In 2019, a captivating video was unveiled by MIT researchers, showcasing the remarkable capabilities of M-Blocks 2.0 cubes. The footage held us spellbound as these cubes exhibited a stunning array of movements. They seamlessly rearranged themselves into precise formations, effortlessly scaled atop one another, gracefully spun through the air, and engaged in an assortment of hive-like behaviors.
Watching these M-Blocks in action, we couldn’t help but draw comparisons to the beloved Transformers from fiction. These robotic blocks demonstrated an uncanny resemblance to their fictional counterparts, exhibiting independent locomotion, the ability to autonomously connect with one another to form predetermined structures, and even navigate along designated paths and towards sources of light. It was an extraordinary glimpse into a world where tangible objects appeared to possess a life of their own.
A Novel Approach
The human skin is truly remarkable, exhibiting a myriad of extraordinary capabilities. It possesses an intricate system that enables it to perceive temperature, pressure, and texture with great precision. Furthermore, it demonstrates exceptional elasticity, effortlessly stretching and then reverting to its original form repeatedly. Perhaps most impressively, it acts as a protective shield, effectively separating the body from various harmful elements that exist in the external environment, such as bacteria, viruses, toxins, and even ultraviolet radiation. Considering these astounding attributes, scientists and engineers are fervently driven to replicate the marvel of human skin by developing synthetic alternatives. Their vision encompasses the creation of robots and prosthetic limbs equipped with skin-like qualities, which includes the astounding ability to self-heal—a characteristic that distinguishes human skin as truly exceptional.
“We’ve achieved what we believe to be the first demonstration of a multi-layer, thin film sensor that automatically realigns during healing. This is a critical step toward mimicking human skin, which has multiple layers that all re-assemble correctly during the healing process,” remarks co-author Chris Cooper.
The findings of the study were published in Science.
It is soft and stretchable. But if you puncture it, slice it, or cut it— each layer will selectively heal with itself to restore the overall function,” adds co-author Sam Root. “Just like real skin.”
The skin is composed of multiple layers, each with its distinct characteristics. Remarkably, it has developed intricate immune mechanisms that facilitate the regeneration of tissue, restoring its original layered structure. This remarkable process relies on the sophisticated interplay of molecular recognition and signaling.
“With true ‘skin’ the layers should realign naturally and autonomously,” Cooper adds.
ccording to Root, a research group headed by Professor Zhenan Bao from Stanford University is on the verge of developing a revolutionary multi-tiered synthetic skin. The team’s groundbreaking work could result in the creation of individual layers, each measuring as thin as a micron or even less. In fact, these layers are so remarkably thin that a stack of 10 or more would have a thickness comparable to that of a single sheet of paper.
“One layer might sense pressure, another temperature, and yet another tension,” points out Root.
The composition of distinct layers can be intentionally designed to detect variations in temperature, physical stress, or electric properties.
“We reported the first multi-layer self-healing synthetic electronic skin in 2012 in Nature Nanotechnology,” adds Bao. “There has been a lot of interest around the world in pursuing multi-layer synthetic skin since then.”
The distinguishing feature of their current work lies in the self-recognition and alignment of the layers during the healing process, which enables the restoration of functionality layer by layer. In contrast to existing self-healing synthetic skins that require manual realignment by humans, this innovative approach allows for automatic alignment of like layers. Even a minor misalignment in the layers of existing synthetic skins could potentially jeopardize the recovery of functionality.
The secret lies within the materials themselves. Each layer of this innovative material is built upon a sturdy foundation comprised of long molecular chains that are interconnected through dynamic hydrogen bonds. These bonds bear a resemblance to the ones that hold the DNA double helix together, enabling the material to stretch repeatedly without tearing. While rubber and latex are well-known examples of natural polymers, there is a vast array of synthetic polymers available as well. The key lies in meticulously designing the molecular structures of these polymers and selecting the appropriate combination for each layer. This involves using one polymer for the first layer, another for the second, and so forth.
The researchers employed two specific polymers: polypropylene glycol (PPG) and polydimethylsiloxane (PDMS), commonly known as silicone. Both of these polymers possess rubber-like electrical and mechanical properties, biocompatibility, and the ability to be mixed with nano- or microparticles to facilitate electric conductivity. Crucially, the chosen polymers, along with their respective composites, are immiscible, meaning they do not mix with one another. However, due to the presence of hydrogen bonding, they strongly adhere to one another, resulting in the creation of a robust, multilayer material.
Both PPG and PDMS offer a unique advantage: they soften and become malleable when heated, but solidify as they cool. Consequently, by raising the temperature of the synthetic skin, the researchers were able to expedite the healing process. Normally, healing at room temperature can take up to a week. However, by heating the material to a mere 70°C (158°F), the self-alignment and healing occur within approximately 24 hours. Moreover, the two materials were meticulously engineered to exhibit similar viscous and elastic responses to external stress within an appropriate temperature range.
“Skin is slow to heal, too. I cut my finger the other day and it was still healing four or five days later,” Cooper adds. “For us, the most important part is that it heals to recover functions without our input or effort.”
‘Transformers’ in Real Life?
Having achieved a promising prototype, the scientists embarked on an even more ambitious endeavor. Collaborating with Professor Renee Zhao from Stanford University, they integrated magnetic materials into their polymer layers. This groundbreaking innovation endowed the synthetic skin with the remarkable abilities of autonomous healing and self-assembly, enabling it to seamlessly come together from individual components.
“Combining with magnetic field-guided navigation and induction heating,” adds Zhao, “we may be able to build reconfigurable soft robots that can change shape and sense their deformation on demand.”
“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,” adds Cooper, as he presents short videos (Video 1, video 2)featuring multiple fragments of stratified synthetic skin being submerged in water.
Utilizing magnetic forces, the individual pieces are drawn closer together, gradually reassembling into their original form. As the reassembly progresses, their electrical conductivity is restored, evident by the glowing LED mounted on the material.
The researchers’ upcoming endeavors involve reducing the thickness of the layers to the utmost extent possible and developing layers with diverse functionalities. The current prototype was specifically designed to detect pressure, but additional layers capable of sensing temperature changes or strain could be incorporated.
Looking ahead, the team envisions a future where robots can be ingested in fragments and autonomously assemble inside the human body to perform minimally invasive medical procedures. Other potential applications include the creation of self-repairing electronic skins that conform precisely to robots, granting them a sense of touch and multiple sensory capabilities.
Source: 10.1126/science.adh0619
Image Credit: YOSHIKAZU TSUNO/AFP via Getty Images