Fabrication of alternating p/n-type TE fibers
The flexible TE fibers consisting of single-walled carbon nanotubes (SWCNTs) (Figure S1, S2) and polyvinyl alcohol (PVA) hydrocolloids were fabricated through a continuously alternating extrusion process (Figure 1a, Supplementary video 1). The room temperature extrusion is user-programmed and computer-controlled to automate an extrude-segment assembly line (Figure 1b). The p-type and n-type composite gel in the two separate polytetrafluoroethylene (PTFE) tubes move back and forth to closely align and extrude into a core tube. Both the loss modulus G” and storage modulus G’ of the formulated hydrocolloids are prepared to be very low with G’ < G”, enabling the gel deformable under applied pressure.33 The rheological properties of hydrogel after freezing gelation show much higher G’ and G” of around three orders of magnitude increase. The G’ of the gel is higher than the G” below the critical shear stress point, meaning that the gel will maintain its shape as long as the shear stress is lower than the critical value.34 Apart from the versatile tunability of the hydrocolloids, another notable advantage is that the migration of the solvent is severely restricted by the PVA polymer networks. Hence, alternatingly extruded gels are shown to preserve the p-n junctions and exhibit fairly clear interfaces even under continual compressive and shear stresses along the core tube (Figure 1c, top inset). In comparison, untailored solid and liquid matrixes face the issue of cross-mixing or even blending when they are successively extruded into a single tube (Figure 1c, bottom inset).
As indicated by the rheology properties evolution, the SWCNT/PVA and polyethyleneimine (PEI) doped compounds exhibit dramatic decrease in fluidity (Figure S3), similar to the PVA hydrogel. This aids in the restriction of the constituents i.e. SWCNTs and dopants within the specific segments of TE fiber (Figure S4). Moreover, the PVA-based hydrogel has the ability to adhere and heal when subjected to a freezing gelation process to realize firm and close contact between the adjacent gel segments. Figure S5 shows as-prepared alternating PVA gels with different user-defined segment lengths.
Mechanical and TE properties of TE fibers
With the bypassing of the traditional arduous synthesis and assembly of p/n-type TE fiber of limited scale, the proposed alternating extrude-segment assembly line demonstrates the feasibility of digitalized manufacturing of continuous meter-scale TE fibers (Figure 2a). Figure 2a presents a TE fiber under UV light which reveals an alternately distributed yellow (p-type) and green (n-type) TE segments. Figure S6 shows the profile of the TE fiber, exhibiting a relatively uniform diameter and evenly distributed CNTs. Moreover, PVA-based composites have been demonstrated to be of high strength and toughness.35, 36 Similarly, the as-prepared TE fibers also possess inherent mechanical merits as the TE fibers with or without PEI present high tensile strength over 20 MPa (Figure 2b). Notably, this signifies that the bonding between the adjacent p/n-type segments is very strong and able to hold weights, as heavy as 500 g (Figure 2b, inset). As a weaving material, the flexibility of the TE fiber is an essential requirement for body conformation, comfort and aesthetics. To verify its flexibility, the fiber was knotted into different styles or with other fiber (Figure 2c, inset). Moreover, the resistance changes under continuous bending of a TE fiber were recorded in Figure 2c to demonstrate its satisfactory electrical robustness.
In order to switch the majority carriers in the SWCNTs from holes to electrons, sufficient amine-rich PEI molecules should be added to inject ample electrons into the SWCNTs. Thus, the weight ratio of the SWCNTs and PEI profoundly affects the properties of the TE fiber. Figure 2d presents the Seebeck coefficient, S and electrical resistivity, change as the weight ratio of the PEI : SWCNTs increase gradually (the weight ratio of SWCNTs : PVA is fixed as 1 : 2). The pristine SWCNTs/PVA composite without dopant shows a positive Seebeck coefficient (39.5 mV/K), indicating the composite is p-type. As the PEI content increases, the Seebeck coefficient gradually shifts to negative (–48 mV/K at PEI : SWCNTs weight ratio of 30%), which indicates the switching to n-type. The resistance of the composite increases with the PEI content. Since high Seebeck coefficient facilitates the increase of temperature-gradient induced TE voltage while high resistance results in low current, the weight ratio of PEI and SWCNTs is chosen to be 25% to ensure a minimal tradeoff between the TE voltage and current. Figure 2e and 2f show the voltage-current varying at different temperature differences for 1 cm long p-type and n-type TE fibers, respectively. It can be seen that both the voltage and current increase linearly with the temperature differences. Figure S7 show the TE voltages of the p-type and n-type TE fibers vary with the temperature difference.
Conformal heat energy harvesting of TE textiles
Alternating p/n-type segmented TE fibers were plain weaved into flexible TE textile. Moreover, the TE textiles can be processed to be of different colors for aesthetic purpose and/or coated with other materials to be waterproof and washable as shown in Figure 3a and Supplementary video 2. The p/n-type units are electrically connected in series, similar to the commercial TE generators which have been demonstrated to be an efficient design (Figure 3b). In this configuration, the same designated p/n-type length were used to ensure not only the pitch weave matches the segment but also to ascertain successive p-n junctions alternate between hot and cold surfaces. Consequently, the carriers in each TE units will flow in same direction along the fiber, so that voltage multiplication can be achieved. Depending on different connections between the terminals of each fiber, on-demand voltage or current multiplication can be achieved by series or parallel electrical connections.
