Novel Wireless-Communicating Textiles Made from Multi-Material and Minimally-Invasive Fibers

1.1. Smart Textiles in Biomedical Applications

Smart textiles are based on research occurring in different disciplines: chemistry, physics, material science, and computer science and technology. Significant for this research is the interdisciplinary approach and the interaction between basic research and design activities. Textiles of today are materials with applications in almost all our activities, we wear clothes all the time and we are surrounded with textiles in almost all our environments. The integration of multifunctional use in such a common material has become a special area of interest in recent years. Fiber yarns, and fabrics [1] with added functionality have been developed for a wide range of applications in sport training [2,3] and healthcare industries [4], fashion [5], work ware and military [6,7]. It has been shown that various optical and electrical functionalities may be delivered at length scales and in a mechanically flexible form associated with fibers [8–10]. Besides sensing, reacting, and conducting electricity, the next-generation textile will be able to perform computational operations [10], thus shifting textiles from passive functionalities to a more dynamical role. Active functionalities in a smart textile may include power generation or storage, human interface elements, bio-sensing devices, radio frequency (RF) emission/reception, various assistive technologies such as personal emergency awareness systems and response communication. This new generation of textiles will favor the interaction between users and their environment by combining novel materials and integrated computing power, and should find numerous applications in almost every aspect of our lives.

Textile-based sensing has always been a large field of research in the biomedical community and has constantly attracted considerable attention. Up to date it was demonstrated that capacitive switches can be used for electrocardiogram (ECG) [11] and electromyography (EMG)—measuring activity produced by skeletal muscles [12], and electroencephalography (EEG) sensing [13]; fabrics incorporating thermocouples can be used for sensing temperature [14]; luminescent elements integrated in fabrics could be used for bio-photonic sensing [15]; shape-sensitive fabrics can sense movement [16], and can be combined with EMG sensing to calculate muscle fitness, etc. Thus, there is a great variety of textile bio-sensors that could be integrated into textiles, suitable for a variety of ubiquitous mobile applications pertaining, for example, to 24/7-connected healthcare and body area networks, which require delivery of wireless RF signals, as schematized in Figure 1. These applications in connected healthcare rely on WLAN, cellular networks, or other wireless communication bands to be effective 24 h per day, and thus require RF antenna integration into textiles.

1.2. Textile Antennas and Multi-Material Fiber Approach

In the past few years, several prototypes of RF textiles have been demonstrated, still the current trend of wireless textile communications devices is geared towards obtrusive monolithic patch designs. The most common approach relies on regular conductive threads [17] or coated polymer fibers [18] sewed in a shape of a patch antenna on top of the textile. There were also several attempts to integrate microstrip antennas or patch-like sensors not just between the layers or on top of a textile, but also literally inside the textile via 3D weaving [19] technique. Recently, Whittow et al. [20] demonstrated a textile patch antenna produced by inkjet-printing technique, though the authors admit that such antennas are prone to damage, operation frequency shifts and losses due to compressing and tensioning of the conducting surface.

Investments in both commercial and academic research have been driven mainly by leveraging the CMOS approach and further miniaturization of the electronics to obtain smart textile functionalities. In the CMOS approach, planar electronics and optoelectronic devices based on conventional microelectronics manufacturing processes are inserted in a hybrid manner into the threads of the textile. With this approach, practical issues have hindered the wide adoption of smart textile technology, such as:

Problem of manufacturability: The process for mass producing traditional textiles does not translate well to smart textiles containing CMOS components—cutting patterns from larges rolls of cloth and sewing them together to make garments breaks and reforms electronic connections in an uncontrolled manner. Generally, since smart fabric elements have to be introduced onto a textile surface in a separate post-processing step, this makes fabrication of RF textiles labor intensive and expensive.

Problem of environmental endurance: Clothing and some other textile products must be washable, which subjects the electronics elements to water and chemical immersion, physical stress, and extreme temperatures—the current state of the art tends to be too fragile for this treatment, especially for smart textiles containing CMOS components. The state of the art in flexible electronics and nanotechnology, required to integrate power and computer processing elements into fabric, has not advanced to the point that analog processing or digital logic can be integrated into fabrics.

