Mechanical Properties Of Human Skin Research Articles

Liquid Metal Flexible EMG Gel Electrodes for Gesture Recognition.

Gesture recognition has been playing an increasingly important role in the field of intelligent control and human-computer interaction. Gesture recognition technology based on electromyography (EMG) with high accuracy has been widely applied. However, conventional rigid EMG electrodes do not fit the mechanical properties of human skin. Therefore, rigid EMG electrodes are easily influenced by body movements, and uncomfortable to wear and use for a long time. To solve these problems, a stretchable EMG electrode based on liquid metal nanoparticles was developed in this research. It is conformal with human skin because of its similar mechanical properties to skin. Liquid metal nanoparticles mixed in polymer can be connected to each other to form conductive circuits when pressed by mechanical force. Therefore, this preparation method of liquid metal flexible gel electrodes is low-cost and can be fabricated largely. Moreover, the liquid metal flexible gel electrodes have great stretch ability. Their resistance increases slightly at maximum strain state. Based on these advantages, the flexible gel electrodes are applied to arm to collect EMG signals generated by human hand movements. In addition, the signals are analyzed by artificial intelligence algorithm to realize accurate gesture recognition.

Multiwavelength tissue-mimicking phantoms with tunable vessel pulsation

For the development and routine characterization of optical devices used in medicine, tissue-equivalent phantoms mimicking a broad spectrum of human skin properties are indispensable. Our work aims to develop a tissue-equivalent phantom suitable for photoplethysmography applications. The phantom includes the optical and mechanical properties of the three uppermost human skin layers (dermis, epidermis, and hypodermis, each containing different types of blood vessels) plus the ability to mimic pulsation. While the mechanical properties of the polydimethylsiloxane base material are adjusted by different mixing ratios of a base and curing agent, the optical properties are tuned by adding titanium dioxide particles, India ink, and synthetic melanin in different concentrations. The layered structure of the phantom is realized using a doctor blade technique, and blood vessels are fabricated using molding wires of different diameters. The tissue-mimicking phantom is then integrated into an artificial circulatory system employing piezo-actuated double diaphragm pumps for testing. The optical and mechanical properties of human skin were successfully replicated. The diameter of the artificial blood vessels is linearly dependent on pump actuation, and the time-dependent expansion profile of real pulse forms were mimicked. A tissue equivalent phantom suitable for the ex-vivo testing of opto-medical devices was demonstrated.

Recent development of piezoelectric biosensors for physiological signal detection and machine learning assisted cardiovascular disease diagnosis.

As cardiovascular disease stands as a global primary cause of mortality, there has been an urgent need for continuous and real-time heart monitoring to effectively identify irregular heart rhythms and to offer timely patient alerts. However, conventional cardiac monitoring systems encounter challenges due to inflexible interfaces and discomfort during prolonged monitoring. In this review article, we address these issues by emphasizing the recent development of the flexible, wearable, and comfortable piezoelectric passive sensor assisted by machine learning technology for diagnosis. This innovative device not only harmonizes with the dynamic mechanical properties of human skin but also facilitates continuous and real-time collection of physiological signals. Addressing identified challenges and constraints, this review provides insights into recent advances in piezoelectric cardiac sensors, from devices to circuit systems. Furthermore, this review delves into the integration of machine learning technologies, showcasing their pivotal role in facilitating continuous and real-time assessment of cardiac status. The synergistic combination of flexible piezoelectric sensor design and machine learning holds substantial potential in automating the detection of cardiac irregularities with minimal human intervention. This transformative approach has the power to revolutionize patient care paradigms.

Mechanics of vibrotactile sensors for applications in skin-interfaced haptic systems

Recent advances in haptic interfaces support a range of options in adding sensations of touch to virtual and augmented reality experiences. A fundamental understanding of the mechanics associated with coupling between vibro-tactile actuators and the skin is important in considering device designs and interpreting sensory perceptions. Here, we investigate vibrational dynamics induced by the three main classes of such actuators in bilayer elastomer structures that capture essential mechanical properties of human skin. The measurements rely on three dimensional digital image correlation methods, with corresponding simulations based on finite element analysis techniques. Studies examine the effects of key parameters relevant to the mechanics and resulting sensations, such as those related to contact area, actuation amplitude and spatio-temporal distributions of displacements in terms of both surface and body waves. Results reveal that tactor type actuators operate in a power efficient mode to produce deformations largely oriented out of the plane of the skin, for robust sensations that can, however, depend strongly on mounting strategy. Actuators based on eccentric rotating motors yield deformations with similar magnitudes but with substantial in-plane components and reduced sensations. Key attractive features of these actuators are in small, lightweight designs that facilitate mounting on the skin and deployment into large arrays. A third type of device, linear resonant actuators, produce the weakest sensations and the lowest power efficiencies, with limited potential for practical applications.

