A Weighted Average Phase Velocity Inversion Model for Depth-Resolved Elasticity Evaluation in Human Skin In-Vivo

A three-dimensionally architected electronic skin mimicking human mechanosensation

Human skin sensing of mechanical stimuli originates from transduction of mechanoreceptors that converts external forces into electrical signals. Although imitating the spatial distribution of those mechanoreceptors can enable developments of electronic skins capable of decoupled sensing of normal/shear forces and strains, it remains elusive. We report a three-dimensionally (3D) architected electronic skin (denoted as 3DAE-Skin) with force and strain sensing components arranged in a 3D layout that mimics that of Merkel cells and Ruffini endings in human skin. This 3DAE-Skin shows excellent decoupled sensing performances of normal force, shear force, and strain and enables development of a tactile system for simultaneous modulus/curvature measurements of an object through touch. Demonstrations include rapid modulus measurements of fruits, bread, and cake with various shapes and degrees of freshness.

Dermal stiffness governs the topography of the epidermis and the underlying basement membrane in young and old human skin

The epidermis is a stratified epithelium that forms the outer layer of the skin. It is composed primarily of keratinocytes and is constantly renewed by the proliferation of stem cells and their progeny that undergo terminal differentiation as they leave the basal layer and migrate to the skin surface. Basal keratinocytes rest on a basement membrane composed of an extracellular matrix that controls their fate via integrin-mediated focal adhesions and hemidesmosomes which are critical elements of the epidermal barrier and promote its regenerative capabilities. The distribution of basal cells with optimal activity provides the basement membrane with its characteristic undulating shape; this configuration disappears with age, leading to epidermal weakness. In this study, we present an in-depth imaging analysis of basal keratinocyte anchorage in samples of human skin from participants across the age spectrum. Our findings reveal that skin aging is associated with the depletion of hemidesmosomes that provide crucial support for stem cell maintenance; their depletion correlates with the loss of the characteristic basement membrane structure. Atomic force microscopy studies of skin and in vitro experiments revealed that the increase in tissue stiffness observed with aging triggers mechanical signals that alter the basement membrane structure and reduce the extent of basal keratinocyte anchorage, forcing them to differentiate. Genomic analysis revealed that epidermal aging was associated with mechanical induction of the transcription factor Krüppel-like factor 4. The altered mechanical properties of tissue being a new hallmark of aging, our work opens new avenues for the development of skin rejuvenation strategies.

In vivo stiffness measurement of epidermis, dermis, and hypodermis using broadband Rayleigh-wave optical coherence elastography

Traveling-wave optical coherence elastography (OCE) is a promising technique to measure the stiffness of biological tissues. While OCE has been applied to relatively homogeneous samples, tissues with significantly varying elasticity through depth pose a challenge, requiring depth-resolved measurement with sufficient resolution and accuracy. Here, we develop a broadband Rayleigh-wave OCE technique capable of measuring the elastic moduli of the 3 major skin layers (epidermis, dermis, and hypodermis) reliably by analyzing the dispersion of leaky Rayleigh surface waves over a wide frequency range of 0.1-10 kHz. We show that a previously unexplored, high frequency range of 4-10 kHz is critical to resolve the thin epidermis, while a low frequency range of 0.2-1 kHz is adequate to probe the dermis and deeper hypodermis. We develop a dual bilayer-based inverse model to determine the elastic moduli in all 3 layers and verify its high accuracy with finite element analysis and skin-mimicking phantoms. Finally, the technique is applied to measure the forearm skin of healthy volunteers. The Young's modulus of the epidermis (including the stratum corneum) is measured to be ∼ 4 MPa at 4-10 kHz, whereas Young's moduli of the dermis and hypodermis are about 40 and 15 kPa, respectively, at 0.2-1 kHz. Besides dermatologic applications, this method may be useful for the mechanical analysis of various other layered tissues with sub-mm depth resolution. STATEMENT OF SIGNIFICANCE: To our knowledge, this is the first study that resolves the stiffness of the thin epidermis from the dermis and hypodermis, made possible by using high-frequency (4 - 10 kHz) elastic waves and optical coherence elastography. Beyond the skin, this technique may be useful for mechanical characterizations of various layered biomaterials and tissues.

Highly Elastic, Sensitive, Stretchable, and Skin-Inspired Conductive Sodium Alginate/Polyacrylamide/Gallium Composite Hydrogel with Toughness as a Flexible Strain Sensor

As a classic flexible material, hydrogels show great potential in wearable electronic devices. The application of strain sensors prepared using them in human health monitoring and humanoid robotics is developing rapidly. However, it is still a challenge to fabricate a high-toughness, large-tensile-deformation, strain-sensitive. and human-skin-fit hydrogel with the integration of excellent mechanical properties and high electrical conductivity. In this study, a flexible sensor using a highly strain-sensitive skin-like hydrogel with acrylamide and sodium alginate was designed using liquid metallic gallium as a "reactive" conductive filler. The sensor had a low elastic modulus (30 kPa) similar to that of skin, a high-toughness (2.25 MJ m^-3), self-stiffness, a large tensile deformation (1400%), recoverability, and excellent fatigue resistance. Moreover, the addition of gallium might enhance the electrical conductivity (1.9 S m^-1) of the hydrogel while maintaining high transparency, and the flexible sensor device constructed from it showed high sensitivity to strain (gauge factor = 4.08) and pressure (gauge factor = 0.455 kPa^-1). As a result, the hydrogel sensor could monitor various human motions, including large-scale joint bending and tiny facial expression, breathing, voice recognition, and handwriting. Furthermore, it might even be used for human-computer communication.

Optical coherence elastography to evaluate depth-resolved elasticity of tissue

Skin-elasticity measurements can assist in the clinical diagnosis of skin diseases, which has important clinical significance. Accurately determining the depth-resolved elasticity of superficial biological tissue is an important research direction. This paper presents an optical coherence elastography technique that combines surface acoustic waves and shear waves to obtain the elasticity of multilayer tissue. First, the phase velocity of the high-frequency surface acoustic wave is calculated at the surface of the sample to obtain the Young's modulus of the top layer. Then, the shear wave velocities in the other layers are calculated to obtain their respective Young's moduli. In the bilayer phantom experiment, the maximum error in the elastic estimation of each layer was 2.2%. The results show that the proposed method can accurately evaluate the depth-resolved elasticity of layered tissue-mimicking phantoms, which can potentially expand the clinical applications of elastic wave elastography.