Multi-material fibers have the potential to address some of the major challenges in developing sensor fabrics and bio-sensing textiles connected in real time to mobile communications infrastructures. By combining insulating and conducting materials into a well-defined geometry imbedded in a fiber, many functional electronic devices are achieved, in particular fiber and textile devices that lend themselves to RF-emission adaptable to existing broadband mobile infrastructures. Research in co-drawn metal-insulator-semiconductor photo-detecting fiber devices with mesoscopic-scale cross-sectional features has heralded a novel path to acoustic, thermal, and optical detection [8–10]. In such fibers, various functionalities may be delivered at length scales and in a mechanically flexible form. Thus, multi-material fibers provide a perfect building material for the next generation of biomedical textiles as they can be easily integrated during the weaving process in a cost-effective and minimally-invasive manner while preserving the mechanical and cosmetic properties of the garments.

2.1. Leaky Coaxial Cable Antenna

Leaky Coaxial Cables (LCXs) are known for their capability of distributing radio waves in tunnels, mines, underpasses, and many other confined environments, with smooth electrical field coverage [21]. There are very few studies on the use of small-scale LCXs for novel mobile communications systems such as biomedical textiles. Fiber RF-antennas based on LCXs can be achieved through combinations of materials and careful design to impart fiber RF-functionality, while preserving fiber sub-millimeter size and flexibility suitable for integration into textiles.

LCXs generally consist of three parts: an inner conductor, a dielectric material, and an outer conductor that exhibits slits at a prescribed periodic distance P [21–24], as schematized in Figure 2. The periodic distance P and the relative permittivity ε_r of the dielectric must be designed in accordance with the frequency constraints of the wireless networks as well as with the clothing material and dimensional constraints of the garment industry. Usually, the theoretical modeling considers an infinitely long coaxial cable that supports a single TEM propagation mode. This allows the use of Bloch-Floquet theory, which greatly eases the analysis of the radiation properties. The geometric parameters of the LCX are generally chosen as to allow only the m = −1 fundamental spatial harmonic [21], which radiates at an angle of:

This mode of operation provides a nearly transverse (i.e., broadside) radiation pattern. The frequency range of LCXs is determined by the periodic distance P of the slits and the relative permittivity ε_r of the dielectric, defined for the m = −1 fundamental spatial harmonic by the inequality:

The fiber antenna itself may represent only a few small threads in the textile fabric, thus enabling the use of non-standard textile fiber materials, such as polymer-clad silica fibers, without disrupting the mechanical and cosmetic attributes of the garment. This opens the possibility of leveraging the polymer-clad silica fiber manufacturing capabilities of the fiber optics industry. Such fibers (ε_r ∼ 3.7) provide a periodic distance P in the range from 1 to 40 cm, for carrier frequencies in the range of 0.8–5.0 GHz, respectively, which is compatible with typical clothing dimensions. Therefore, the LCXs can be designed for textile antenna applications with the WLAN 802.11b/g/n 2.4 GHz ISM band in mind, and can be scaled according to any desired carrier frequency of most wireless networks while leveraging the economies of scale of polymer-clad silica fiber industry. A transmission loss of a few dB/m for the coaxial cable is acceptable for textile antenna applications where typical clothing dimensions are below the meter range.

5.1. LCX Fiber Antenna

The radiation pattern measurements of the LCXs were obtained in an anechoic chamber using a wide-band Log periodic directional antenna Aaronia HyperLOG-7060 from 700 MHz to 6 GHz along with a tunable signal generator on the transmission side, while the LCXs acted as the far-field receiver. The LCX fibers were carefully positioned with the slits facing the transmitting antenna at 0° position. Radiation pattern measurements along the E-plane (parallel) and H-plane (perpendicular) polarizations are presented in Figure 6. In agreement with theoretical predictions, the LCXs exhibited an omnidirectional radiation pattern in the H-plane, while the E-plane presented several radiation lobes due to spatial harmonics of the LCX structure. Peak directivities in the E-plane were calculated at 6.8 dBi for the LCX operating at 2.4 GHz. Slight asymmetries in the radiation pattern of the LCX fiber may be attributed to roughness, thickness, and electrical conduction non-uniformities in the silver thin film lining of the LCXs. The sub-millimeter attribute of the coaxial design, the thinness of the conducting layers, and the inherent process variations of Tollen's silver nitrate reduction, render the LCX device sensitive to skin depth effects.

