超软可拉伸导电水凝胶的高灵敏度应变传感机制与应用

研究背景

水凝胶是一种由三维聚合物网络封闭水体而形成的软材料。由于其特殊性，水凝胶已被广泛应用于多种领域，包括可穿戴电子设备和离子电子设备、软机器人和机器、组织程、药物输送、光导纤维、3D打印材料和固态电解质。这些应用通常要求水凝胶具有强大的断裂韧性，以保持结构的完整性，从而在多变的机械载荷和变形条件下经久耐用。人们精心设计了非常规的水凝胶网络，如双网络、无机纳米复合材料、滑环连接、结晶聚合物骨架和疏水性均匀交联等，以赋予水凝胶极高的机械性能。除了良好的机械性能，不同的应用还要求水凝胶具有一系列特定的性能。例如为了满足柔性和可穿戴电子应用对高灵敏度、柔韧性和耐用性的要求，导电水凝胶还需具备软变形性、高离子传导性和防冻性等诱人特性。然而，要满足柔性和可穿戴电子产品的定制化关键需求，要实现理想的水凝胶，并将坚固的抗断裂特性、柔软的顺应变形性、理想的导电性能和高抗冷冻性结合起来，仍然是一项艰巨的挑战。

有研究认为，在水凝胶网络中引入高含量的无机盐是保证导电水凝胶具有优异导电性和抗冻性能的有效方法。然而，在高盐分环境下，水凝胶的初始机械性能可能会下降，因为水凝胶中的初始交联网络很容易受到引入离子的干扰。例如，先前的研究表明，高盐环境可能会导致聚乙烯醇、聚丙烯酰胺(PAM)和壳聚糖等传统水凝胶聚合物网络因盐析效应而发生缠结。在 1.0 M NaCl溶液中膨胀后，水凝胶的韧性和粘弹性显著下降，因为水凝胶网络在盐水环境中完全破裂。最近，作者团队提出了一种通过疏水性均相交联制造牢不可破的水凝胶网络的简便易行的可行策略。疏水性均相交联网络与外部高碱性环境的结合本应为柔性和可穿戴电子设备的导电水凝胶提供一种巧妙的设计。然而，高盐环境对疏水均相交联策略产生的水凝胶拓扑结构的影响仍有待进一步研究。

研究成果

要满足柔性可穿戴电子产品的定制化关键需求，设计出兼具坚固韧性、柔软弹性、理想导电性和抗冷冻性的理想导电水凝胶仍然是一项艰巨的挑战。四川大学谭帅&吉俊懿教授等人提出了一种制备符合这些预期需求的水凝胶的简便可行的策略，即在饱和的高盐溶液中使用稳定胶束的疏水性二乙烯基苯与丙烯酰胺的均匀交联一步制备水凝胶。研究了高碱性环境对水凝胶拓扑结构和机械性能的影响。在水凝胶聚合过程中，高碱性环境会抑制胶束中疏水交联剂的尺寸，从而削弱动态疏水关联，使水凝胶软化。不过，均匀的交联网络确保了超大变形时的抗断裂性。获得的水凝胶具有特殊的机械性能，兼具极软变形性和抗断裂性(杨氏模量，5kPa; 拉性，10200%；韧性，134 kJ m^-2；以及出色的抗裂纹扩展性)。饱和盐水环境还赋予了水凝胶理想的离子导电性 (106 mS cm^-1) 和抗冻性 (<20℃)。所得水凝胶的这些综合性能非常适合柔性电子应用，衍生应变传感器的高灵敏度和耐用性就证明了这一点。相关研究以“High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors”为题发表在ACS NANO期刊上。

图文导读

Figure 1. (a) Schematic illustration of fabrication process for PAMD-NaCl hydrogels, (b) energy dissipation mechanism of PAMD-NaCl hydrogels during deformations, and (c) extreme deformable and antifracture images of the prenotched PAMD-NaCl hydrogels after being stored at ?20 °C for 24 h.

Figure 2. (a) Size distribution change during preparing PAMD-NaCl hydrogels, (b) FT-IR spectra of freeze-dried PAMD and PAMD-NaCl hydrogels, (c) strain dependence of the storage (G′) and viscous (G″) moduli for PAMD and PAMD-NaCl hydrogels, and (d) frequency dependence of the G′, G″, and loss factors tan δ for PAMD and PAMD-NaCl hydrogels.

