Taking inspiration from the remarkable abilities of human skin, scientists at Stanford University have made a significant breakthrough in synthetic skin technology. The researchers have developed a multi-layered thin film sensor that heals by auto-realignment, closely resembling the natural healing process of our skin.
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Human skin has long been the subject of interest and inspiration for engineers with its astounding ability to sense and react to various external stimuli and to heal itself. It's capable of considerable stretch and retraction, and it effectively guards the body against an array of external threats like bacteria, viruses, toxins, and harmful radiation.
Stanford's Breakthrough: Multi-layered Self-healing Synthetic Skin
Taking cues from these attributes, scientists have been attempting to create synthetic skin that could give robots and prosthetic limbs more human-like qualities, including the remarkable ability to heal themselves. Researchers at Stanford University accomplished a significant stride toward achieving this objective.
As explained by Chris Cooper, a Ph.D. candidate at Stanford and co-author of a study published in Science, "We've achieved what we believe to be the first demonstration of a multi-layer, thin film sensor that automatically realigns during healing."
In human skin, the process of healing involves the strategic self-realignment of multiple layers that reassemble correctly. To replicate this in synthetic skin, Cooper and fellow co-author Sam Root emphasize that layering is crucial. Each layer of the synthetic skin they've developed can selectively heal with itself to restore overall function, closely mimicking the behaviour of human skin.
The natural structure of skin is composed of multiple layers that are uniquely capable of molecular recognition and signalling. This results in the intricate process of rebuilding tissue in its original layered structure. With synthetic skin, the goal is to have these layers realign naturally and autonomously, just like human skin, says Cooper.
This is an image captured by a digital microscope, highlighting a depth-profiled view of a laminate film composed of five alternating layers. The layers, made from immiscible dynamic polymer films, have undergone a cycle of damage, autonomous realignment, and self-healing. The film is then intentionally pulled apart on a non-self-healing substrate to showcase the exact location of the initial damage. - (Image Credit: Bao Group, Stanford U.)
A significant improvement
Root continues in explaining that the team might be able to create synthetic skin with individual functional layers as thin as a micron each. This would make the total thickness of ten or more layers no more than a sheet of paper. Each layer could be engineered to sense specific changes, like pressure, temperature, and tension, making the synthetic skin highly functional.
The team, led by Professor Zhenan Bao, was previously known for reporting the first multi-layer self-healing synthetic electronic skin in 2012. However, what sets their current work apart is that the layers can self-recognize and align with like layers during the healing process, restoring functionality layer by layer. This is a significant improvement over existing self-healing synthetic skins that need manual realignment.
The right materials
The secret behind their success lies in the choice of materials. The backbone of each layer comprises long molecular chains periodically connected by dynamic hydrogen bonds, similar to the bonds that hold DNA strands together.
The researchers used PPG (polypropylene glycol) and PDMS (polydimethylsiloxane, better known as silicone), both of which have rubber-like electrical and mechanical properties and are biocompatible.
These materials can be mixed with nano- or microparticles to enable electric conductivity. Importantly, while they do not mix, these polymers adhere well to each other, creating a durable, multilayer material.
When heated, these polymers soften and flow, facilitating the healing process. At room temperature, healing may take up to a week, but at 70°C (158°F), the self-alignment and healing occur within about 24 hours.
The Stanford team then integrated magnetic materials into their polymer layers, allowing the synthetic skin to self-assemble from separate pieces. This exciting development could pave the way for reconfigurable soft robots that can change shape and sense their deformation on demand.
Amazing Potential Applications and Future Directions
With an initial prototype in place, the researchers aim to make the layers as thin as possible and create layers of varying function. The current prototype can sense pressure, and they plan to add layers to sense changes in temperature or strain.
“Our long-term vision is to create devices that can recover from extreme damage. For example, imagine a device that when torn into pieces and ripped apart, could reconstruct itself autonomously,” Cooper says, while showing several pieces of stratified synthetic skin immersed in water.
Drawn together magnetically, the pieces inch toward one another, eventually reassembling. As they heal, their electrical conductivity returns, and an LED attached atop the material glows to prove it.
Pieces of synthetic skin are drawn together magnetically; electrical conductivity returns as they heal, and the LED lights. - (Image Credit: Bao Group, Stanford U.)
From self-assembling robots that could perform non-invasive medical treatments inside the body to multi-sensory, self-healing electronic skins that give robots a human-like sense of touch, the possibilities are truly astounding.
In terms of future vision, the team imagines, potentially, robots that could be swallowed in pieces and then self-assemble inside the body to perform non-invasive medical treatments.
All in all, the science of synthetic skin is rapidly progressing, bringing us closer to the era of fully-functional humanoid robots, more advanced prosthetics and new methods to perform complex medical treatments. If you are interested in more details about the underlying research, be sure to check out the paper published in the peer-reviewed journal Science, listed below.
Sources and further reading:
Autonomous alignment and healing in multilayer soft electronics using immiscible dynamic polymers (Science)
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