Skin of the future? - Decoding electronic skin and a comprehensive review of the latest developments

INTRODUCTION

As one of the body’s most versatile organs, the skin plays a crucial role in maintaining overall health through its numerous functions. One such unique function is the ability to sense different stimuli, such as touch, pressure, pain, and temperature. Most importantly, it can withstand stress with its ability to stretch and self-repair. Taking inspiration from nature, scientists have developed electronic skin using advanced materials and nanotechnology to simulate human skin and its abilities.

“Smart skin” or “Electronic skin” refers to a super-thin electronic gadget that adheres to the skin like a tattoo and can measure a wide range of parameters in real-time analysis. E-skin originally was defined as a “large-area, flexible array of sensors with data processing capabilities, which can be used to cover the entire surface of a machine or even a part of a human body.”^[1] These are designed to monitor skin health by analyzing various parameters, such as pressure, temperature, hydration, oil levels, pH balance, and ultraviolet exposure. Furthermore, research on smart skin is increasingly focusing on the detection of biomarkers such as electrolytes, metabolites, proteins, and hormones in human sweat. Electronic skin can also be equipped with transdermal drug delivery systems to improve its real-time, non-invasive, dynamic therapeutic capabilities. What sets the E-skin apart from traditional wearables is its ability to track data at the molecular level, making it invaluable for health monitoring, early diagnosis, and even personalized treatments.

E-skin has been studied widely for its application in the field of robotics since the 1970s. Later, these tactile sensors were extrapolated for their applications in prostheses and mechanical arms. Thereafter, the applications of E-skin have grown in multitude for its functionality in robotics, prostheses, health monitoring, and therapeutics, making it one of the trending areas of research. The scope of an E-skin can range from a small patch to a large area covering a limb, with corresponding uses for health monitoring and robotics, respectively. It enables the detection of action-related information, such as slipping and grasping, while providing detailed tactile feedback, including texture and the hardness/ softness of contact forces. It can even accurately localize the pressure and give sensory feedback in real time. Using multifunctional sensors, it can accurately monitor various health-related biometrics in real time. This can be of use in remotely monitoring a patient’s vitals, sending data directly to the physician, thereby helping with real-time diagnosis and timely treatment.

OVERVIEW/ARCHITECTURE OF ELECTRONIC SKIN

Fabricating a high-performing, mechanically flexible, and stretchable E-skin similar to the human epidermis is a challenging task. A simple outline of the anatomy of E-skin is shown in Figure 1. The three main components of E-skin include a substrate with a power supply, interconnected sensors, and a wireless transmission network.^[2] These building blocks should have certain requirements to meet the definition of an ideal E-skin, which are summarized in Table 1.

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Figure 1:

A framework depiction of an electronic skin with a flexible substrate with a power supply, inbuilt sensors, and data transmission systems.

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Energy harvesting/power supply

One of the major areas of challenge in fabricating an E-skin is providing a long-lasting power supply, as wearables require independence from interchangeable batteries. The use of multiple microsensors and finite signal transmission networks rapidly increases the energy consumption. Although there has been significant progress in the available power units, they still carry certain restrictions such as frequent charging and battery replacement.^[25] Hence, the future of E-skin relies on creating a device that has minimal power consumption, low cost, is comfortable to wear, and is self-powered. So far, numerous studies have been conducted to harvest energy from various environmental sources^[26] as listed in Figure 2. The highest power conversion efficiency was recorded with flexible photovoltaic cells (InGaP-GaAs tandem solar cell).^[29] Other approaches included Cu(In, Ga) Se₂ solar cells,^[30] perovskite-based solar cells,^[31] dye-sensitizer electrolytes,^[32] and quantum dots.^[33] The most common strategies used for mechanical-to-electrical energy conversion were the piezoelectric effect and the triboelectric effect. The materials studied, operating based on the piezoelectric effect, were ZnO nanostructures,^[34] lead zirconate titanate films,^[35] and barium titanate nanoparticle films.^[36]

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Figure 2:

Environmental energy sources for electronic skin.

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Triboelectricity works on the principle of contact electricity, i.e., when two mechanical surfaces come in contact with each other, an electrical impulse is generated. Fan et al., reported the first flexible triboelectric nanogenerator, using PET polymer substrates and Au/Pd-Au metal sheets.^[37]

ADVANCES IN ELECTRONIC SKIN TECHNOLOGY

E-skin devices are subjected to continuous wear and tear as intimate contact and movement with skin causes fatigue and microcracks.^[42] Usage of self-healing materials increases longevity and heals any microcracks, thus avoiding major structural damage. This was achieved using microcapsules, which released a healing agent in the form of a liquid monomer upon contact with the microcracks.^[43] Other methods studied include the incorporation of conductive fillers into a self-healing network, use of covalent/non-covalent/hydrogen bonds,^[44] and use of metal droplets distributed in a silicon elastomer.^[45,46]

To mimic human skin, E-skin should be comfortable to wear for a prolonged period of time and thus, biocompatible. Hence, breathable materials with interconnected pores can be substituted as substrates to provide better permeability, as demonstrated by Shao et al.,^[47] who developed a PVDF nanofiber membrane.

