Plastocapillarity: Partial and full Newtonian drop embedding into immiscible yield stress substrates.

HYPOTHESIS: Recent advances have been made in elastocapillarity; reversible 3D deformations of solid substrates with low elastic moduli from the surface tension of deposited drops. This study explores permanent deformations caused by liquid drops on immiscible yield stress substrates. We hypothesize that the substrate's rheological properties play a major role in determining the shape and stability of the drop-substrate interface, and govern partial or full embedding into the substrate. EXPERIMENTS: Substrate yield stress magnitudes are modified through altering the mixture ratios of petroleum jelly to paraffin oil. Water drops are deposited on substrates and deformation profiles of the deformed interface are quantified. FINDINGS: Above a critical Bingham-Capillary number, which characterizes the ratio of yield stress magnitude to surface tension, deposited water drops deform the substrate surface permanently, but minimally. Below this value, drops become increasingly embedded as the substrate yield stress magnitude decreases, with larger indentation depths and increased circumferential ridge heights. With sufficiently low yield stress magnitudes, where surface tension forces dominate over yield stress forces, the plastically deformed ridges fully encapsulate the liquid drop surface, resulting in full drop embedding within the substrate. These results advance knowledge of interfacial wetting on soft yield stress substrates and has implications for binary fluids, functional materials, and new drug delivery systems.

Heterogeneous drying stresses in stratum corneum.

We study the drying of stratum corneum, the skin's outermost layer and an essential barrier to mechanical and chemical stresses from the environment. Even though stratum corneum exhibits structural features across multiple length-scales, contemporary understanding of the mechanical properties of stratum corneum is based on the assumption that its thickness and composition are homogeneous. We quantify spatially resolved in-plane traction stress and deformation at the interface between a macroscopic sample of porcine stratum corneum and an adherent deformable elastomer substrate. At length-scales greater than a millimeter, the skin behaves as a homogeneous elastic material. At this scale, a linear elastic model captures the spatial distribution of traction stresses and the dependence of drying behavior on the elastic modulus of the substrate. At smaller scales, the traction stresses are strikingly heterogeneous and dominated by the heterogeneous structure of the stratum corneum.

Non-invasive in vivo quantification of human skin tension lines.

Human skin is a composite tissue that exhibits anisotropic mechanical properties. This anisotropy arises primarily from the alignment of collagen and elastin fibers in the dermis, which causes the skin to exhibit greater tension in one direction, making it appear stiffer. A diverse number of skin tension guidelines have been developed to assist surgeons in making incisions that produce the least conspicuous scars. However, skin anisotropy is believed to vary from subject to subject, and no single guideline is universally recognized as the best to implement for surgical applications. To date, no system exists that can rapidly and non-invasively measure lines of skin tension in vivo. In this article, we evaluate the ability of a new aspiration system to measure the anisotropy of human skin. The device painlessly applies a radial stress of 17 kPa to a region of skin, and captures radially asymmetric skin deformations via a dermal camera. These deformations are used to quantify orientations of strain extrema and the direction of greatest skin stiffness. The ratio of these asymmetric strains varies between 1 and -0.75. A simple 2D transverse isotropic model captures this behavior for multiple anatomical sites. Clinical trials reveal that skin tension line orientations are comparable with existing skin tension maps and generally agree across subjects, however orientations statistically differ between individuals. As such, existing guidelines appear to provide only approximate estimates of skin tension orientation. STATEMENT OF SIGNIFICANCE: Skin tension lines (STL) in human skin arise primarily from collagen fiber alignment in the dermis. These lines are used by surgeons to guide incisions that produce the least conspicuous scars. While numerous anatomical STL maps exist, no single guideline is universally recognized as the most reliable. Moreover, manual methods of quantifying STL are imprecise. For the first time, we have developed a device capable of rapidly and non-invasively measuring STL orientations in vivo, using a single test. Our results are used to establish a simple constitutive model of mechanical skin anisotropy. Clinical trials further reveal STL orientations are comparable with existing maps, but statistically differ between individuals. Existing guidelines therefore appear to provide only approximate estimates of STL orientation.

Control of human skin wettability using the pH of anionic surfactant solution treatments.

The outermost layer of human skin, or stratum corneum, acts as a protective barrier between underlying living tissue and the external environment. The wettability of this tissue layer can influence spreading of chemicals and the adhesion of pathogenic microorganisms. We show in this article that the wettability of isolated human stratum corneum can be controlled through treatment with solutions of the anionic surfactant, sodium lauryl sulfate, buffered to different pH values. Relative to control treatments with the buffer solution alone, surfactant solution treatments under acidic conditions cause delipidated stratum corneum to become more hydrophobic. In contrast, alkaline conditions cause the stratum corneum to become more hydrophilic; irrespective of lipid composition. This transition is consistent with a reorientation of bound surfactants at the tissue interface. Under acidic conditions, electrostatic binding of negatively charged surfactant head groups with positively charged keratin in the stratum corneum would increase tissue hydrophobicity due to the exposed hydrophobic tails. However, a hydrophobic based attraction of the apolar surfactant tails to the stratum corneum surface under alkaline conditions would leave the hydrophilic surfactant head groups exposed, causing increased tissue hydrophilicity. Changes in wettability with pH become diminished when lipids ordinarily found in stratum corneum are present, suggesting the lipids partially inhibit surfactant binding. Profilometry studies of the tissue topography highlight that surfactant induced changes in stratum corneum surface roughness cannot account for the observed changes in wettability.

