Predicting the macroscopic response of electrospun membranes based on microstructure and single fibre properties

Nonlinear behavior of stochastic athermal fiber networks with elastic-plastic fibers

Stochastic fiber networks form the structural component of network materials, which are broadly encountered in engineering and biology. Apparent elastic-plastic behavior, characterized by a yield point and softening at larger strains, is observed in some of these materials. A range of mechanisms, some of which being unrelated to fiber plasticity, may cause this behavior. In this work we investigate network plasticity caused by the plastic deformation of fibers and develop a comprehensive perspective on its relationship with network structural parameters. We determine the scaling of the yield stress and yield strain with network parameters emphasizing differences between the affine and non-affine deformation regimes. The non-linear response of the network is more complex when fiber plasticity takes place than in the purely elastic case. We describe four non-linear regimes and their dependence on network parameters. Further, we evaluate the dissipation and residual strains resulting upon loading-unloading cycles for a variety of networks and discuss design strategies for maximizing energy dissipation. Finally, we provide guidelines for the interpretation of experimental results and discuss ways to distinguish between various mechanisms that may cause a yield point and apparent elastic-plastic behavior.

Multiscale characterization of electrospun non-wovens for corneal regeneration: Impact of microstructure on mechanical, optical and biological properties

The multiscale approach in designing substrates for regenerative medicine endows them with beneficial properties determining their performance in the body. Substrates for corneal regeneration should reveal the proper transparency, mechanical properties and microstructure to maintain the functionality of the regenerated tissue. In our study, series of non-wovens with different fibres orientation (random (R), aligned (A)), topography (shish-kebab (KK), core-shell (CS)) and thickness were fabricated via electrospinning. The samples were assessed for mechanical (static tensile test) and optical properties (spectroscopy UV-Vis). The research evaluated the impact of different microstructures on the viability and morphology of three cell lines (Hs 680, HaCaT and RAW 264.7). The results showed how the fibres arrangement influenced mechanical behaviour of the non-wovens. The randomly oriented fibres were more elongated (up to 50 mm) and had a lower maximum tensile force (up to 0.46 N). In turn, the aligned fibres were characterized by lower elongation (up to 19 mm) and higher force (up to 1.45 N). The conducted transparency tests showed the relation between thickness (of the non-woven and fibres) and morphology of the substrate and light transmission. To simulate the in vivo conditions, prior to the light transmission studies, samples were immersed in water. All the samples exhibited high transparency after immersion in water (>80%). The impact of various morphologies was observed in the in vitro studies. All the samples proved high cells viability. Moreover, the substrate morphology had a significant impact on the orientation and arrangement of the fibroblast cytoskeleton. The aligned fibres were oriented in exactly the same direction. The conducted research proved that, by altering the non-wovens microstructure, the properties can be adjusted so as to induce the desirable cellular reaction. This indicates the high potential of electrospun fibres in terms of modulating the corneal cell behaviour in response to the implanted substrate.

Wicking fingering in electrospun membranes

Pronounced fingering of the waterfront is observed for in-plane wicking in thin, aligned electrospun fibrous membranes. We hypothesize that a perturbation in capillary pressure triggers the onset of fingering, which grows in a non-local manner based on the waterfront gradient. Vertical and horizontal wicking in thin electrospun membranes of poly(ethylene-co-vinyl alcohol) (EVOH) fibers with varying fiber alignment and degree of orientation is studied with backlight photography. A non-local transport model considering the gradient of the waterfront is developed, where fiber orientation is modeled with a correlated random field. The model shows that a transition from straight to highly fingered waterfront occurs during water uptake as observed in the experiment. The size and shape of the fingers depend on fiber orientation. Based on good model agreement, we show that, during wicking in thin electrospun membranes, fingering is initially triggered by a perturbation in capillary pressure caused by the underlying anisotropic and heterogeneous membrane structure which grows in a non-local manner depending on the waterfront gradient.