In vivo measurement of the anisotropic mechanical properties of human skin by indentation test

A novel approach is presented to determine the anisotropic mechanical properties of human skin invivo by indentation method. In this paper, the skin is considered as an anisotropic elastomer due to fiber reinforcement with an additional property of limiting extensibility. The associated theory of indentation method is presented and specialized to the particular case of the constitutive behaviors of skin so as to establish the theory basis of this work. The indentation tests are performed on the inner forearm in the relaxant and uniaxially pre-stretched state, respectively. Different from the conventional indentation tests, a special optical system is used to capture the contact morphology in real time. The results demonstrate that the eccentricity of the elliptical contact region changes with the pre-stretch on the ground that the eccentricity has a dependence on the degree of anisotropy.

Visual indentation apparatus and finite element modelling as a method to characterize 3D mechanical properties of facial skin in vivo

Knowledge of the mechanical properties of human skin has essential applications in cosmetic, dermatology and surgical fields. In this study, a hybrid experimental–numerical method was developed to characterize the human skin mechanical properties in vivo. An indentation experiment was physically conducted and simulated by finite element (FE) analysis. In the physical experiment, an indenter made of transparent glass was adopted so that the elliptical eccentricity in the contact area between skin and indenter could be monitored in real-time. In FE simulation, the dermis and subcutaneous tissue were modeled as anisotropic hyperelastic material and isotropic elastic material, respectively. By fitting the FE results to the experimental data, the three-dimensional (3D) mechanical properties of skin, including the dermis and subcutaneous tissue were determined. Furthermore, to demonstrate the ineffectiveness of the hybrid experimental–numerical method, a sensitivity analysis was conducted. In this sensitivity analysis, it is shown that the different mechanical parameters have a different contribution to the final FE result. Finally, the effect of subcutaneous tissue for in vivo skin indentation measurement was also evaluated.

Bio Phantoms Mimicking the Dielectric and Mechanical Properties of Human Skin Tissue at Low-Frequency Ranges

Tissue phantoms are widely used as substitute materials for real tissue validation of various newly emerging biomedical technologies such as ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI). However, there is no specific recipe for fabricating skin-mimicking phantoms which can mimic both the mechanical and dielectric properties of human skin at lower frequency ranges. 
 
 The objective of this paper is to present a variety of tissue-mimicking materials for filling this research gap in the lower frequency range from 20 Hz to 300 kHz. The starting point of our experiments is based on the oil-in-gelatin based tissue-mimicking materials (TMMs) that have shown to mimic the dielectric properties of human skin in higher frequency ranges. This paper examines the mechanical and dielectric performance of five major classes of tissue-mimicking materials (1) Oil-in-gelatin, (2) lignin and graphene nanopowder in gelatin, (3) gelatin and distilled water, (4) mixed oil in gelatin and distilled water, and (5) lignin in gelatin and distilled water. 
 
 Mechanical and electrical testing was performed using compression testing and parallel plate method respectively. The effect of electrode polarization was considered in the measured data and the intrinsic impedance values were found to be following the Cole-Cole equation. The Young's modulus range of all tissue-mimicking materials was within the range of skin.

Friction Dynamics on Human Skin Surfaces.

This study includes an evaluation of friction on the fingers, palms, and forearms of 20 subjects using a sinusoidal motion friction evaluation system. The effects of the changing water content in the stratum corneum at each position as well as the skin elastic modulus on the friction parameters were analyzed to show the factors governing friction dynamics. We observed a significant delay time (δ) and a stick-slip phenomenon during the sinusoidal motion friction processes. These dynamic phenomena are due to the softness and viscoelastic properties of human skin. The findings regarding the response of human skin to frictional stimulation under accelerated conditions facilitates a better understanding of the dynamic and mechanical properties of human skin.

Clinical Challenge and Call for Research on Keloid Disorder: Meeting Report from The 3rd International Keloid Research Foundation Symposium, Beijing 2019

Clinical Challenge and Call for Research on Keloid Disorder: Meeting Report from The 3rd International Keloid Research Foundation Symposium, Beijing 2019

Zwitterionic Skins with a Wide Scope of Customizable Functionalities.

