Bio-inspired e-skin takes cues from nature for precision healthcare

July 18, 2023

(Nanowerk Highlights) Electronic skin (e-skin) devices that mimic the human skin’s ability to sense biomechanical and bioelectrical signals from the body show great promise for health and medical monitoring applications.

An international research team recently developed an innovative multi-layered e-skin patch called the SPRABE skin with skin-like properties that can collect different biosignals relevant to health status.

They report their findings in Advanced Functional Materials (“Stretchable, Breathable and Adhesive Skin with Multimodal Sensing Capabilities for Human-Centered Healthcare”).

Inspired by the multi-layered structure and various functions of human skin, the SPRABE skin design consists of special layers fabricated using scalable manufacturing techniques such as electrospinning and spray coating. The upper protective layer consists of a porous fibrous polymer scaffold that protects against external damage from abrasion, adhesion, water, etc., similar to the layer of the epidermis that protects our skin.

The new strain-sensing layer in the center is a nanocomposite of carbon nanotubes coated with 2D nanosheets of a recently discovered material called MXene. This network changes electrical resistance when stretched due to micro-cracks that form between the components. This allows the SPRABE skin to detect various biomechanical signals from body movements. MXenes are a class of cutting-edge nanomaterials with promising characteristics such as high conductivity and flexibility. SPRABE skin design. a) Schematic illustration of the bioinspired design of the SPRABE shell. SPRABE skin mimics attributes such as flexibility, breathability, self-adhesive, self-protection, and skin-inspired biomechanical/bioelectric sensing capabilities. b) SPRABE leather fabrication process based on electrospinning and spraying (above). Note that, the TPU membrane is electro-rotated to either side of the S-layer to act as both a P-layer and an I-layer. The molecular structures of the TPU and WPU polymer chains, and the schematic structure of the MXene were used. (Reprinted with permission by Wiley-VCH Verlag)

The lower electrode layer in contact with the skin contains conductive MXene flakes in a biocompatible polymer composite optimized to provide strong adhesion. This allows SPRABE skin to collect subtle biopotential signals on the skin surface such as ECG, EMG and EEG, which are important for clinical monitoring and diagnostics. An insulating layer between the sensing layers prevents electrical interference between the biomechanical and biopotential modes.

The rational combination of special materials in its layered architecture allows SPRABE skin to imitate the soft, elastic and breathable properties of human skin for comfortable wearing. Mechanical tests show that the e-skin patch can be stretched extensively to approximately 4000% elongation and reliably restores its original shape without permanent deformation or delamination between layers. The porous fibrous scaffold provides good permeability for moisture transport for skin-like breathability and prevents heat build-up under prolonged wear.

In testing, the protective coating effectively protected the strain-sensing coating from electrical damage under harsh conditions such as abrasion, stripping of tape and water immersion which would degrade exposed sensors. This durability allows for stable operation over time.

The main advantage of the SPRABE peel is the soft adhesive electrode interface which conforms to the skin and maintains firm contact even during strenuous activity. The strong adhesion is made possible by the compatibility between the components of the electrode coating and their molecular interactions with the skin. Importantly, the adhesive strength remains consistently high even after repeated attach/remove cycles and on damp, sweaty and hairy skin surfaces. This robust interface is essential for obtaining high-fidelity biomechanical and biopotential signals without interference and noise during natural movement and activity, unlike rigid gel electrodes.

For biomechanical sensing, the optimized nanocomposite strain sensing layer exhibits very high sensitivity over a wide elongation range of up to 485%, tens of times greater than previous e-skin sensors. The researchers found that the microcracks formed between the carbon nanotubes and MXene nanosheets when stretched increase the electrical resistance in proportion to elongation. Varying the ratio of MXene to carbon nanotubes sets the sensitivity.

Remarkably, the adhesive SPRABE leather patch detects subtle radial pulse waves at the wrist with details such as dicrotic grooves, even when underwater or under vibrations flooding the non-adhesive sensor. It also easily distinguishes leg movements such as walking, jumping, squatting based on different resistance signal patterns of knee deformation.

In tests for biopotential sensing, the SPRABE skin electrode impedance was much lower and more stable relative to frequency than standard gel electrodes. It manages to obtain comparable or better ECG, EMG and EEG signals than rigid electrodes under static and dynamic conditions. ECG waveform features such as the QRS complex and T wave remain clear when the subject moves the arm vigorously in the air or underwater, thanks to uninterrupted skin contact.

EMG tests also demonstrated higher fidelity of the SPRABE skin in monitoring patterns of muscle activation from subtle finger movements to heavy grips, promising prosthetic control. EEG signals can effectively sense the open/closed state of a subject’s eyes for brain monitoring applications.

Finally, the researchers demonstrated a versatile wireless multi-sensing platform based on the SPRABE skin for healthcare applications. The E-skin patch simultaneously measures ECG and foot movement continuously during long periods of running and jogging at different intensities on the treadmill. The e-skin closely conforms to body contours and stays put during exercise, enabling simultaneous high-quality recording of heart, muscle and movement data for health assessment.

In short, this multi-functional electronic skin technology has wide-ranging healthcare and medical applications ranging from daily health monitoring to diagnostic devices that assess heart health, nerve function and locomotor biomechanics. The bio-inspired multilayer design and scalable manufacturing allow for integrated, wearable e-skin patches that can conveniently monitor a wide range of biochemical and biomechanical signals with clinical-grade sensitivity, even under conditions demanding vigorous movement and environmental exposure. Further development of the multi-modal sensing concept that is soft, adhesive, and can lead to personalized health tracking in everyday life and high-performance wearable medical devices.

Michael Berger

– Michael is the author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: A Small Future And
Nanoengineering: Skills and Tools for Making Technology Invisible
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