Tailoring the multiscale architecture of electrospun membranes to promote 3D cellular infiltration

Controlling the architecture of engineered scaffolds is of outmost importance to induce a targeted cell response and ultimately achieve successful tissue regeneration upon implantation. Robust, reliable and reproducible methods to control scaffold properties at different levels are timely and highly important. However, the multiscale architectural properties of electrospun membranes are very complex, in particular the role of fiber-to-fiber interactions on mechanical properties, and their effect on cell response remain largely unexplored. The work reported here reveals that the macroscopic membrane stiffness, observed by stress-strain curves, cannot be predicted solely based on the Young's moduli of the constituting fibers but is rather influenced by interactions on the microscale, namely the number of fiber-to-fiber bonds. To specifically control the formation of these bonds, solvent systems of the electrospinning solution were fine-tuned, affecting the membrane properties at every length-scale investigated. In contrast to dichloromethane that is characterized by a high vapor pressure, the use of trifluoroacetic acid, a solvent with a lower vapor pressure, favors the generation of fiber-to-fiber bonds. This ultimately led to an overall increased Young's modulus and yield stress of the membrane despite a lower stiffness of the constituting fibers. With respect to tissue engineering applications, an experimental setup was developed to investigate the effect of architectural parameters on the ability of cells to infiltrate and migrate within the scaffold. The results reveal that differences in fiber-to-fiber bonds significantly affect the infiltration of normal human dermal fibroblasts into the membranes. Membranes of loose fibers with low numbers of fiber-to-fiber bonds, as obtained from spinning solutions using dichloromethane, promote cellular infiltration and are thus promising candidates for the formation of a 3D tissue.

A computational framework for modeling cell-matrix interactions in soft biological tissues

Living soft tissues appear to promote the development and maintenance of a preferred mechanical state within a defined tolerance around a so-called set point. This phenomenon is often referred to as mechanical homeostasis. In contradiction to the prominent role of mechanical homeostasis in various (patho)physiological processes, its underlying micromechanical mechanisms acting on the level of individual cells and fibers remain poorly understood, especially how these mechanisms on the microscale lead to what we macroscopically call mechanical homeostasis. Here, we present a novel computational framework based on the finite element method that is constructed bottom up, that is, it models key mechanobiological mechanisms such as actin cytoskeleton contraction and molecular clutch behavior of individual cells interacting with a reconstructed three-dimensional extracellular fiber matrix. The framework reproduces many experimental observations regarding mechanical homeostasis on short time scales (hours), in which the deposition and degradation of extracellular matrix can largely be neglected. This model can serve as a systematic tool for future in silico studies of the origin of the numerous still unexplained experimental observations about mechanical homeostasis.

Anisotropic electrospun honeycomb polycaprolactone scaffolds: Elaboration, morphological and mechanical properties

Tissue engineering technology requires porous scaffolds, based on biomaterials, which have to mimic as closely as possible the morphological and anisotropic mechanical properties of the native tissue to substitute. Anisotropic fibrous scaffolds fabricated by template-assisted electrospinning are investigated in this study. Fibers of electrospun Polycaprolactone (PCL) were successfully arranged spatially into honeycomb structures by using well-shaped 3D micro-architected metal collectors. Fibrous scaffolds present 2 × 4 mm² wide elementary patterns with low and high fiber density areas. Distinct regions of the honeycomb patterns were analyzed through SEM images revealing different fiber diameters with specific fiber orientation depending on the regions of interest. Tensile test experiments were carried out with an optical observation of the local deformation at the pattern scale, allowing the determination and analysis, at small and large deformation, of the axial and transverse local strains. The honeycomb patterned mats showed significantly different mechanical properties along the two orthogonal directions probing an anisotropic ratio of 4.2. Stress relaxation test was performed on scaffolds at 15% of strain. This measurement pointed out the low contribution of the viscosity of about 20% in the mechanical response of the scaffold. An orthotropic linear elastic model was consequently proposed to characterize the anisotropic behavior of the produced patterned membranes. This new versatile method to produce architected porous materials, adjustable to several polymers and structures, will provide appealing benefits for soft regenerative medicine application and the development of custom-made scaffolds.