With growing interest in the fields of wearable devices, it is crucial yet rather challenging to develop skinlike soft conductive materials with customizable functionalities and human tissue-compatible mechanical properties. Previously reported electronic skins struggle to meet the demands for transparence, mechanical adaptability, and stable conductivity during deformation. The recent rise of ionic skins with inorganic salts or ionic liquids doping provides the intrinsic stretchability, however, dilemmas remain for their limited functionalities such as a monotonous appearance and a narrow scope of mechanical and sensory properties. Herein, we design a type of zwitterionic hydrogels from the perspective of molecular interactions, which successfully combines ultrastretchability (>10000% strain), high strength (∼300 kPa), self-healability (at room temperature within 12 h), 3D printability, distinct stimuli-responsibility, biocompatibility, and antibacterial activity. The wide spectrum of such excellent properties has been rarely reported before and along with the ability to fabricate bioinspired intelligent skins recreating multiple sensations and mechanical properties of human skin, covering a broad range of sensitivity, and displaying tunable visual effect. We believe this work will inspire the programming of stimuli-responsive skinlike materials and contribute to the smart devices for information transformation between natural and artificial interfaces.

Experimentally derived viscoelastic properties of human skin and muscle in vitro

Measurement of the mechanical properties of human skin in vivo is challenging. Moreover, those with regard to excitation frequency have been rarely reported thus far. In this study, a vibration-based experimental method was employed to measure the viscoelastic properties with regard to the excitation frequency. Pieces of human skin and skeletal muscle excised from cadavers immediately post mortem were stored in a sealed container. As the experiment began, they were removed from the container and used to measure the viscoelastic properties as time elapsed. Young's moduli of the samples of human skin tissue that were immediately removed from the container were found to be similar to those obtained with in-vivo indentation methods. They were also found to be approximately one-third of those of human skeletal muscles. The viscoelastic properties of human skin were found to remain almost constant within the frequency range up to 120 Hz and are similar to those of porcine tissue. Young's moduli of the human skin and skeletal muscle were also found to reach the maximum values approximately five days post mortem. However, the loss factor of the human skin did not vary significantly as time elapses.

Preparation of Artificial Skin that Mimics Human Skin Surface and Mechanical Properties.

We have developed an artificial skin that mimics the morphological and mechanical properties of human skin. The artificial skin comprises a polyurethane block possessing a microscopically rough surface. We evaluated the tactile sensations when skin-care cream was applied to the artificial skin. Many subjects perceived smooth, moist, and soft feels during the application process. Cluster analysis showed that these characteristic tactile feels are similar to those when skin-care cream is applied to real human skin. Contact angle analysis showed that an oil droplet spread smoothly on the artificial skin surface, which occurred because there were many grooves several hundred micrometers in width on the skin surface. In addition, when the skin-care cream was applied, the change in frictional force during the dynamic friction process increased. These wetting and frictional properties are important factors controlling the similarity of artificial skin to real human skin.

General theory of skin reinforcement

Macroscopic mechanical properties of human skin in vivo cannot be considered independent of adjacent subcutaneous white adipose tissue (sWAT). The layered system skin/sWAT appears as the hierarchical structural composite in which single layers behave as fiber-reinforced structures. Effective macroscopic mechanical properties of such composites are mainly determined either by the properties of the skin or by those of the sWAT, dependent on the conditions of mechanical loading. Mechanical interactions between the skin and the adjacent sWAT associated with a mismatch in the mechanical moduli of these two layers can lead to production of the skin wrinkles. Reinforcement of the composite skin/sWAT can take place in different ways. It can be provided through reorientation of collagen fibers under applied loading, through production of new bonds between existing collagen fibers and through induction of additional collagen structures. Effectiveness of this type of reinforcement is strongly dependent on the type of mechanical loading. Different physical interventions induce the reinforcement of at least one of these two layers, thus increasing the effective macroscopic stiffness of the total composite. At the same time, the standalone reinforcement of the skin appears to be less effective to achieve a delay or a reduction of the apparent signs of skin aging relative to the reinforcement of the sWAT.

Skin health monitoring

Ultrathin and conformal piezoelectric transducers enable high-resolution mapping of the mechanical properties of human skin.

Bioinspired design of nanostructured elastomers with cross-linked soft matrix grafting on the oriented rigid nanofibers to mimic mechanical properties of human skin.

