Researchers have been developing robots for decades that can assemble cars, pick items from conveyor belts, and battle each other in televised competitions. More recently, another growing category of robots – softbotics – has caught increasing attention due to their ability to interact with human safely and seamlessly. They have the potential to attach to your skin, crawl on the floor of your house, and assist you in daily chores. Development of softbotic machines and robots relies on the advancement of soft functional materials that mimic the properties of biological tissue. Such materials should be highly elastic and deformable but also provide electrical connectivity between batteries, microelectronics, motors, and other hardware for computing and robotic functionality.
In the recent decade, researchers have demonstrated novel materials that combine soft tissue like compliance with high electrical conductivity. This is typically accomplished by incorporating ultrathin circuit materials or networks of conductive particles into a soft polymer matrix. However, these materials are usually vulnerable to physical damage because of their low modulus and stiffness. Under the wear and tear of daily use, these soft materials are susceptible to mechanical and electrical failure caused by punctures, cuts, or severe impact. This can make their use prohibitive for applications in softbotics since the soft materials are exposed and there is no protective hardcase to prevent damage.
Inspired by the ability of natural biological tissue to heal itself when damaged, I attempted to discover a material that is of similar mechanical properties to human skin, capable of delivering high electrical current to power-hungry devices like motors, and able to heal itself spontaneously after damage without the need of external stimuli. Combining these three desirable properties in a single material could allow softbotic systems to be much more robust for everyday applications in robotics and human assistance.
I began by investigating materials that are both soft and self-adhere to restore their material integrity after being torn or severed. Among the various self-healing soft materials that I found in the literature, I was most intrigued by a poly(vinyl alcohol)–sodium borate (PVA–Borax) gel that had been reported in 2014 by Spoljaric and others in the European Polymer Journal. The polymer chains within the PVA-Borax gel are connected through hydrogen bonds that readily form after the material is severed and brought back into contact.
To make the gel electrically conductive, I adopted the common practice in my lab of embedding the polymer matrix with a suspension of silver microflakes and microscale droplets of liquid metal alloy. The liquid metal droplets serve two roles. First, they fuse the silver flakes together to form electrically conductive pathways that can be stretched with the surrounding gel. Second, they allow the pathways to form new connections after severed surfaces are brought back int contact. Together, the combination of self-healing PVA-Borax gel and self-wetting liquid metal give the composite its unique resilience to tearing or other mechanical damage.
One challenge in working with gels is there tendency to dry out in air. When hydrated with water, the weight of the PVA-Borax gel can reduce by up to 6 % within 24 hours. To address this, I replaced water with ethylene glycol, an organic solvent whose evaporation rate is negligible in ambient conditions. Compared to water-filled hydrogels, this organogel exhibits constant weight, which enables stable mechanical and electrical properties over time.
To demonstrate the versatility of the material for use in real-world conditions, I performed a few simple but illustrative implementations. First, I created a fully untethered, snail-inspired crawling robot composed of an on-board battery and an electrical motor embedded in a soft silicone exterior. The self-healing conductive composite is used as a connector between the motor and the battery. Working with my lab mates, we demonstrated that the crawling speed of the robot reduces by more than 50% when the connector is partially severed. After the severed halves of the conductive strip are manually reconnected, the snail robot recovers 68% of its initial speed. As another representative implementation, we showed that the conductive gel composite can be used to create reconfigurable circuits for powering a toy car and some on-board electronics. Finally, we showed that the composite can be used as a reconfigurable bioelectrode for measuring muscle activity on different locations of the body through electromyography. The material shows the ability to obtain high-signal-to-noise-ratio EMG readings and demonstrates the ability to reconfigure its size and shape to adapt to different locations of the body. Together, these demonstrations highlight the potential applications of our self-healing, electrically conductive organogel composite in soft robotics, soft electronics and healthcare.