(Nanowerk News) Human skin is amazing. It feels temperature, pressure, and texture. It can stretch and bounce back, time and time again. And it provides a barrier between the body and the bad things in the world — bacteria, viruses, toxins, ultraviolet radiation, and more. Therefore, engineers are interested in making synthetic leather. They envision robots and prosthetic limbs that have skin-like qualities—not least the skin’s incredible ability to heal.
“We have achieved what we believe is the first demonstration of a multi-layered thin film sensor that automatically realigns during curing. This is an important step towards mimicking human skin, which has many layers that all reattach properly during the healing process,” says Chris Cooper, Ph.D. candidate at Stanford University who, along with postdoctoral researcher Sam Root, are co-authors of the new study at Science announce advance (“Automatic alignment and healing in multi-layered soft electronics using immiscible dynamic polymers”).
Layering is essential to mimic the many qualities of leather. “It’s soft and stretchy. But if you poke it, slice it, or cut it—each layer heals itself selectively to restore overall function,” says Root. “Just like real leather.”
The skin is also made up of layers. It has just developed an immune mechanism that rebuilds tissues with the original layered structure through complex processes involving molecular recognition and signaling.
“With a true ‘skin’, the layers have to be laid back naturally and independently,” says Cooper.
Root says the team, led by Professor Zhenan Bao at Stanford University, may be able to create graded synthetic skins with individual functional layers as thin as a micron, perhaps less. Thin enough that a stack of 10 or more layers wouldn’t be much thicker than a sheet of paper. “One layer may feel pressure, another temperature, and another tension,” says Root. Materials of different layers can be engineered to sense thermal, mechanical, or electrical changes.
“We reported the first multi-layer self-healing synthetic electronic skin in 2012 Natural Nanotechnology (“An electrically and mechanically self-healing composite with pressure-sensitive and flexion properties for electronic skin applications”),” Bao said. “There has been a lot of worldwide interest in pursuing multi-layered synthetic leather since then.” What sets their current work apart is that the layers self-recognize and align with like layers during the healing process, restoring layer-by-layer functionality as they heal. Existing self-healing synthetic skins must be rearranged manually—by humans. Even a slight misalignment of the layers can compromise functional recovery.
The secret is in the ingredients. The backbone of each layer is formed from long molecular chains linked periodically by dynamic hydrogen bonds, similar to the double helix bonds of DNA strands, which allow the material to be stretched repeatedly without tearing. Rubber and latex are two well-known natural polymers, but there are also countless synthetic polymers. The key is designing the molecular structure of the polymer and choosing the right combination for each layer—the first layer of one polymer, the second layer of another, and so on.
The researchers used PPG (polypropylene glycol) and PDMS (polydimethylsiloxane, better known as silicon). Both have rubber-like electrical and mechanical properties and biocompatibility and can be mixed with nano- or micro-particles to enable electrical conductivity. Critically, the selected polymers and their respective composites are immiscible – they have not mixed with each other, because of hydrogen bonds, they adhere well to each other to create a durable multi-layered material.
Both polymers have the advantage that when heated they soften and flow, but solidify when cold. So, by heating the synthetic skin, the researchers were able to speed up the healing process. At room temperature, healing can take up to a week, but when heated to only 70 °C (158 F), it self-aligns and heals in about 24 hours. Both materials are carefully designed to have a similar viscous and elastic response to external stress over the appropriate temperature ranges.
“The skin is also slow to heal. I cut my finger a few days ago and it is still healing four or five days later,” said Cooper. “For us, the most important part is healing to restore function without our input or efforts.”
One Step Further
With a successful prototype, the researchers then took it a step further, working with Professor Renee Zhao at Stanford University, adding a magnetic material to their polymer coating, allowing the synthetic skin to not only cure but also self-assemble from the separate pieces. “Combining magnetic field-guided navigation and induction heating,” says Zhao, “we may be able to create reconfigurable soft robots that can change shape and sense deformation on demand.”
“Our long-term vision is to create devices that can recover from extreme damage. For example, imagine a device that when shredded and shredded, can reconstruct itself autonomously,” said Cooper, showing a short video of several graded pieces of synthetic leather submerged in water. Magnetically pulled together, the pieces edge past one another, eventually coming back together. As they cure, their electrical conductivity returns, and an LED mounted over the material glows to prove it.
Between the next steps, the researchers will work to make the layers as thin as possible and create layers with multiple functions. The current prototype is engineered to sense stress, and additional layers engineered to sense changes in temperature or strain may be included.
In terms of future vision, the team envisions, potentially, robots that could be swallowed to pieces and then self-assemble inside the body to perform non-invasive medical treatments. Other applications include multi-sensory, self-healing electronic skins that conform to robot shapes and give them a sense of touch.