Two-conductor transmission lines, such as LCXs, have a dominant quasi-TEM mode of propagation. The quasi-TEM assumption is generally valid for structures made with low-loss dielectrics and near-perfect conductors [27,28]. Due to the thinness of the LCX silver thin film lining, as shown in Figure 4b, which is significantly thinner than the skin depth of silver at 2.4 GHz frequency, the condition of near-perfect conductor is not strictly enforced. Finite element calculations were carried out using COMSOL Multiphysics software to verify that a TEM mode could still propagate despite the thinness of the silver layer. Effective mode indices of n = 1.94 − 0.17j and n = 1.90 − 0.16j were found for the window-less and the windowed portions of the LCX, respectively. The simulated electric field structure and intensities shown in Figure 7 displaying quasi-TEM mode attributes and the indices nearing the expected value of n = 1.94 are strong indicators of quasi-TEM mode propagation. The results confirm that a TEM-like mode propagates inside the windowless segments of the LCX, whereas coupling to radiation takes place in the windowed segments.

5.2. Dipole Fiber Antenna

Radiation patterns of the textile-integrated dipole fiber antennas were also measured in the anechoic chamber at 2.45 GHz frequency, using the technique outlined in Section 5.1. We have also directly compared their performance with the commercial dipole WiFi duck antennas. These antennas are widely used in multiple wireless consumer devices (WiFi access point, routers, etc.) and for communication systems as they are cheap, provide omnidirectional radiation pattern. Results of these measurements are shown in Figure 8, both commercial (Figure 8 left) and fiber (Figure 8 right) dipoles demonstrate classical λ/2 dipole radiation patterns.

5.3. Gain Measurements

Efficiency of an antenna (η) is related to its gain (G) and directivity (D) as G = ηD and relates the power delivered to the antenna to the power radiated or dissipated within the antenna η = P_radiated/P_input. In order to measure antenna gain experimentally the following technique, based on the well-known Friis equation, was used. Two antennas were mounted on bases, oriented in the direction of the maximum transmitted signal, connected to an Agilent 8722ES network analyzer, and used for line-of-sight transmission in an unobstructed lab environment over a distance of 150 cm, greater than the far field criterion at f = 2.45 GHz, and a height of 120 cm off the ground. Additionally, an RF absorber was placed on the ground in order to exclude any multipath signals, schematic of the setup is shown in Figure 9.

In this case, the ratio of power available at the input of the receiving antenna, P_R, to output power of the transmitting antenna, P_T, is given by the Friis equation:

Thus, it becomes possible to determine the gain of the antenna-under-test either by using a well-characterized transmitting antenna with known gain or by using to identical unknown antennas. In our case, we again used the Aaronia HyperLog 7060 antenna as a transmitter and dipole antennas acting as unknown receivers. Measured S-parameter curves for the textile-integrated LCX and dipole fiber antennas are shown in Figure 10; S₁₁ curve (blue) corresponds to the return losses of the Aaronia HyperLog 7060 and S₂₂ curve (red) to the return losses of the textile-integrated fiber antenna, the green S₂₁ curve represents the power transmitted from one antenna to another.

Using the described technique, we have calculated the gain of the textile-integrated fiber dipole as 3.34 dBi and the gain of the commercial duck antenna as 3.45 dBi, which is close to the 3.5 dBi value provided by the manufacturer. Gain of the LCX fiber antenna was estimated as 1.62 dBi, which, although lower than typical values for the antennas in mobile applications, is still sufficient to act as a receiver.

These antennas have omnidirectional radiation patterns; therefore part of the radio waves emitted by the antennas would be absorbed by the human body. The rate at which energy is absorbed by the human body is measured by the Specific Absorption Rate (SAR), and its maximum levels for modern wearable devices have been set by governmental regulating agencies in many countries. In the USA, the Federal Communications Commission (FCC) has set a SAR limit of 1.6 W/kg, averaged over a volume of 1 g of tissue, for the head. In Europe, the limit is 2 W/kg, averaged over a volume of 10 g of tissue. Antenna simulations performed with the ANSYS HFSS software show that, with the current dipole configuration, the SAR is less than 1.6 W/kg when radiated power is −8 dBm at 2 mm distance from the skin surface of a human body. This would allow the RF-textile to maintain good-to-excellent signal strength (>−60 dBm) on the receiver part for most commercial WIFI router antennas.