Deeply etched forehead creases indicate aging. Various treatments such as filler injections, fat grafting, and facelift surgery are used to remove them. However, knowledge of the anatomical structures associated with subcutaneous tissue changes and the superficial musculoaponeurotic system is lacking, and there is no consensus about the appropriate treatment. We have investigated the subcutaneous structures involved in forehead creases; this will help to establish selection criteria for improved treatment. The forehead sections of five unfixed adult Asian cadavers were obtained. Tissues containing forehead creases were removed from the periosteum and were examined using gross observation, radiography, histology, and nano-computed tomography. All methods revealed that the dermis in the skin crease area, namely the fold visible from the body surface, was bound to the frontalis muscle by a three-dimensional fibrous structure between the fatty septa. This structure was dense near the skin folds and sparse and thin in other areas. In particular, it was tightly bound to the dermis immediately below the crease, with collagen fibers traversing toward the epidermis. In addition, there were fewer skin appendages near the crease than in the normal area, or they were absent altogether; the epidermis was thicker, and the dermal papillae were more developed. It is thought that the density and firmness of the fibrous fatty septal structures between the dermis-frontalis muscle and the specific structures of the epidermis and dermis immediately below the crease account for the characteristic plastic forehead creases.

This work combines experiments and finite element simulations to study the effect of pre-imposed cyclic loading on surface instability of silicon rubber under compression. We first fabricate cuboid blocks of silicon rubber and pinch them cyclicly a few times. Then, an in-house apparatus is set to apply uniaxial compression on the silicon rubber under exact plane strain conditions. Surprisingly, we find multiple creases on the surface of silicone rubber, significantly different from what have been observed on the samples without the cyclic pinching. To reveal the underlying physics for these experimentally observed multiple creases, we perform detailed nanoindentation experiments to measure the material properties at different locations of the silicon rubber. The modulus is found to be nonuniform and varies along the thickness direction after the cyclic pinching. According to these experimental results, three-layer and multilayer finite element models are built with different materials properties informed by experiments. The three-layer finite element model can excellently explain the nucleation and pattern of multiple surface creases on the surface of compressed silicone rubber, in good agreement with experiments. Counterintuitively, the multilayer model with gradient modulus cannot be used to explain the multiple creases observed in our experiments. According to these simulations, the experimentally observed multiple creases should be attributed to a thin and stiff layer formed on the surface of silicon rubber after the pre-imposed cyclic loading.

The architectural features of branching morphogenesis demonstrate exquisite reproducibility among various organs and species despite the unique functionality and biochemical differences of their microenvironment. The regulatory networks that drive branching morphogenesis employ cell-generated and passive mechanical forces, which integrate extracellular signals from the microenvironment into morphogenetic movements. Cell-generated forces function locally to remodel the extracellular matrix (ECM) and control interactions among neighboring cells. Passive mechanical forces are the product of in situ mechanical instabilities that trigger out-of-plane buckling and clefting deformations of adjacent tissues. Many of the molecular and physical signals that underlie buckling and clefting morphogenesis remain unclear and require new experimental strategies to be uncovered. Here, we highlight soft material systems that have been engineered to display programmable buckles and creases. Using synthetic materials to model physicochemical and spatiotemporal features of buckling and clefting morphogenesis might facilitate our understanding of the physical mechanisms that drive branching morphogenesis across different organs and species.