Biomimetic design of the flexible BTS polymer. (A) Schematic illustration of the plant cell wall structure and of pectin. (B) Design of the ABA-type block copolymer, with m = 5, n = 5, p = 100, and r = 85.7. The hydrophilic A block, rich in carboxyl (red) and hydroxyl (blue) groups, is designed to electrostatically interact with metallic cations. The hydrophobic B block, composed of poly(n-butyl acrylate), is designed to provide mechanical flexibility and stretchability to the network. As shown in the particle size measurement (bottom right), the synthesized polymer exists in a colloidal state in ethanol due to phase separation. (C) Hypothesized mechanism governing the temperature response of the synthetic block copolymer. Rearrangement of the potential wells at low and high temperature in which the cations are confined (top). When an external electric field is applied, temperature rises cause an increase in ion migration through the hydrophilic channels formed between the colloidal particles in the polymer matrix (bottom). (D) Mechanical flexibility and stretchability of the synthetic block copolymer. The dried pectin film is prone to tear after a slight bending deformation (left), whereas the composite polymer is robust to repeated stretching motions because of the soft B block (right). The polymer is stretched 300% from its original shape. Credit: Science Advances (2023). https://www.science.org/doi/10.1126/sciadv.ade0423

Film formation behavior and component analysis of the block copolymer. (A) Digital images of the cross-linked block copolymer solution drop-casted on a flexible plastic substrate. The amount of cross-linking metal ions used for all tests is fixed to 100% unless otherwise stated. a.u., arbitrary units; EtOH, ethanol. (B) Time series of ATR-FTIR spectra after polymer deposition. During 1 hour of air dry, ethanol starts to evaporate, and subsequently, water is absorbed and saturated by the hygroscopic regions in the polymer matrix. *Peak assignments of the ATR-FTIR spectra (table S4). (C) TGA profile of the polymer film before and after washing in DI water. Titration analysis is performed to determine the percent of IPC formed in the composite film based on the amount of bounded metal ions after wash. Inset: Corresponding FTIR spectra of the films. (D) Arrhenius plot of different block copolymers. Activation energy compared between different polymer films with varying ratio of carboxyl to hydroxyl functional groups (type 1: m = 5, n = 5, p = 100; type 2: m = 4, n = 7, p = 100; type 3: m = 10, n = 0, p = 100). Inset: Corresponding thermal response of each type of polymer compared. Measurements are performed using the electrode design fabricated in fig. S6B with an AC voltage bias of 300 mV at 200 Hz. (E) Relation between the concentration of metal ions in the composite film and the amount of water adsorbed. Weight percentage of the polymer, Ca²⁺, and water is calculated on the basis of the TGA profile and the coupled gas-phase FTIR spectra of the dried polymer films (fig. S5) with varying amount of CaCl₂ (table S5). A linear relationship is observed between the CaCl₂ concentration and water absorbed by the composite film. (F) Impedance spectra of the polymer film with and without Ca²⁺. Credit: Science Advances (2023). https://www.science.org/doi/10.1126/sciadv.ade0423

Characterization of the flexible BTS polymer sensor. (A) Current variation as a function of temperature, measured between 15° and 45°C with an applied voltage of 300 mV at 200 Hz (bottom) and the corresponding current response calculated at different hydration levels (top). Darker blue lines represent higher hydration, and lighter blue lines represent lower hydration. Inset: Device schematic and configuration for electrical measurement. (B) Response comparison between state-of-the-art temperature sensing materials and devices, normalized with signals at 15°C. (C) Frequency-dependent current measured between 1 Hz and 5 MHz at varying temperatures (top) and the corresponding current response calculated when 300 mV was applied (bottom). Maximum response of 161.3 is obtained at 17.8 Hz. (D) Cyclic stability test over 100 cycles of continuous temperature oscillation between 15° and 45°C. (E) Temperature error extracted from the thermal sensor as a function of normal pressure and (F) bending strain (curvature). All plots represent data from polymer solution cross-linked with 100% concentration of CaCl₂. Credit: Science Advances (2023). https://www.science.org/doi/10.1126/sciadv.ade0423

Temperature sensing array based on flexible BTS polymer. (A) Schematic diagram of a flexible thermal sensor for large-area, multipixel temperature mapping. The sensor is placed on top of a glass plate where a heat source in the middle is used to generate a thermal gradient across the surface. Inset: Time versus temperature profile of the heat source (top) and the measured temperature value from each sensor (bottom). (B) Spatiotemporal reconstruction of the temperature evolution across the glass plate. Data are extracted from five sensor pixels along the radius of the glass slide. Credit: Science Advances (2023). https://www.science.org/doi/10.1126/sciadv.ade0423

IR sensor based on flexible BTS polymer. (A) Schematic of the fabricated long-wave, thermal IR detector. (B) Current as a function of IR power irradiated at a wavelength of 8.5 μm. Inset: Representative current profile during a 4-s period of IR exposure in air with different power. Current is measured using an applied voltage of 1 V at 200 Hz. (C) Responsivity of the IR detector at different wavelengths. Inset: Wavelength-dependent absorption intensity of the block copolymer. (D) Real-time detection of thermal radiation generated by a hand wave motion. Covering the hand with a sheet of paper, the rise in current is reduced due to limited transmission of irradiation power. Covering the hand with an aluminum sheet, no change or decrease in current is observed due to IR reflection. Credit: Science Advances (2023). https://www.science.org/doi/10.1126/sciadv.ade0423