic bi-layer hydrogel by combining a pH-responsive rhodamine-B (RhB)-functionalized fluorescent hydrogel layer and a photothermal-responsive shape-changing melanin-added poly (N-isopropylacrylamide) (PNIPAM) hydrogel layer with fluorescent color-changing and shape-changing bi-functional synergy. This bi-layer hydrogel can obtain fast and complex actuations under irradiation with 808 nm near-infrared (NIR) light due to both the melanin-composited PNIPAM hydrogel with high efficiency of photothermal conversion and the anisotropic structure of this bi-hydrogel. Furthermore, the RhB-functionalized fluorescent hydrogel layer can provide rapid pH-responsive fluorescent color change, which can be integrated with NIR-responsive shape change to achieve bi-functional synergy. As a result, this bi-layer hydrogel can be designed using various biomimetic devices, which can show the actuating process in the dark for real-time tracking and even mimetic starfish to synchronously change both the color and shape. This work provides a new bi-layer hydrogel biomimetic actuator with color-changing and shape-changing bi-functional synergy, which will inspire new strategies for other intelligent composite materials and high-level biomimetic devices. Full article
(This article belongs to the Special Issue Bio-Inspired Polymeric Gels and Their Applications)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>(<b>a</b>) The preparation diagram of the bi-layer hydrogel. (<b>b</b>) The chemical reaction and the illustration of the chemical structures of the RFH layers. (<b>c</b>) The chemical reaction and the illustration of the chemical structures of the MPH layers.</p> Full article ">Figure 2
<p>(<b>a</b>) The SEM images of the bi-layer hydrogel. (<b>b</b>) The SEM images of the RFH and MPH. (<b>c</b>) Frequency sweep data of MPH in terms of G′ and G″ at 20 °C and 40 °C.</p> Full article ">Figure 3
<p>(<b>a</b>) The mechanism of pH-responsive fluorescent color-changing performance of the RFH. (<b>b</b>) The pH-dependent fluorescent emission spectra of RFH (λex = 365 nm). (<b>c</b>) The six cycles between pH = 3 and 10 buffer solution of the RFH. (<b>d</b>) The color-changing process of the RFH from pH = 10 to 3.</p> Full article ">Figure 4
<p>(<b>a</b>) Swelling ratio curve of MPH (2 mg/mL). (<b>b</b>) Absorbance of different concentrations of melanin solutions. (<b>c</b>) Temperature elevation curves of MPH with different melanin concentrations exposed to 808 nm NIR light (7.5 W/cm<sup>2</sup>). (<b>d</b>) IR thermal images of MPH (2.0 mg/mL) irradiated by different NIR light power. All scale bars are 5 mm.</p> Full article ">Figure 5
<p>(<b>a</b>–<b>c</b>) The bending process of the bi-layer hydrogel actuator under 7.5 W/cm<sup>2</sup> of NIR light irradiation. (<b>d</b>–<b>f</b>) The folding process of the bi-layer hydrogel actuator under 60 W/cm<sup>2</sup> of NIR light irradiation. All scale bars are 5 mm.</p> Full article ">Figure 6
<p>(<b>a</b>) The bending process of the bi-layer hydrogel actuator under 7.5 W/cm<sup>2</sup> of NIR light irradiation at 20 °C and pH = 3 in the dark. (<b>b</b>) The bending process of the bi-layer hydrogel actuator under 7.5 W/cm<sup>2</sup> of NIR light irradiation at 20 °C and pH from 10 to 3 in the dark. (<b>c</b>) The illustration of bi-layer hydrogel fluorescence intensity in different directions under UV light irradiation. (<b>d</b>) The bending process of the bi-layer hydrogel actuator under 7.5 W/cm<sup>2</sup> of NIR light irradiation with single-direction UV light irradiation. All scale bars are 5 mm.</p> Full article ">Figure 7
<p>(<b>a</b>) The tracing process of the biomimetic “gripper” in the dark. (<b>b</b>) The tracing process of the biomimetic “starfish” in the dark. (<b>c</b>) The synchronous color-changing and shape-changing process of biomimetic “starfish” in the dark. All scale bars are 5 mm.</p> Full article ">Scheme 1
<p>Schematic illustration of the composite actuator structure and fluorescent color-changing and shape-changing bi-functional synergistic biomimetic performance.</p> Full article ">
15 pages, 6139 KiB  
Article
A Nanoclay-Enhanced Hydrogel for Self-Adhesive Wearable Electrophysiology Electrodes with High Sensitivity and Stability
by Fushuai Wang, Lang Yang, Ye Sun, Yiming Cai, Xin Xu, Zhenzhong Liu, Qijie Liu, Hongliang Zhao, Chunxin Ma and Jun Liu
Gels 2023, 9(4), 323; https://doi.org/10.3390/gels9040323 - 11 Apr 2023
Cited by 8 | Viewed by 2646
Abstract
Hydrogel-based wet electrodes are the most important biosensors for electromyography (EMG), electrocardiogram (ECG), and electroencephalography (EEG); but, are limited by poor strength and weak adhesion. Herein, a new nanoclay-enhanced hydrogel (NEH) has been reported, which can be fabricated simply by dispersing nanoclay sheets [...] Read more.