The global mechanical properties and multi-scale failure mechanics of heterogeneous human stratum corneum.

UNLABELLED: The outermost layer of skin, or stratum corneum, regulates water loss and protects underlying living tissue from environmental pathogens and insults. With cracking, chapping or the formation of exudative lesions, this functionality is lost. While stratum corneum exhibits well defined global mechanical properties, macroscopic mechanical testing techniques used to measure them ignore the structural heterogeneity of the tissue and cannot provide any mechanistic insight into tissue fracture. As such, a mechanistic understanding of failure in this soft tissue is lacking. This insight is critical to predicting fracture risk associated with age or disease. In this study, we first quantify previously unreported global mechanical properties of isolated stratum corneum including the Poisson's ratio and mechanical toughness. African American breast stratum corneum is used for all assessments. We show these parameters are highly dependent on the ambient humidity to which samples are equilibrated. A multi-scale investigation assessing the influence of structural heterogeneities on the microscale nucleation and propagation of cracks is then performed. At the mesoscale, spatially resolved equivalent strain fields within uniaxially stretched stratum corneum samples exhibit a striking heterogeneity, with localized peaks correlating closely with crack nucleation sites. Subsequent crack propagation pathways follow inherent topographical features in the tissue and lengthen with increased tissue hydration. At the microscale, intact corneocytes and polygonal shaped voids at crack interfaces highlight that cracks propagate in superficial cell layers primarily along intercellular junctions. Cellular fracture does occur however, but is uncommon. STATEMENT OF SIGNIFICANCE: Human stratum corneum protects the body against harmful environmental pathogens and insults. Upon mechanical failure, this barrier function is lost. Previous studies characterizing the mechanics of stratum corneum have used macroscopic testing equipment designed for homogenous materials. Such measurements ignore the tissue's rich topography and heterogeneous structure, and cannot describe the underlying mechanistic process of tissue failure. For the first time, we establish a mechanistic insight into the failure mechanics of soft heterogeneous tissues by investigating how cracks nucleate and propagate in stratum corneum. We further quantify previously unreported values of the tissue's Poisson's ratio and toughness, and their dramatic variation with ambient humidity. To date, skin models examining drug delivery, wound healing, and ageing continue to estimate these parameters.

The effects of barrier disruption and moisturization on the dynamic drying mechanics of human stratum corneum.

We study the dynamic drying mechanics of human stratum corneum, the most superficial layer of skin and essential physical and chemical barrier to the external environment. Barrier disruption caused by a depletion of lipids ordinarily found in healthy stratum corneum can occur with ageing, aggressive cleansing or with dry skin disorders and diseases such as atopic dermatitis and psoriasis. We establish the effects of severe barrier disruption on the dynamic drying mechanics of human stratum corneum by measuring variations in thickness and spatially resolved in-plane displacements in healthy and lipid depleted tissue samples drying in controlled environmental conditions. In-plane displacements recorded at regular intervals during drying are azimuthally averaged and fitted with a profile based on a linear elastic model. The measured thickness of the tissue sample is accounted for in each model fit. Dynamic variations in the drying stress and elastic modulus of the tissue are then established from the model fits. We find that barrier disruption causes dramatic reductions in drying timescales, increases in the elastic modulus of the tissue and larger drying stresses. We expect these changes to increase the propensity for cracking and chapping in skin. The maximum elastic modulus and drying stress of barrier disrupted stratum corneum (ESC=85.4±6.8 MPa, PSC=10.9±0.9 MPa) is reduced to levels comparable with stratum corneum containing lipids (ESC=26.1±3.2 MPa, PSC=2.58±0.45 MPa) after treatment with a 5% aqueous solution of glycerol. Neither 2% nor 5% glycerol solutions slow the accelerated drying timescales in barrier disrupted stratum corneum.

Surfactant treatments influence drying mechanics in human stratum corneum.

We describe a high-throughput method capable of quantifying the elastic modulus and drying stress of ex vivo samples of human stratum corneum. Spatially resolved drying deformations in circular tissue samples are measured, azimuthally averaged and fitted with a profile based on a linear elastic model. Our approach enables the comparison of the physical effects of different cleansers. We find that cleansing can cause dramatic changes to the mechanical properties of stratum corneum. In some cases, cleansing can lead to an order of magnitude increase in elastic modulus and drying stress. We expect that these mechanical properties have a direct impact on cracking and chapping skin as well as the milder sensation of perceived tightness often experienced after washing. Mechanical drying studies are also combined with drop wetting studies and pyranine staining experiments. This combination of techniques allows one to establish a multidimensional profile of stratum corneum including stiffness, susceptibility to drying, hydrophilicity and barrier functionality.