The skin is the most visible vital organ of the human body. The skin includes the epidermis, the dermis and the hypodermis. The epidermis comprises stratified numerous squamous epithelial cells. The dermis comprises of fibroplast cells as well as inflammatory and immune cells, blood vessels and nervous tissue. Elastin, collagen and other extracellular fibers provide flexibility and strength to the skin. The hypodermis connects the skin and the rest of the body and is made up of a layer of subcutaneous fat cell that functions as a cushioning layer and offers thermal insulation. Today, the latest innovation technologies have attempted to imitate the human skin.
A research team led by Stanford Chemical Professor, Zhenan Bao, has successfully designed self-healing skin that can sense even slight pressure changes and heals on itself when cut or torn. The research team used the self-healing capability of a supramolecular organic polymer and the conductivity of embedded nickel microstructure nanoparticles resulting in a material with a good level of conductivity. The team ruptured the material with a scalpel and discovered that it returned back to its original strength and conductivity. The initial conductivity is completely restored with around 90% efficiency after a healing time of 15 seconds and mechanical properties restored after 10 minutes. The team concluded that the material can be used as a sensor in prosthetics and also in electrical wires and devices.
A research study conducted by Cabibihan J-J et al in 2011 investigated several configurations of synthetic finger phalanges for their skin compliance behavior and compared the same with the phalanges of a human hand and a prosthetic hand. The paper addressed the need for advanced methods and designs to ensure that prosthetic hands and arms are not obvious in social meetings. Handshake tests were done to determine the areas on the human hand that experienced high-contact forces. With the help of finite element simulations, the force displacement results of the synthetic finger phalanx designs were compared to experimental results from prosthetic and human finger phalanges.
During a handshake, it was observed that at the areas where it is possible to achieve full grasping of the other person’s hand, the mid-phalanges of the little, ring and middle fingers were chosen. The simulation model results show that by introducing a 2 mm height pocket on the synthetic finger phalange internal structure, the skin compliance of a silicone material improved to 235% and that of a polyurethane material to 436%. Furthermore, an indentation of a 2N force on the synthetic skin with an open pocket can attain a displacement over 2 mm while that on a commercially available prosthetic hand can achieve 0.2 mm.
Electronic Skin vs. Human Skin
Pang C et al (2012) designed a highly sensitive and flexible strain-gauge sensor using reversible interlocking of nanofibers. These flexible electronic sensors are capable of differentiating between twisting and shear forces, sensing the gentle steps of a ladybird and can be strapped to the wrist. The device includes two interlocked arrays of polymeric nanofibres coated with high aspect ratio Pt supported on layers of thin polydimethylsiloxane. The response of the sensor revealed high repeatability up to 10,000 cycles with excellent switching behavior. It has been proved in this study that the sensor can monitor signals from the effect of a bouncing droplet of water on a superhydrophobic surface to human heartbeat. The research is an attempt to mimic the complicated characteristics of the human skin.
Integrating Artificial Skin with the Human Body
A large amount of research has been done on large area, stretchable and flexible large area electronics. This type of research has focused on engineering efforts for the design of functional systems that adopt bio-inspired designs or the need to be intimately combined with the human body. Some examples include bio-integrated electronics for high resolution mapping of electrophysiology in the brain and heart (see an example of this in a video below).
Electronic skin measures heart rate, brain waves, and muscle activity
Kim DH et al (2008) developed a simple approach to stretchable, high-performance and foldable integrated circuits (ICs). The systems combine inorganic electronic materials, which include aligned nanoribbon arrays of single crystalline silicon with elastomeric and ultrathin plastic substrates. Three-dimensional computational and analytical modeling of the mechanics as well as electronic behaviors of these ICs was performed.
The Coleman Lab at the University of California San Diego along with the research group of John Rogers at the University of Illinois at Urbana Champaign is involved in the development of stretchable, foldable electrode arrays that are capable of non-invasive measurement of neural signals such as EEG without using gel. The electrodes are based on layouts developed recently for silicon electronics providing linear elastic responses to applied force with the capability to twist, fold and deform into a number of curved shapes.
The main benefit of stretchable electronics is that it can wrap curvilinear, arbitrary surfaces and also achieve mechanical properties similar to the tissues in the human body. Specifically, the signal-to-noise ratio of recorded signals gains advantage from low output impedances between the skin and the electrodes enabled by the conformal interface.
The mechanics and material ideas show mechanically invisible, intimate, consistent and tight attachment of high-performance electronic functionality with the skin surface in ways that surpass limitations of previous approaches. Future research efforts will focus on materials and devices that can accommodate efflux of dead cells from the skin surface as well as transpiration are needed to support ongoing research in this direction.
Sources and Further Reading
Benjamin C-K, Tee CW, Allen R, Bao Z. An electrically and mechanically self-healing composite with pressure- and flexion-sensitive properties for electronic skin applications. Nature Nanotechnology. 2012;7:825–832.
Kim D-H, Ahn J-H, Choi WM, Kim H-S, Kim T-H, Song J, et al. Stretchable and Foldable Silicon Integrated Circuits. Science. 2008;320(5875): 507–511.
Cabibihan J-J, Pradipta R, Ge SS. Prosthetic finger phalanges with lifelike skin compliance for low-force social touching interactions, Journal of Neuroengineering and Rehabilitation. 2011;8:16.
Kim D-H, Lu N, Ma R, Kim Y-S, Kim R-H, Wang S, et al. Epidermal Electronics. Science. 2011;333(6044):838–843.
Pang C, Lee GY, Kim TI, Kim SM, Kin HN, Ahn SH, et al. A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres. Nature Materials. 2012;11:795–801.
NS139 Casey G (2002) The physiology of the skin. Nursing Standard. 16, 34, 47-51.