Hydrogel-based wet electrodes are the most important biosensors for electromyography (EMG), electrocardiogram (ECG), and electroencephalography (EEG); but, are limited by poor strength and weak adhesion. Herein, a new nanoclay-enhanced hydrogel (NEH) has been reported, which can be fabricated simply by dispersing nanoclay sheets (Laponite XLS) into the precursor solution (containing acrylamide, N, N′-Methylenebisacrylamide, ammonium persulfate, sodium chloride, glycerin) and then thermo-polymerizing at 40 °C for 2 h. This NEH, with a double-crosslinked network, has nanoclay-enhanced strength and self-adhesion for wet electrodes with excellent long-term stability of electrophysiology signals. First of all, among existing hydrogels for biological electrodes, this NEH has outstanding mechanical performance (93 kPa of tensile strength and 1326% of breaking elongation) and adhesion (14 kPa of adhesive force), owing to the double-crosslinked network of the NEH and the composited nanoclay, respectively. Furthermore, this NEH can still maintain a good water-retaining property (it can remain at 65.4% of its weight after 24 h at 40 °C and 10% humidity) for excellent long-term stability of signals, on account of the glycerin in the NEH. In the stability test of skin–electrode impedance at the forearm, the impedance of the NEH electrode can be stably kept at about 100 kΩ for more than 6 h. As a result, this hydrogel-based electrode can be applied for a wearable self-adhesive monitor to highly sensitively and stably acquire EEG/ECG electrophysiology signals of the human body over a relatively long time. This work provides a promising wearable self-adhesive hydrogel-based electrode for electrophysiology sensing; which, will also inspire the development of new strategies to improve electrophysiological sensors. Full article
(This article belongs to the Special Issue Bio-Inspired Polymeric Gels and Their Applications)
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Figure 1

Figure 1
<p>(<b>a</b>) FT-IR spectra of NEH, XLS, and PAM. (<b>b</b>) SEM and (<b>c</b>) EDS mapping of PAM and NEH.</p> Full article ">Figure 2
<p>(<b>a</b>) Tensile strength and breaking elongation results of NEH with different contents of nanoclay. (<b>b</b>) The tensile stress–strain curves of NEH at 0 days and 7 days, compared with HCEP.</p> Full article ">Figure 3
<p>(<b>a</b>) Adhesive property for NEH with different percentage contents of nanoclay (The pink column is the selected nanoclay content for the following study). (<b>b</b>) Comparison of adhesive property on the glass substrate for 24 h at 25 °C, 65% humidity.</p> Full article ">Figure 4
<p>The weight loss curves of NEH and HCEP. (<b>a</b>) For 24 h, at 40 °C, 10% humidity, and (<b>b</b>) for 7 d at 20 °C, 20% humidity.</p> Full article ">Figure 5
<p>(<b>a</b>) Comparison of skin–electrode contact impedance at the forearm between the CEP and NEH electrode for 6 h. (<b>b</b>) Measurement for the electrode pair impedance of the NEH electrode and CEP.</p> Full article ">Figure 6
<p>(<b>a</b>) Schematic illustration for self-adhesive electrode mold. (<b>b</b>) Schematic illustration of the structure of the NEH self-adhesive electrode. (<b>c</b>) Image of the NEH self-adhesive electrode manufacturing process. (<b>d</b>) Image of NEH self-adhesive electrode adhering to the skin of forehead.</p> Full article ">Figure 7
