In situ measurement of viscoelastic properties of cellular monolayers via graphene strain sensing of elastohydrodynamic phenomena.

Recent advances recognize that the viscoelastic properties of epithelial structures play important roles in biology and disease modeling. However, accessing the viscoelastic properties of multicellular structures in mechanistic or drug-screening applications has challenges in repeatability, accuracy, and practical implementation. Here, we present a microfluidic platform that leverages elastohydrodynamic phenomena, sensed by strain sensors made from graphene decorated with palladium nanoislands, to measure the viscoelasticity of cellular monolayers in situ, without using chemical labels or specialized equipment. We demonstrate platform utility with two systems: cell dissociation following trypsinization, where viscoelastic properties change over minutes, and epithelial-to-mesenchymal transition, where changes occur over days. These cellular events could only be resolved with our platform's higher resolution: viscoelastic relaxation time constants of λ = 14.5 ± 0.4 s-1 for intact epithelial monolayers, compared to λ = 13.4 ± 15.0 s-1 in other platforms, which represents a 30-fold improvement. By rapidly assessing combined contributions from cell stiffness and intercellular interactions, we anticipate that the platform will hasten the translation of new mechanical biomarkers.

Controlling the adhesion and differentiation of mesenchymal stem cells using hyaluronic acid-based, doubly crosslinked networks.

We have created hyaluronic acid (HA)-based, cell-adhesive hydrogels that direct the initial attachment and the subsequent differentiation of human mesenchymal stem cells (MSCs) into pre-osteoblasts without osteogenic supplements. HA-based hydrogel particles (HGPs) with an average diameter of 5-6 µm containing an estimated 2.2 wt% gelatin (gHGPs) were synthesized by covalent immobilization of gelatin to HA HGPs prepared via an inverse emulsion polymerization technique. Separately, a photocrosslinkable HA macromer (HAGMA) was synthesized by chemical modification of HA with glycidyl methacrylate (GMA). Doubly crosslinked networks (DXNs) were engineered by embedding gHGPs in a secondary network established by HAGMA at a particle concentration of 2.5 wt%. The resultant composite gels, designated as HA-gHGP, have an average compressive modulus of 21 kPa, and are non-toxic to the cultured MSCs. MSCs readily attached to these gels, exhibiting an early stage of stress fiber assembly 3 h post seeding. By day 7, stellate-shaped cells with extended filopodia were found on HA-gHGP gels. Moreover, cells had migrated deep into the matrix, forming a three dimensional, branched and interconnected cell community. Conversely, MSCs on the control gels lacking gelatin moieties formed isolated spheroids with rounded cell morphology. After 28 days of culture on HA-gHGP, Type I collagen production and mineral deposition were detected in the absence of osteogenic supplements, suggesting induction of osteogenic differentiation. In contrast, cells on the control gels expressed markers for adipogenesis. Overall, the HA-gHGP composite matrix has great promise for directing the osteogenic differentiation of MSCs by providing an adaptable environment through the spatial presentation of cell-adhesive modules.