Human skin exhibits highly nonlinear elastic properties that are essential to its physiological functions. It is soft at low strain but stiff at high strain, thereby protecting internal organs and tissues from mechanical trauma. However, to date, the development of materials to mimic the unique mechanical properties of human skin is still a great challenge. Here we report a bioinspired design of nanostructured elastomers combining two abundant plant-based biopolymers, stiff cellulose and elastic polyisoprene (natural rubber), to mimic the mechanical properties of human skin. The nanostructured elastomers show highly nonlinear mechanical properties closely mimicking that of human skin. Importantly, the mechanical properties of these nanostructured elastomers can be tuned by adjusting cellulose content, providing the opportunity to synthesize materials that mimic the mechanical properties of different types of skins. Given the simplicity, efficiency, and tunability, this design may provide a promising strategy for creating artificial skin for both general mechanical and biomedical applications.

Comparison of different viscoelastic models for the characterisation of mechanical properties of human skin in vivo by indentation test

Comparison of different viscoelastic models for the characterisation of mechanical properties of human skin in vivo by indentation test

Mechanical properties of human skin in vivo: a comparative evaluation in 300 men and women

Previous studies have shown that the clinical genesis and onset of facial wrinkles as well as the morphology of the extracellular matrix differ between the sexes. The aim of this present clinical study was to do the first systematic assessment of gender-related differences in skin elasticity, with special focus on age-related changes. 300 healthy male and female subjects (20-74years) were selected following strict criteria including age, sun behavior or smoking habits. Skin mechanical properties were assessed at the cheek, neck, volar forearm and dorsum of the hand using a non-invasive suction device. Data analysis shows a significant negative correlation for all parameters and subject's age. At young age results of the relative parameters are higher for women, whereas absolute parameters are higher for men. Parameters referring to the recovery phase change stronger with aging. The present study verifies the progressive decline of the skin mechanical properties with aging. However, the elastic ability of the skin to recover after stretching is more strongly affected by the aging process than the firmness of the skin. Further, the mechanical properties change differently in men and woman over lifetime.

Tribological method to objectify similarity of vague tactile sensations experienced during application of liquid cosmetic foundations

To develop objective estimation methods for the tactile sensations experienced during the application of liquid cosmetic foundation (i.e., emulsion foundations (EFs)), a tribotester based on the mechanical properties of human skin was used in lubrication experiments with EF samples. From the time-varying signals of the tangential spring deflection obtained in reciprocating rubbing tests for small sample amounts, trajectories (i.e., plots of the tangential spring deflection against the tangential drive position) were obtained. Using characteristic trajectory shapes (e.g., a shape like a ribbon with jagged edges) makes it possible to evaluate the similarity of vague tactile sensations without sensory testing.

Non contact method for in vivo assessment of skin mechanical properties for assessing effect of ageing

The assessment of human tissue properties by objective and quantitative devices is very important to improve the understanding of its mechanical behaviour. The aim of this paper is to present a non contact method to measure the mechanical properties of human skin in vivo.A complete non contact device using an air flow system has been developed. Validation and assessment of the method have been performed on inert visco-elastic material. An in vivo study on the forearm of two groups of healthy women aged of 23.2±1.6 and 60.4±2.4 has been performed. Main parameters assessed are presented and a first interpretation to evaluate the reduced Young's modulus is proposed.Significant differences between the main parameters of the curve are shown with ageing. As tests were performed with different loads, the influence of the stress is also observed. We found a reduced Young's modulus with an air flow force of 10mN of 14.38±3.61kPa for the youngest group and 6.20±1.45kPa for the oldest group. These values agree with other studies using classical or dynamic indentation. Non contact test using the developed device gives convincing results.

Quantifying the mechanical properties of human skin to optimise future microneedle device design

Microneedle devices are a promising minimally invasive means of delivering drugs/vaccines across or into the skin. However, there is currently a diversity of microneedle designs and application methods that have, primarily, been intuitively developed by the research community. To enable the rational design of optimised microneedle devices, a greater understanding of human skin biomechanics under small deformations is required. This study aims to develop a representative stratified model of human skin, informed by in vivo data. A multilayer finite element model incorporating the epidermis, dermis and hypodermis was established. This was correlated with a series of in-vivo indentation measurements, and the Ogden material coefficients were optimised using a material parameter extraction algorithm. The finite element simulation was subsequently used to model microneedle application to human skin before penetration and was validated by comparing these predictions with the in-vivo measurements. Our model has provided an excellent tool to predict micron-scale human skin deformation in vivo and is currently being used to inform optimised microneedle designs.