Weaving a New World: Wearable Thermoelectric Textiles

Propelled by exciting leaps in materials science and fabrication technology, there is an expanding market for development of functional or “active” textiles materials which can provide a secondary function beyond traditional textile use [1,2]. One need not look farther than the recent commercial surge of smart watch technologies to recognize the growing demand for smart devices in the fashion industry. In this burgeoning field, researchers are utilizing a variety of innovative approaches, including combining textiles with Thermo Electric (TE) modules [3,4], a solid-state technology which provides temperature modulation (heating or cooling). The concept of a wearable TE textile is innovative and fun with huge potential, but might be unfamiliar to general readers since it has only recently emerged into mainstream markets. With this in mind, this report will provide an overview of the important concepts, relevant history, and current research status of TE technologies, highlighting the major challenges facing the field.


Introduction
Imagine a smart shirt that can automatically sense and maintain a person's temperature within a small homeostatic window, while also generating the necessary power for itself simply from body heat! The idea of this may sound fanciful, but the possibility of realizing this technology is closer than one might expect.
Propelled by exciting leaps in materials science and fabrication technology, there is an expanding market for development of functional or "active" textiles -materials which can provide a secondary function beyond traditional textile use [1,2]. One need not look farther than the recent commercial surge of smart watch technologies to recognize the growing demand for smart devices in the fashion industry. In this burgeoning field, researchers are utilizing a variety of innovative approaches, including combining textiles with Thermo Electric (TE) modules [3,4], a solid-state technology which provides temperature modulation (heating or cooling). The concept of a wearable TE textile is innovative and fun with huge potential, but might be unfamiliar to general readers since it has only recently emerged into mainstream markets. With this in mind, this report will provide an overview of the important concepts, relevant history, and current research status of TE technologies, highlighting the major challenges facing the field. TE technology has been commercially deployed for over a hundred years with two main applications: a.
Power generation: A TE generator can convert heat directly into electricity via the See beck effect and b.
Thermal management: A TE device can be used for active cooling or heating by applying an electrical current and leveraging the Peltier effect. Due to growing global demand for sustainable energy solutions, TE generators have emerged as an attractive non-fossil-based technology for improving the efficiency of energy consumption, enabling co-generation from thermal "waste" heat [5]. Peltier-based thermal management also provides unique advantages due to the lack of mechanical noise and toxic chemical refrigerants such as freon [5][6][7]. Furthermore, TE devices can effectively modulate temperature without any moving parts such as compressors, fans, or coolants, and so they have also been commercially applied for portable refrigeration and water purification applications [7]. Historically, the research community has focused most of its resources on improving performance in semiconductor-based TE devices for power generation and thermal management [8]. Owing to the exceptional performance of bismuth telluride (Bi 2 Te 3 )-based TE modules at room temperature, this technology has been commercially deployed in aerospace and automobile applications as both an energy source and solid-state cooling system, respectively [6]. More recently, flexible TE generators have emerged as promising candidates to supply power in wearable devices due to their ability to generate electricity from body heat continuously and semi-permanently. However, thus far, all varieties of commercialized TE modules have been based on rigid and brittle semiconducting materials, barring such devices from penetrating into the market for flexible or wearable devices [9]. In order to enable the application of TE technology to wearable devices, the design of flexible TE materials has been prioritized, along with the development of fabrication techniques suitable for preparing TE modules on flexible substrates. In pursuit of this goal, organic TE materials (such as conducting polymers) [10][11][12][13], nanocarbon materials [14,15], and hybrid organic-inorganic composite materials [16][17][18]  performance of organic materials still lags behind that of Bi 2 Te 3based modules, preventing scale-up of these materials from lab-scale to commercial implementation. [10] Furthermore, fabrication of flexible TE modules remains a challenging issue. Due to the special care that must be taken to avoid deteriorating mechanical and electrical performance in these soft materials, consideration of geometric design and electrode deposition technique are only two of many such challenges [19]. Textilebased TE modules are a highly promising candidate to overcome these challenges for wearable applications. While previous flexible TE modules have generally been developed as adhesive patches to be placed on the skin or attached via a wristband [20], textile-based TE modules can be directly fabricated as TE clothes via direct weaving of thermopiles into a textile. They are wholly new and innovative conceptually and thus come with a new host of challenges on both the material and device sides. For example, high mechanical strength and flexibility are further required for these woven modules in order to take advantage of a wide variety of innovative stitches [3]. Integration into daily life adds complexities as well; wearable TE textiles must be durable enough to withstand daily use and regular washing/drying conditions. Additionally, the thermal and electrical stability of textile-based modules is of critical importance for the effective design of devices capable of maintaining a stable electrical current or the cooling/heating states. These effects can be exacerbated by property mismatches between TE materials and subtractive textile layers. To provide perspective on the future of this field, some central challenges are summarized:

Material design
a.
Achieve high performance in flexible TE materials (ZT greater than 0.5). b.
Develop both n-and p-type TE materials that have harmonically matched performances. c.
Guarantee chemical stability, mechanical stability, and non-toxicity.

Module design
a.
Geometric design of wearable TE module with high power density.

b.
Fabricate metal electrodes efficiently to minimize the parasitic loss of thermal and electrical energy.

c.
Maintain the performance and mechanical durability of TE module in bent/strained state.

d.
Withstand washing/drying conditions without performance degradation.

e.
Measure and evaluate the performance of materials and modules by standard techniques.

Perspective
Despite facing a new set of challenges, textile-based wearable TEs are an exciting realm of research with huge potential and significant interest from both commercial and academic sectors. Realization of practical devices would enable personal electricity generation from body heat or smart temperature conditioners for both active cooling and heating purposes. Based on the current research trends, we expect wearable TE textiles to continue to gather momentum and appear increasingly in mainstream markets in the near future.