<p>(<b>a</b>) Schematic illustration of electrode placement for the eye-open/eye-close EEG signal recording. (<b>b</b>) EEG power spectral density (PSD) of the eyes-open and eyes-closed periods. (The dashed box is to highlight the peak at 10 Hz.) (<b>c</b>) One channel EEG signal recorded during eyes blinking and rest. The time–frequency analysis of the signal from 0.5–45 Hz during (<b>d</b>) eyes open and (<b>e</b>) closed.</p> Full article ">Figure 8
<p>EEG signal recorded with NEH self-adhesive electrode after 3 days.</p> Full article ">Figure 9
<p>(<b>a</b>) ECG measurement with the CEP attached to the wrist. (<b>b</b>) ECG measurement with the NEH self-adhesive electrode attached to the wrist. ECG was obtained in three states: (<b>c</b>) static, (<b>d</b>) bending arm at 0.1 Hz, (<b>e</b>) bending arm at 0.2 Hz. The arrows represent peak-to-peak values of signals acquired at different actions. The capitalized letters represent different actions: static (T1), bending arm at 0.1 Hz (T2), static (T3), and bending arm at 0.2 Hz (T4).</p> Full article ">Scheme 1
<p>(<b>a</b>) Schematic illustration for the double-crosslinked network of Nanoclay-Enhanced Hydrogel and the fabricating process. (<b>b</b>) Application of the NEH self-adhesion electrode for electrophysiology signal acquisition.</p> Full article ">
14 pages, 5242 KiB  
Article
An Anisotropic Hydrogel by Programmable Ionic Crosslinking for Sequential Two-Stage Actuation under Single Stimulus
by Yanjing Zhang, Xingyu Cao, Yuyu Zhao, Huahuo Li, Shengwei Xiao, Zhangxin Chen, Guobo Huang, Ye Sun, Zhenzhong Liu and Zhicai He
Gels 2023, 9(4), 279; https://doi.org/10.3390/gels9040279 - 29 Mar 2023
Cited by 4 | Viewed by 2366
Abstract
As one of the most important anisotropic intelligent materials, bi-layer stimuli-responsive actuating hydrogels have proven their wide potential in soft robots, artificial muscles, biosensors, and drug delivery. However, they can commonly provide a simple one-actuating process under one external stimulus, which severely limits [...] Read more.
As one of the most important anisotropic intelligent materials, bi-layer stimuli-responsive actuating hydrogels have proven their wide potential in soft robots, artificial muscles, biosensors, and drug delivery. However, they can commonly provide a simple one-actuating process under one external stimulus, which severely limits their further application. Herein, we have developed a new anisotropic hydrogel actuator by local ionic crosslinking on the poly(acrylic acid) (PAA) hydrogel layer of the bi-layer hydrogel for sequential two-stage bending under a single stimulus. Under pH = 13, ionic-crosslinked PAA networks undergo shrinking (-COO−/Fe3+ complexation) and swelling (water absorption) processes. As a combination of Fe3+ crosslinked PAA hydrogel (PAA@Fe3+) with non-swelling poly(3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate) (PZ) hydrogel, the as-prepared PZ-PAA@Fe3+ bi-layer hydrogel exhibits distinct fast and large-amplitude bidirectional bending behavior. Such sequential two-stage actuation, including bending orientation, angle, and velocity, can be controlled by pH, temperature, hydrogel thickness, and Fe3+ concentration. Furthermore, hand-patterning Fe3+ to crosslink with PAA enables us to achieve various complex 2D and 3D shape transformations. Our work provides a new bi-layer hydrogel system that performs sequential two-stage bending without switching external stimuli, which will inspire the design of programmable and versatile hydrogel-based actuators. Full article