With continuing advancement in the field of E-skin technology and artificial intelligence, integration of both these fields is the need of the hour. Although the current research efforts are focused on fabricating E-skins for the collection of various data, research on translating these inputs into meaningful outcomes is lacking. This is possible only by creating a closed-loop system with integrated machine learning for better data processing and pattern recognition.^[48]

APPLICATIONS OF ELECTRONIC SKIN

E-skin can be broadly categorized into two types [Figure 3], depending on their applications: (a) Artificial E-skins for robotics and prosthetics, which can replicate the functions of human skin, and (b) biomedical E-skins for real-time collection and analysis of physiological data for diagnosing and providing timely therapeutic interventions.

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Figure 3:

Types of E-skin.

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Robots equipped with artificial E-skins can obtain precise information, such as motion control and somatosensation, from their surroundings, which helps in the execution of complex tasks. Furthermore, they can be integrated with prosthetic devices to reciprocate essential human bodily functions like sensation. This is particularly useful in patients with neurological deficits, as in the case of leprosy.

Biomedical E-skins can be either diagnostic or therapeutic. Biomedical E-skins can be beneficial for collecting and monitoring real-time electrophysiological data such as brain signals, sweat collection and analysis, blood pressure, cardiac and muscle activities on a day-to-day basis in medicine and healthcare. Real-time sweat analysis is a novel noninvasive alternative to invasive blood testing owing to its convenience in sample collection and real-time monitoring of biochemical parameters.^[49] Koh et al., have developed a closed microfluidic system with inbuilt micropores that harvest the sweat from sweat pores, which are stored and later analyzed for biomarkers.^[27] These wearable devices enabled the wireless measurement and transmission of various physiological parameters, including total sweat loss, lactate, pH, chloride, and glucose concentrations, using colorimetric detection.^[27,49,50] This could also benefit in the detection of various analytes that are excreted by sweat, such as alcohol and heavy metals.^[51] Flexible and wearable electronics could offer numerous benefits to the field of sports medicine, in view of its comfortability and continuous fitness tracking. It could be used for the identification of various chemical metabolites and cardiac markers that could predict disease events before the onset of symptoms.

E-skin patches based on functional hydrogels are proven to be promising candidates as soft bio-integrated electronics for accelerated wound healing and wound repair, owing to their high water content and low Young’s modulus comparable to those of biological tissues. The various hydrogels studied for this purpose include poly(2-hydroxyethyl acrylate) (PHEA) hydrogel, Ag flake hydrogel, PEDOT: Polystyrenestyrenesulfonate hydrogel, polydopamine (PDA) hydrogel, and PVA hydrogel. The efficacy of these was studied on mouse models, where wound healing was promoted by electrical field-stimulated fibroblast migration, proliferation, and differentiation and also by iontophoretic drug delivery.^[52]

CHALLENGES AND LIMITATIONS

Fabricating an E-skin comes with a lot of challenges due to the ultrafine build nature of flexible electronics. The currently available planar electronics are rigid, lacking flexibility. The research requirements vary according to the intended use of E-skin.

As artificial E-skins are not intended for wearing on human skin, their requirements are those of robustness, better adhesion to devices, and longevity.

On the other hand, biomedical E-skins are desired to collect reliable electrophysiological data by direct integration on human skin. To achieve this and to reduce the side effects, it is crucial that both materials are cohesive with matched elastic moduli. In addition to this, biomedical E-skins need to be biocompatible, breathable, and self-healing for long-term operation.

Although there are laudable benefits of biomedical E-skin, we should take it with a pinch of salt for all the masked potential hazards. It could have various biophysical effects on the biological functioning of human skin, ranging from inflammation, interference with keratinocyte migration, affecting the skin barrier, and the normal desquamation of stratum corneum, to altered function of various receptors resulting in pigmentation and accelerated photoaging.^[53] It could have other effects related to allergy and disease, which are yet to be known.

Another paramount challenge is in developing a data transmission system with higher bandwidth and better energy efficiency. In addition, managing the storage demands of a huge database can be quite challenging.

FUTURE PROSPECTS

There have been significant advancements over the recent years in the technologies related to the fabrication of E-skin. The foremost challenge is to make an ideal E-skin with perception abilities, mimicking human skin in all aspects. Integrating a variety of sensors into one device can make it multifunctional.

Owing to the wear and tear associated with prolonged use of E-skin, the new generation of E-skin should have self-healing capacity, as discussed above.

Another key advancement is flexible printed circuit technology, which will replace the traditional rigid circuit boards and hence reduce the overall power consumption.

More recently, machine learning has been integrated with E-skin systems to enhance device control accuracy and to process data efficiently. This convergence can be of help in promoting the translation of these devices to clinical practice and patient care.

CONCLUSION

The advancements in E-skin materials and structures have been remarkable, opening exciting new avenues for research. However, the development of sophisticated materials with superior dexterity, sensing capabilities, and malleability remains a pressing need. To translate these innovations into affordable and accessible patient care solutions, further research and funding are essential.

Nevertheless, the future of E-skin holds tremendous promise, with vast applications across various fields, including dermatology.