Figure 3. (a, b) Photographs of the PAMD-NaCl hydrogels underweight-hanging, compressing, cutting, blending, poking, knotting, and stretching at 25 °C (a) and ?20 °C (b). (c) Adhesive performance of the PAMD-NaCl hydrogels on diverse surfaces.

Figure 4. (a) Compressive stress?strain curve of the PAMD-NaCl hydrogels, (b) cyclic compressive stress?strain curves of the PAMD-NaCl hydrogels with an 80% strain, (c) nominal tensile stress?strain curves of the PAMD and PAMD-NaCl hydrogels, (d) cyclic tensile stress?strain curves of the PAMD-NaCl hydrogels with a 1500% strain, (e) nominal tensile stress?strain curves of the prenotched and unnotched PAMD-NaCl hydrogels for pure shear tests, and (f) comprehensive performance comparison in soft compliance (low E), stretchability, racture toughness, antifreezing properties, ion conductivity, and reversible cyclic deformations between PAMD-NaCl hydrogels and representative hydrogel sensors of the past 3 years.

Figure 5. (a?e) Polarized microscope images of the PAMD-NaCl hydrogels in various states (top) and with 45° deflection (bottom). S denotes stretching.

Figure 6. (a) Ion conductivity of the PAMD-NaCl hydrogels being bent at various degrees. (b) Ion conductivity of the PAMD-NaCl hydrogels during cyclic bending. (c) Ion conductivity of the PAMD-NaCl hydrogels during cooling. (d) Response and recovery times of the sensor based on the PAMD-NaCl hydrogels. (e?g) Resistance changes of the PAMD-NaCl hydrogels at room temperature and ?20 °C during deformation: (e) stretch and release; (f) press or thrust and release; (g) twist or bend and release. (h) Blowing the hydrogel surface and tapping the substrate. (i) Resistance change of the PAMD-NaCl hydrogels before and after a notch was introduced (λ = 2). (j)Resistance change of the prenotched PAMD-NaCl hydrogels during the 3600th cyclic deformation (λ = 2). (k) Resistance change of the PAMD-NaCl hydrogels during large cyclic deformation (λ = 102) before and after a notch was introduced. (l) Resistance changes and GFs of the PAMD-NaCl hydrogel sensor under a large applied tensile strain. (m) Brightness change of an LED controlled by deformation of the PAMD-NaCl hydrogels at room temperature and ?20 °C.

Figure 7. Sensing examples of the PAMD-NaCl hydrogel attached at the throat (a), forehead (b), wrist (c, d), finger (e), knee (f), and foot (g) for human motion detections at 25 and ?20 °C.

总结与展望

本文提出了一种简便的策略来制造水凝胶，这种水凝胶具有断裂韧性、极高的软变形性、理想的有利性能和优异的抗冻性等综合特性。这种水凝胶是通过丙烯酷胺和 DVB/SDS 胶束在饱和氯化钠溶液中原位聚合一步制备而成的，其原理是在水凝胶中形成动态疏水缔合和均匀的聚合物交联，从而实现有效的能量耗散。通过疏水均匀交联策略，阐明了高碱性环境对水凝胶拓扑结构和性能的影响。在水凝胶制备过程中，只需添加盐类就能显著改变所制备水凝胶的性能。在水凝胶聚合过程中，DVB/SDS 胶束在高盐分环境下的尺寸比在非盐分环境下的尺寸小，这削弱了水凝胶网络中疏水缔合的物理相互作用使水凝胶在小变形(杨氏模量5kPa时软化。尽管如此，由于存在均匀的交联网络，所获得的水凝胶仍表现出优异的抗断裂性、极可逆变形性和抗裂纹扩展性 (韧性，134J m^-2；拉伸性，102)。此外，饱和NaCl水凝胶理想的离子传导性 (106 mS cm^-1) 和抗冻性 (<20℃)。基于这些优异的综合性能，所获得的水凝胶作为应变传感器显示出了高灵敏度和耐久性。这项研究工作深入推进了人们对疏水均匀交联水凝胶拓扑结构的理解，并为柔性电子应用中理想导电水凝胶的设计提供了优雅而简单的方法。

文献链接

High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors

https://doi.org/10.1021/acsnano.3c09884