Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA.
Soft Matter. 2018 May 2;14(17):3296-3303. doi: 10.1039/c8sm00337h.
Soft and stretchable materials play an important role in the emerging fields of soft robotics, human-machine interfaces, and stretchable electronics. Hydrogels are compelling materials because they are soft, chemically tunable, biocompatible, and ionically conductive. Hydrogels have been used as components of skin mountable sensors, such as electrocardiogram (ECG) electrodes, and show promise in emerging devices as stretchable, transparent electrodes. Ultimately, these types of devices interface the hydrogel with rigid metallic electrodes to connect with electronic circuitry. Here, we show it is possible to interface hydrogel with liquid metal (eutectic gallium indium, EGaIn) electrodes to create completely soft and deformable electrodes that provide low resistance traces through the gel without altering its mechanical properties. As a case study, we created and tested electrodes for ECG monitoring. ECG electrodes require low impedance at biomedically relevant frequencies (1-50 Hz). Potentiostatic electrochemical impedance spectroscopy measurements show that capacitive effects at the hydrogel-EGaIn interface dominate the impedance at these low frequencies, yet can be reduced by interfacing the metal with acidic or basic hydrogels that remove the native oxide skin from the metal. Increasing the ionic strength of the hydrogel also helps in lowering the impedance of the metal-hydrogel electrodes. The resulting devices have signal-to-noise ratios that exceed commercial ECG electrodes. The softness of these hydrogels can be modified without compromising the electrical properties to create truly soft electrodes. Interfacing liquid metal conductors with hydrogels represents a potential strategy of creating soft electrodes for various bioelectronic applications, e-skins, and next-generation soft robotics.
柔软和可拉伸材料在软机器人、人机界面和可拉伸电子等新兴领域中起着重要作用。水凝胶是一种很有前途的材料，因为它们具有柔软、可化学调控、生物相容性和离子导电性。水凝胶已被用作可贴附于皮肤的传感器的组件，如心电图（ECG）电极，并在新兴设备中作为可拉伸透明电极显示出潜力。最终，这些类型的设备将水凝胶与刚性金属电极进行接口连接，以与电子电路连接。在这里，我们展示了将水凝胶与液态金属（共晶镓铟，EGaIn）电极进行接口连接以创建完全柔软和可变形的电极是可能的，这些电极在不改变其机械性能的情况下通过凝胶提供低电阻迹线。作为一个案例研究，我们创建并测试了用于心电图监测的电极。心电图电极在生物医学相关频率（1-50Hz）下需要低阻抗。恒电位电化学阻抗谱测量表明，凝胶-EGaIn 界面的电容效应主导了这些低频下的阻抗，但可以通过将金属与酸性或碱性水凝胶接口连接来降低阻抗，这些水凝胶可以去除金属的自然氧化皮。增加水凝胶的离子强度也有助于降低金属-水凝胶电极的阻抗。所得到的器件的信噪比超过了商业心电图电极。这些水凝胶的柔软度可以在不影响电性能的情况下进行修改，从而创建真正柔软的电极。将液态金属导体与水凝胶接口连接代表了一种为各种生物电子应用、电子皮肤和下一代软机器人创建软电极的潜在策略。