(This article belongs to the Special Issue Bio-Inspired Polymeric Gels and Their Applications)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>(<b>a</b>) Comparison of FTIR spectra of PZPAA@Fe<sup>3+</sup> bi-hydrogel in different area. (<b>b</b>) SEM image and EDS measurement of bi-layer hydrogel. (<b>c</b>,<b>d</b>) Equilibrium water contents (EWC) and (<b>e</b>) swelling/shrinking behavior of PZ hydrogel and PAA@Fe<sup>3+</sup> hydrogel from pH = 3 to pH = 13.</p> Full article ">Figure 2
<p>(<b>a</b>) Schematic illustration of the shape deformation of PZ-PAA@Fe<sup>3+</sup> bi-layer hydrogel strip. The bi-layer hydrogel at pH = 13 first bent toward the PAA@Fe<sup>3+</sup> side (+), then became straight, and finally bent toward the PZ side (−). (<b>b</b>) Bending deformation of PZ-PAA@Fe<sup>3+</sup> bi-layer hydrogel at pH = 13 and SEM images of the cross sections at different stages. All scale bars are 50 μm.</p> Full article ">Figure 3
<p>(<b>a</b>) Optical photos of bending process of PZ-PAA@Fe<sup>3+</sup> bi-layer hydrogel strip in pH = 13 at 25 °C. Bending kinetics of bi-layer hydrogel strips affected by (<b>b</b>) different pH and (<b>c</b>) different thickness ratios of PZ-PAA@Fe<sup>3+</sup>. All scale bars are 10 mm.</p> Full article ">Figure 4
<p>(<b>a</b>) Optical photos of bending process of PZ-PAA@Fe<sup>3+</sup> bi-layer hydrogel strip in pH = 13 at 55 °C. (<b>b</b>) Bending kinetics of bi-layer hydrogel strips affected by temperature. (<b>c</b>) Reversible bending behavior of the bi-layer hydrogel between pH = 3 and pH = 13. All scale bars are 10 mm. All scale bars are 10 mm.</p> Full article ">Figure 5
<p>(<b>a</b>) Tensile stress-strain curves of PZ-PAA bi-layer hydrogels coordinating at different Fe<sup>3+</sup> concentrations. (<b>b</b>) Stress and strain at break, (<b>c</b>) elastic modulus, and (<b>d</b>) tensile toughness of PZ-PAA@Fe<sup>3+</sup> bi-layer hydrogel.</p> Full article ">Figure 6
<p>Fabrication of PZ-PAA@Fe<sup>3+</sup> heterogeneous hydrogel actuators through painting Fe<sup>3+</sup> solutions onto PZ-PAA surface (PAA side) and their 2D and 3D deformation in response to pH = 13. (<b>a</b>) Schematic illustration of the locally Fe<sup>3+</sup> Coordinated PZ-PAA@Fe<sup>3+</sup> heterogeneous hydrogel. Controllable actuation of (<b>b</b>) dot- and (<b>c</b>) stripe-patterned hydrogels.</p> Full article ">Figure 7
<p>(<b>a</b>) Asymmetric deformation from planar sheet to lantern and folded box. (<b>b</b>) “Capturing-releasing” process of a plastic block by a cross-shaped hydrogel in pH = 13. (<b>c</b>) Multi-step deformation of the biomimetic hydrogel flower (the hydrogel petals are treated with 0.075 and 0.10 M Fe<sup>3+</sup>, respectively).</p> Full article ">Scheme 1
<p>Schematic illustration of the fabrication process of PZ-PAA@Fe<sup>3+</sup> bi-layer hydrogel and the programmable heterogeneous hydrogel actuators.</p> Full article ">
10 pages, 2077 KiB  
Communication
Bionic Aerogel with a Lotus Leaf-like Structure for Efficient Oil-Water Separation and Electromagnetic Interference Shielding
by Fengqi Liu, Yonggang Jiang, Junzong Feng, Liangjun Li and Jian Feng
Gels 2023, 9(3), 214; https://doi.org/10.3390/gels9030214 - 10 Mar 2023
Cited by 7 | Viewed by 2148
Abstract
Increasing pollution from industrial wastewater containing oils or organic solvents poses a serious threat to both the environment and human health. Compared to complex chemical modifications, bionic aerogels with intrinsic hydrophobic properties exhibit better durability and are considered as ideal adsorbents for oil-water [...] Read more.
