(Nanowerk Spotlight) Electronic skin (e-skin) devices that mimic human skin’s ability to sense biomechanical and bioelectrical signals from the body show great promise for healthcare and medical monitoring applications.
An international team of researchers recently developed an innovative multi-layered e-skin patch called SPRABE-skin with skin-like properties that can collect different biosignals relevant to health status.
Inspired by the multilayer structure and various functions of the human skin, the SPRABE-skin’s design consists of specialized layers made using scalable manufacturing techniques like electrospinning and spray coating. The top protective layer composed of a porous fibrous polymer scaffold guards against external damage from abrasion, adhesion, water, etc., analogous to the epidermis layer protecting our skin.
The novel strain sensing layer in the middle is a nanocomposite of carbon nanotubes layered with 2D nanosheets of a recently discovered material called MXene. This network changes electrical resistance upon stretching due to microcracks forming between the components. This allows the SPRABE-skin to detect various biomechanical signals from the body’s movements. MXenes are a cutting-edge class of nanomaterials with promising characteristics like high conductivity and flexibility.
The design of the SPRABE-skin. a) Schematic illustration of the bioinspired design of SPRABE-skin. The SPRABE-skin mimics the skin-like attributes of stretchability, breathability, self-adhesiveness, self-protection, and skin-inspired biomechanical/bioelectrical sensing capabilities. b) Fabrication process of the SPRABE-skin based on electrospinning and spraying (top). Note that, the TPU membranes were electrospun onto both sides of the S-layer to serve as P-layer and I-layer. Molecular structure of polymer chains of TPU and WPU, and the schematic structure of MXene used. (Reprinted with permission by Wiley-VCH Verlag)
The bottom electrode layer that makes contact with the skin contains conductive MXene flakes in a biocompatible polymer composite optimized to provide robust adhesion. This allows the SPRABE-skin to collect subtle biopotential signals on the skin’s surface like ECG, EMG, and EEG, which are important for clinical monitoring and diagnostics. An isolating layer in between the sensing layers prevents electrical interference between the biomechanical and biopotential modes.
The rational combination of specialized materials in its layered architecture allows the SPRABE-skin to mimic the soft, elastic, and breathable nature of human skin for comfortable wearability. Mechanical tests showed that the e-skin patch can be strained extensively to around 4000% elongation and reliably recover its original shape without permanent deformation or delamination between layers. The porous fibrous scaffold gives it good permeability to water vapor transportation for skin-like breathability and preventing heat buildup when worn for long durations.
In tests, the protective layer effectively shielded the strain sensing layer from electrical damage under harsh conditions like abrasion, tape peeling, and water immersion that would degrade exposed sensors. This durability allows stable operation over time.
A major advantage of the SPRABE-skin is its soft adhesive electrode interface that conforms to skin and maintains robust contact even during vigorous activity. Strong adhesion is enabled by compatibility between the electrode layer components and their molecular interaction with skin. Importantly, adhesive strength remained consistently high even after repeated cycles of attaching/detaching and on damp, sweaty and hairy skin surfaces. This robust interface is critical for acquiring high fidelity biomechanical and biopotential signals without interference and noise during natural motions and activities, unlike rigid gel electrodes.
For biomechanical sensing, the optimized nanocomposite strain sensing layer exhibited ultrahigh sensitivity over a wide elongation range up to 485%, tens of times greater than previous e-skin sensors. The researchers found that microcracks formed between the carbon nanotubes and MXene nanosheets when stretched increased electrical resistance proportional to elongation. Varying the MXene to carbon nanotubes ratio tuned the sensitivity.
Remarkably, the self-adhesive SPRABE-skin patch detected subtle radial pulse waves at the wrist with details like the dicrotic notch, even when underwater or under vibration that overwhelmed non-adhesive sensors. It also readily distinguished leg motions like walking, jumping, squatting based on distinct resistance signal patterns from knee deformation.
In tests for biopotential sensing, the SPRABE-skin’s electrode impedance was much lower and more stable versus frequency than standard gel electrodes. It successfully acquired ECG, EMG and EEG signals comparable to or better than rigid electrodes under static and dynamic conditions. ECG waveform features like QRS complex and T-wave remained clear when the subject moved the arms vigorously in air or underwater, thanks to uninterrupted skin contact.
EMG tests also demonstrated the SPRABE-skin’s higher fidelity in monitoring muscle activation patterns from subtle finger gestures to strenuous gripping, promising for prosthetic control. EEG signals could effectively sense the subject’s eyes open/closed states for brain monitoring applications.
Finally, the researchers demonstrated a versatile wireless multi-sensing platform based on the SPRABE-skin for healthcare applications. E-skin patches simultaneously measured ECG and leg movements continuously over extended periods of running and jogging at different intensities on a treadmill. The e-skins conformed well to body contours and stayed adhered through exercise, enabling high-quality simultaneous recording of cardiac, muscular and motion data for health assessment.
In summary, this multi-functional electronic skin technology has wide ranging healthcare and medical applications from daily wellness monitoring to diagnostic devices assessing heart health, neural function and locomotion biomechanics. The bioinspired multilayer design and scalable manufacturing enables an integrated wearable e-skin patch that can comfortably monitor diverse biochemical and biomechanical signals with clinical-level sensitivity, even under demanding conditions of vigorous movement and environmental exposure. Further development of this soft, adhesive, multi-modal sensing concept could lead to personalized health tracking in daily life and high performance wearable medical devices.
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