The heat flow direction for such in-plane configured TE textile is in thickness direction, different from conventional flexible TE generators that harvest heat in an in-plane direction. As the textile is directly made by weaving the TE fibers (diameter
of only 0.8 mm，length of p/n-type segment is around 2 mm) without incorporating other yarns, the thickness of the textile is quite small, leading to a small temperature gradient and hence a modest TE performance. For a single p/n TE pair, both the voltage and current monotonically increase as the temperature of the substrate increases from 5 to 20 °C higher than the ambient (Figure 3c). The temperature difference between the top and bottom of the textile is calculated to be around 1 °C at 20 °C, which is more than one order magnitude lower than the recorded one.
For a TE fiber composed of 8 pairs of p/n couples, the open-circuit voltage approximately amplified proportionally, for instance, the voltage increases from ~ 0.12 mV to ~ 0.52 mV at same apparent temperature difference of 20 °C (Figure 3d).
Meantime, the TE currents are almost same at corresponding temperature difference, agreeing with the tandem electrical circuit. We also develop a finite element model that allows us to calculate the temperature and electrical potential distributions along the fiber. The simulated results correspond well to the experiments (Figure S8). Further, the heat energy harvesting ability on curve surface was conceptually demonstrated by wrapping a piece of cloth composed of 33 TE fibers on a filled with water at different temperature (Figure S9). The TE fibers are woven in plain with in-series electrical connection at the terminals. As expected, the voltages are amplified by around 33-fold compared with a single fiber, while the current remains the same (Figure 3e). This demonstration not only shows the multiplication effect of the energy harvesting ability from one-dimensional fiber to two-dimensional textile, but also proves the conformability of the textile to a nonplanar surface.
TE textiles for heat and light sensing
Taking the advantage of the continuous p-type and n-type TE fiber, 10 TE fibers are easily woven into a patch of cross-stitch, as shown in Figure 4a. Every fiber composes of five p/n pairs with each p/n segment of 1 cm in length. All of the fibers cross at the p/n joints, as shown in Figure S10a. When a specific joint node contacts a hot object, the contact point displays a much higher temperature than the adjacent node as simulated in the inset in Figure 4b. Because of the enlarged segment length and well thermal insulated cotton cross-stitch yarns, the adjacent node around the contact point stay at a constant temperature. A touch induced temperature gradient along the p/n TE fiber manifests an electrical potential difference between the terminals of the contacted fiber. Conversely, the non-contacted fibers do not generate electrical signals (Figure S10b).
To locate the geometry coordinates, the warp and weft TE fibers are x and y axes defined. With an ambient temperature of 22 °C, the signal intensities when contacted with different temperatures were investigated (Figure 4b). As the object temperatures increase from 5 to 70 °C, the voltage along the x axis decreases from 0.85 to - 2 mV. Meantime, the voltage along the y axis, which is underneath the direct contacted fiber, indicates a relatively small change from –0.3 to 1.0 mV. Both the correspondences between the voltage signals and the object temperatures are stable and nearly linear, laying the foundations for touch positioning. Figure 4c and S11a-b show the signals of each fibers when node (3,3) was touched by a finger. The contacted X–3 and Y–3 fibers generate a strong signal as opposed to the absence of signal for the non-contact ones. The signal-to-noise ratio was calculated (Figure S11c-d), revealing a well discerning capability. The position addressable ability of this panel is also verified (Figure S12). With the 5 5 pixels touch panel, we further realize a hand-writing alphabetical ‘NUS’ inputs (Figure 4d-e and Supplementary video 3).
Benefitting from the superior light absorbing ability of carbon material (Figure 4f), the fibers easily heat up when irradiated by light. Inset in Figure 4f shows the temperature evolution of a fiber when irradiated by a light beam of 1000 W/m2. The photothermal heat will induce a temperature gradient between the light exposed and non-exposed regions to generate electrical signals (Figure 4g). Utilizing the photothermal light sensing property, TE fibers on the 6 faces of a cube was constructed and successfully realized light communication to accurately perceive the incident light orientation (Figure 4h, Supplementary video 4).
Modularized TE textiles for multitasking robot
The TE fiber composed garments can provide modularized solutions to equip the wearer with various functions, offering a new strategy for future conformal robotic electronics. Figure 5a shows a robotic arm capable of hot/cold perception, phototaxis and energy harvesting rendered by TE fiber glove, band wrist and sleeve, respectively. With a minimal pair of a single crossed p/n TE fiber stitched at the glove fingertips, the robotic arm can sense an object’s temperature and adaptively grasp or loosen its grip accordingly (Figure 5b and 5c). The phototaxis demonstration was realized by 8 TE fibers, evenly distributed around a wristband. When a light beam shines from a certain orientation, the particular irradiated fiber will generate a TE signal which in turn gives an electrical cue to rotate the arm to face the light source (Figure 5e-g). Supplementary video 5 shows the feedback control of a robot arm wearing TE garments including reflex of hot/cold subject and phototaxis. To manifest the body heat harvesting, a TE cloth was worn on a person’s arm. The local body surface temperatures at different states i.e. sleeping, working and exercising generate different TE voltages of 5, 7 and 12 mV, respectively (Figure 5d). Such conformable body heat harvesting textile can also be scaled up to be integrated into other wearable self-powering systems to charge batteries or power electronics directly. With such a suit of TE textiles, the garment enables a robot to multitasking, not limited to, sensing and energy harvesting.