Increasing pollution from industrial wastewater containing oils or organic solvents poses a serious threat to both the environment and human health. Compared to complex chemical modifications, bionic aerogels with intrinsic hydrophobic properties exhibit better durability and are considered as ideal adsorbents for oil-water separation. However, the construction of biomimetic three-dimensional (3D) structures by simple methods is still a great challenge. Here, we prepared biomimetic superhydrophobic aerogels with lotus leaf-like structures by growing carbon coatings on Al2O3 nanorod-carbon nanotube hybrid backbones. Thanks to its multicomponent synergy and unique structure, this fascinating aerogel can be directly obtained through a simple conventional sol-gel and carbonization process. The aerogels exhibit excellent oil-water separation (22 g·g−1), recyclability (over 10 cycles) and dye adsorption properties (186.2 mg·g−1 for methylene blue). In addition, benefiting from the conductive porous structure, the aerogels also demonstrate outstanding electromagnetic interference (EMI) shielding capabilities (~40 dB in X-band). This work presents fresh insights for the preparation of multifunctional biomimetic aerogels. Full article
(This article belongs to the Special Issue Bio-Inspired Polymeric Gels and Their Applications)
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Figure 1

Figure 1
<p>(<b>a</b>) Schematic diagram of CACAs preparation process. (<b>b</b>) Macro photo (inset) and SEM image of lotus leaves [<a href="#B32-gels-09-00214" class="html-bibr">32</a>]. Copyright 2019, Elsevier. (<b>c</b>) SEM and (<b>d</b>) TEM images of CACAs. (<b>e</b>) FT-IR patterns of raw materials and CACAs. (<b>f</b>) XRD patterns before and after carbonization. (<b>g</b>) Compression strength of CACAs with different CNTs content.</p> Full article ">Figure 2
<p>(<b>a</b>) Superhydrophobicity and (<b>b</b>) WCA of CACAs towards liquids with different PH. (<b>c</b>) Dynamic adsorption process of chloroform by CACAs. The adsorption processes of (<b>d</b>) n-hexane and (<b>e</b>) chloroform (both stained with Sudan red) from the surface and bottom of the water, respectively.</p> Full article ">Figure 3
<p>(<b>a</b>) The absorption capacity of CACAs towards different types of oils and organic solvents. Absorption recyclability of the CACAs through (<b>b</b>) distillation (n-hexane) and (<b>c</b>) combustion (DMF) for desorption. (<b>d</b>) Combustion cycle process of CACAs for DMF. (<b>e</b>) Adsorption and separation process of CACAs for methylene blue.</p> Full article ">Figure 4
<p>(<b>a</b>) Conductivity and SSA values of CACAs with different CNTs contents. (<b>b</b>) EMI SE<sub>T</sub> and (<b>c</b>) SE<sub>A</sub> properties of CANAs with various CNTs contents measured in X-band. (<b>d</b>) EMI SE<sub>T,</sub> SE<sub>R</sub> and SE<sub>A</sub> value of CANAs with various CNTs contents at 9 GHz.</p> Full article ">
17 pages, 4067 KiB  
Article
Eco-Friendly Starch Composite Supramolecular Alginate–Ca2+ Hydrogel as Controlled-Release P Fertilizer with Low Responsiveness to Multiple Environmental Stimuli
by Supattra Tiamwong, Pratchayaporn Yukhajon, Pittayagorn Noisong, Maliwan Subsadsana and Sira Sansuk
Gels 2023, 9(3), 204; https://doi.org/10.3390/gels9030204 - 7 Mar 2023
Cited by 6 | Viewed by 2366
Abstract
Environmentally friendly fertilizers (EFFs) have been developed to improve fertilizer efficiency and minimize adverse environmental impacts, but their release behavior under various environmental conditions has been less explored. Using phosphorus (P) in the form of phosphate as a model nutrient, we present a [...] Read more.
