Soft electrochemical bubble actuator with liquid metal electrode using an embodied hydrogel pneumatic source.

Soft pneumatic actuators-such as those used for soft robotics-achieve actuation by inflation of pneumatic chambers. Here, we report the use of the electrochemical reduction of water to generate gaseous products that inflate pneumatic chambers. Whereas conventional pneumatic actuators typically utilize bulky mechanical pumps, the approach here utilizes only electricity. In contrast to dielectric actuators, which require ∼kV to actuate, the electrochemical approach uses a potential of a few volts. The applied potential converts liquid water-a safe, abundant, and cheap fuel-into hydrogen gas. Since the chambers are constructed of hydrogel, the body of the actuator provides an abundant supply of water that ultimately converts to gas. The use of liquid metal for the electrode makes the entire device soft and ensures intimate contact between the chamber walls and the electrode during inflation. The device can inflate in tens of seconds, which is slower than other pneumatic approaches, but much faster than actuating hydrogels via principles of swelling. The actuation volume can be predicted and controlled based on the input parameters such as time and voltage. The actuation shape and position can also be controlled by the position of the electrodes and the geometry of the device. Such actuators have the potential to make tether-less (pump-free), electrically-controlled soft devices that can even operate underwater.

Energy Harvesting and Storage with Soft and Stretchable Materials.

This review highlights various modes of converting ambient sources of energy into electricity using soft and stretchable materials. These mechanical properties are useful for emerging classes of stretchable electronics, e-skins, bio-integrated wearables, and soft robotics. The ability to harness energy from the environment allows these types of devices to be tetherless, thereby leading to a greater range of motion (in the case of robotics), better compliance (in the case of wearables and e-skins), and increased application space (in the case of electronics). A variety of energy sources are available including mechanical (vibrations, human motion, wind/fluid motion), electromagnetic (radio frequency (RF), solar), and thermodynamic (chemical or thermal energy). This review briefly summarizes harvesting mechanisms and focuses on the materials' strategies to render such devices into soft or stretchable embodiments.

A Soft Variable-Area Electrical-Double-Layer Energy Harvester.

The technological promise of soft devices-wearable electronics, implantables, soft robotics, sensors-has accelerated the demand for deformable energy sources. Devices that can convert mechanical energy to electrical energy can enable self-powered, tetherless, and sustainable devices. This work presents a completely soft and stretchable (>400% strain) energy harvester based on variable-area electrical-double-layer (EDL) capacitors (≈40 µF cm-2 ). Mechanically varying the EDL area, and thus the capacitance, disrupts equilibrium and generates a driving force for charge movement through an external circuit. Prior EDL capacitors varied the contact area by depressing water droplets between rigid electrodes. In contrast, here, the harvester consists of liquid-metal electrodes encased in a hydrogel. Deforming the device by ≈25% strain generates a power density ≈0.5 mW m-2 . This unconventional approach is attractive because: (1) it does not need an external voltage supply to provide charge; (2) the electrodes themselves deform; and (3) it can work under various modes of deformation such as pressing, stretching, bending, and twisting. The unique ability of the harvester to operate underwater shows promising applications in wearables that contact sweat, underwater sensing, and blue energy harvesting.