Intelligent textiles, variously known as smart fabrics, electronic textiles, or e-textiles, have attracted considerable attentions worldwide due to their potential to bring revolutionary impacts on human life. Despite many promising progresses in this exciting newly emerged research field, there still exist a number of important obstacles. One of the most challenging issues is the conflict between the flexibility of the textiles and the need to incorporate sensing and computation modules. To address this critical issue, an innovative intelligent textile technology is proposed. The central hypothesis is that practical intelligent textiles can be realized by integrating fabrics with flexible transducers/electronics that are made using a unique, ¡®flexible-skin¡¯ technology.
The unique features of the silicon-based flexible skins are extremely desirable for intelligent textiles. A novel approach of making intelligent textiles by integrating the silicon-based flexible skins with textiles is proposed. The most important advantage of this novel technology is its compatibility with current MEMS and IC technologies, since MEMS devices (micromachined transducers) and ICs can be fabricated on the silicon wafer before the formation of the skin. This not only saves significant R&D efforts by avoiding re-invention, but also enables the integration of abundant sensing and computational capabilities offered by the silicon-based technology.
Figure 1. Top and cross section views of the proposed flexible skin to be woven into textiles.
In order to be integrated with textiles, the original flexible skins are modified and a new perforated structure is proposed as schematically shown in Fig. 1. The new flexible skin consists of 4 components: (1) silicon islands that host sensors, electronics, and bonding pads; (2) metal interconnect wires between silicon islands; (3) polymer layers that sandwich silicon islands and metal wires; and (4) stitching holes etched through polymer layers, which allow the direct sewing into fabrics. One of the fabricated silicon flexible skins is shown in Fig. 2 (a). It can be easily twisted and bent without breaking the interconnect traces and silicon islands as shown in Fig 2 (b)
Figure 2. (a) A silicon flexible skin with stitching holes; (b) a folded silicon flexible skin.
The flexible transducers/circuits made by the proposed method can be directly embroidered into textiles. It is worth noting that the stitching methods and patterns have substantial impact on the mechanical properties and robustness of the assembled fabrics. Numerous stitching patterns and methods, which have already been developed in the textile industry, will be an excellent resource to exploit. The following picture shows one skin stitched onto the surface of textiles using conductive yarns. The electrical contact was made by conductive epoxy.
Figure 3. A silicon flexible skin stitched onto the surface of a piece of KEVLARÒ fabric.
To study the mechanical properties of the assembled intelligent textiles, diffused silicon strain gauges were integrated on silicon islands as shown in the following picture. These strain gauges enable in-situ measurement of the strains experienced by the silicon islands during stretching, twisting, or bending.
Figure 4. Flexible skin with integrated strain gauges
Various functional yarns based on optical fibers, conductive polymer, metal filaments, or even carbon nanotubes have been developed for intelligent textile applications. However, due to the physical shape and temperature constraint, silicon-based sensors and CMOS circuits cannot be integrated, limiting the functionality of current smart yarns. To address this issue, we invented a SOI-CMOS compatible technology to fabricate smart yarns . This compatibility allows the integration of high performance CMOS circuits by taking advantage of SOI-CMOS foundries. The simplified fabrication process is illustrated in Fig. 5.
Figure 5. Simplified fabrication process: (a) Thermal oxide mask was used here for Boron diffusion; (b) Si island on device layer was patterned and the exposed BOX layer was removed; (c) Al deposition and patterning to form traces and pads; (d) 1st 3 ¦Ìm parylene deposition; (e) patterning the parylene openings and exposed metal traces; (f) XeF2 etching to completely undercut the handling wafer, making device free standing; (g) 2nd 10 ¦Ìm parylene deposition to seal the previously opened parylene windows; (h) patterning the parylene layer, open the contact pads and releasing the device; (i) PDMS was injected as a supporting core.
Using this new technology, we successfully demonstrated smart yarns integrated with silicon strain gauges and MOSFETs. Fig. 6 (a) shows a strand of parylene smart yarn which measures 7.5 cm in length and around 100 µm in diameter. Fig. 6 (b) illustrates a kink-free knot made using a strand of smart yarn. Fig. 6 (c) shows the cross section of a strand of PDMS-filled smart yarn device. The integrated silicon strain gauges and MOSFETs are shown in Fig. 7. The whole device is highly transparent even after being filled with PDMS. We expect that this technology will have significant impact on the field of intelligent textiles and some medical applications.
Figure 6. (a) A bent smart yarn device; (b) SEM image of a kink-free knot made by a strand of PDMS filled yarn; (c) Cross-sectional SEM image of a strand of smart yarn device filled with PDMS.
Figure 7. Micrographs of silicon strain gauge and MOSFET integrated in the smart yarns.
This material is based upon work supported by the National Science Foundation under Grants No. 0501314 and No. 0747620. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
R. B. Katragadda
and Y. Xu, ¡°A Novel Intelligent Textile Technology Based on Silicon Flexible
Skins¡±, Proceeding of the International symposium on wearable computer 2005
(ISWC05), Osaka, Japan,
2. R. B. Katragadda and Y. Xu, ¡°A novel intelligent textile technology based on silicon flexible skins¡±, Sensors and Actuators, vol 143, pp. 169-174, 2008.
3. Hongen Tu and Yong Xu, ¡°A SOI-CMOS compatible smart yarn technology¡±, The 17th International Conference on Solid-State Sensors, Actuators and Microsystems, June 16-20, 2013, Barcelona. Spain
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