Environmentally friendly fertilizers (EFFs) have been developed to improve fertilizer efficiency and minimize adverse environmental impacts, but their release behavior under various environmental conditions has been less explored. Using phosphorus (P) in the form of phosphate as a model nutrient, we present a simple method for preparing EFFs based on incorporating the nutrient into polysaccharide supramolecular hydrogels using Cassava starch in the Ca2+-induced cross-link gelation of alginate. The optimal conditions for creating these starch-regulated phosphate hydrogel beads (s-PHBs) were determined, and their release characteristics were initially evaluated in deionized water and then under various environmental stimuli, including pH, temperature, ionic strength, and water hardness. We found that incorporating a starch composite in s-PHBs at pH = 5 resulted in a rough but rigid surface and improved their physical and thermal stability, compared with phosphate hydrogel beads without starch (PHBs), due to the dense hydrogen bonding-supramolecular networks. Additionally, the s-PHBs showed controlled phosphate-release kinetics, following a parabolic diffusion with reduced initial burst effects. Importantly, the developed s-PHBs exhibited a promising low responsiveness to environmental stimuli for phosphate release even under extreme conditions and when tested in rice field water samples, suggesting their potential as a universally effective option for large-scale agricultural activities and potential value for commercial production. Full article
(This article belongs to the Special Issue Bio-Inspired Polymeric Gels and Their Applications)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Photographs of wet and dry (<b>A</b>,<b>B</b>) PHBs and (<b>C</b>,<b>D</b>) s-PHBs, respectively.</p> Full article ">Figure 2
<p>SEM images of (<b>A</b>) PHBs-DI, (<b>B</b>) PHBs-pH = 5, (<b>C</b>) s-PHBs-DI, and (<b>D</b>) s-PHBs-pH = 5 for whole bead (<b>a</b>,<b>d</b>,<b>g</b>,<b>j</b>), outer surface (<b>b</b>,<b>e</b>,<b>h</b>,<b>k</b>), and cross-section inner surface (<b>c</b>,<b>f</b>,<b>i</b>,<b>l</b>), respectively.</p> Full article ">Figure 3
<p>FTIR spectra of (<b>a</b>) KH<sub>2</sub>PO<sub>4</sub>, (<b>b</b>) alginate, (<b>c</b>) starch, (<b>d</b>) PHBs-DI, (<b>e</b>) PHBs-pH = 5, (<b>f</b>) s-PHBs-DI, and (<b>g</b>) s-PHBs-pH = 5.</p> Full article ">Figure 4
<p>Plots for (<b>A</b>) swelling and (<b>B</b>) degradation behavior of (<b>a</b>) PHBs-DI, (<b>b</b>) PHBs-pH = 5, (<b>c</b>) s-PHBs-DI, and (<b>d</b>) s-PHBs-pH = 5.</p> Full article ">Figure 5
<p>TGA curves of (<b>a</b>) KH<sub>2</sub>PO<sub>4</sub>, (<b>b</b>) alginate, (<b>c</b>) starch, (<b>d</b>) PHBs-DI, (<b>e</b>) PHBs-pH = 5, (<b>f</b>) s-PHBs-DI, and (<b>g</b>) s-PHBs-pH = 5.</p> Full article ">Figure 6
<p>(<b>A</b>) Cumulative release ratio of (<b>a</b>) KH<sub>2</sub>PO<sub>4</sub> powder<sub>,</sub> (<b>b</b>) fertilizer 18-46-0, (<b>c</b>) PHBs-DI, (<b>d</b>) PHBs-pH = 5, (<b>e</b>) s-PHBs-DI, and (<b>f</b>) s-PHBs-pH = 5 in aqueous solution and (<b>B</b>) their plots of average phosphate release in the last 24 h.</p> Full article ">Figure 7
<p>Linear plots of kinetic models for phosphate release of (<b>a</b>) KH<sub>2</sub>PO<sub>4</sub>, (<b>b</b>) DAP, (<b>c</b>) PHBs-DI, (<b>d</b>) PHBs-pH = 5, (<b>e</b>) s-PHBs-DI, and (<b>f</b>) s-PHBs-pH = 5: (<b>A</b>) first-order, (<b>B</b>) Higuchi, (<b>C</b>) Ritger–Peppas, and (<b>D</b>) parabolic diffusion.</p> Full article ">Figure 8
<p>Effect of pH on phosphate release; (<b>A</b>) the cumulative release ratio and (<b>B</b>) their corresponding plots for phosphate release in the last 24 h of (<b>a</b>) PHBs and (<b>b</b>) s-PHBs at different pHs. Effect of temperature on phosphate release; (<b>C</b>) the cumulative release ratio and (<b>D</b>) their corresponding plots for average phosphate release in the last 24 h of (<b>a</b>) PHBs and (<b>b</b>) s-PHBs at different temperatures.</p> Full article ">Figure 9
<p>Effect of ionic strength on phosphate release; (<b>A</b>) the cumulative release ratio and (<b>B</b>) their corresponding plots for average phosphate release in the last 24 h of (<b>a</b>) PHBs and (<b>b</b>) s-PHBs at different NaCl concentrations. Effect of water hardness on phosphate release; (<b>C</b>) the cumulative release ratio and (<b>D</b>) their corresponding plots for average phosphate release in the last 24 h of (<b>a</b>) PHBs and (<b>b</b>) s-PHBs at different CaCO<sub>3</sub> concentrations.</p> Full article ">Figure 10
<p>(<b>A</b>) Cumulative release ratio of PHBs and s-PHBs and (<b>B</b>) their plots for phosphate release in the last 24 h in rice field water samples located in different provinces; (<b>a</b>) Khon Kaen, (<b>b</b>) Nakhon Ratchasima, (<b>c</b>) Buriram, (<b>d</b>) Maha Sara Kham, and (<b>e</b>) Surin.</p> Full article ">
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