The human skin is truly remarkable, possessing the ability to sense temperature, pressure, and texture, while also serving as a protective barrier against external threats like bacteria, viruses, and UV radiation. Engineers are fascinated by its properties and are actively working on creating synthetic skin that can mimic these qualities, including its incredible healing abilities.
A recent study conducted by researchers at Stanford University has made significant progress in this field. They have developed a multi-layer, thin film sensor that can automatically realign itself during the healing process, bringing us closer to replicating the complexity of human skin. The team, led by Professor Zhenan Bao, envisions synthetic skin that consists of multiple layers, each capable of independently healing and restoring its functionality, just like real skin.
The key to mimicking the properties of natural skin lies in layering. The synthetic skin being developed can be soft, stretchable, and self-healing. If one layer is punctured, sliced, or cut, it will selectively heal with itself, thereby restoring the overall function. This mimics the behavior of human skin, which rebuilds tissue with its original layered structure through a sophisticated process involving molecular recognition and signaling.
The aim is to create multi-tiered synthetic skin with individually functional layers that are incredibly thin, potentially as thin as a micron or even less. Each layer could be specialized to sense different stimuli such as pressure, temperature, or tension, and their materials can be engineered to detect thermal, mechanical, or electrical changes.
The ultimate goal is to develop robots and prosthetic limbs with skin-like qualities that can provide sensory feedback, protection, and healing capabilities akin to human skin. The recent breakthrough in creating a self-realigning, multi-layered sensor brings us closer to achieving this vision. With further advancements in this field, synthetic skin may revolutionize the field of robotics and prosthetics, enabling more natural and intuitive interactions between humans and machines.
A novel approach
In 2012, the team led by Professor Zhenan Bao at Stanford University reported the development of the first multi-layer self-healing synthetic electronic skin, which garnered significant attention worldwide. Since then, there has been growing interest in the pursuit of multi-layer synthetic skin. However, their latest work distinguishes itself by introducing a crucial advancement—the layers of the synthetic skin can now self-recognize and align with similar layers during the healing process, restoring functionality layer by layer. This is a remarkable improvement as existing self-healing synthetic skins require manual realignment, which can compromise the functional recovery if not done precisely.
The key to this breakthrough lies in the materials used. Each layer of the synthetic skin is composed of long molecular chains connected by dynamic hydrogen bonds, similar to the structure of DNA. These hydrogen bonds enable the material to stretch repeatedly without tearing, akin to the properties of natural polymers like rubber and latex. However, synthetic polymers offer countless possibilities as well. By carefully designing the molecular structures of polymers and selecting the appropriate combinations for each layer, the researchers achieve the desired properties. For their study, they used PPG (polypropylene glycol) and PDMS (polydimethylsiloxane, commonly known as silicone). These polymers possess rubber-like electrical and mechanical properties, biocompatibility, and can be mixed with nano- or microparticles to enable electrical conductivity. Crucially, the chosen polymers and their composites are immiscible—they do not mix with each other. However, due to the hydrogen bonding, they adhere well to create a durable, multi-layered material.
Both PPG and PDMS have the advantage of softening and flowing when warmed, and solidifying upon cooling. By heating the synthetic skin, the researchers could accelerate the healing process. While healing at room temperature can take up to a week, heating it to just 70°C (158°F) facilitates self-alignment and healing within approximately 24 hours. The materials were carefully designed to exhibit similar viscous and elastic responses to external stress over an appropriate temperature range.
Chris Cooper emphasizes that human skin also takes time to heal naturally, and their focus is on enabling the synthetic skin to heal and recover its functions without human intervention. By achieving this autonomous healing capability, the researchers are one step closer to replicating the remarkable healing abilities of real skin.
A step further
After successfully developing a prototype of the self-healing synthetic skin, the researchers collaborated with Professor Renee Zhao at Stanford University to explore additional possibilities. They introduced magnetic materials into the polymer layers of the synthetic skin, enabling not only the healing process but also self-assembly of separate pieces. This advancement opens up the potential for creating reconfigurable soft robots that can change shape and sense deformations through the combination of magnetic field-guided navigation and induction heating.
The researchers envision devices that can recover from extreme damage as part of their long-term vision. They demonstrated this concept by presenting a video where several pieces of stratified synthetic skin were placed in water. Through the magnetic attraction between the pieces, they gradually moved towards each other and eventually reassembled. As the healing process progressed, the electrical conductivity of the material was restored, as evidenced by an LED attached to the top of the synthetic skin glowing.
This breakthrough showcases the potential for creating autonomous devices that can self-repair and reconfigure themselves even when subjected to severe damage. By incorporating magnetic materials and the ability to self-assemble, the synthetic skin paves the way for the development of highly adaptable and resilient soft robots with shape-shifting capabilities.
Moving forward, the researchers are focusing on refining their approach by making the layers of the synthetic skin even thinner. They aim to achieve layers that are as thin as possible while maintaining their functionality. Additionally, they plan to develop layers with different sensing capabilities. While the current prototype can sense pressure, they envision incorporating layers that can detect changes in temperature or strain.
Looking ahead, the team envisions remarkable possibilities for their technology. One potential application is the development of robots that can be ingested in separate pieces and then autonomously self-assemble inside the human body. This concept could revolutionize non-invasive medical treatments by allowing robots to navigate and perform procedures internally.
Furthermore, the researchers foresee the creation of multi-sensory electronic skins that have the ability to self-heal and conform to the shape of robots. These electronic skins would provide robots with a sense of touch, enabling them to interact with their environment more effectively.
These future directions highlight the potential for the integration of synthetic skin technology into various fields, including medicine and robotics. The ability to create functional, self-healing layers with different sensing capabilities opens up a wide range of applications and paves the way for exciting advancements in the field.
Source: Stanford University