Bulk matrix stiffness has emerged as a key mechanical cue in stem cell differentiation. Here, we show that the commitment and differentiation of human mesenchymal stem cells encapsulated in physiologically soft (∼0.2-0.4 kPa), fully synthetic polyisocyanopeptide-based three-dimensional (3D) matrices that mimic the stiffness of adult stem cell niches and show biopolymer-like stress stiffening, can be readily switched from adipogenesis to osteogenesis by changing only the onset of stress stiffening. This mechanical behaviour can be tuned by simply altering the material's polymer length whilst maintaining stiffness and ligand density. Our findings introduce stress stiffening as an important parameter that governs stem cell fate in a 3D microenvironment, and reveal a correlation between the onset of stiffening and the expression of the microtubule-associated protein DCAMKL1, thus implicating DCAMKL1 in a stress-stiffening-mediated, mechanotransduction pathway that involves microtubule dynamics in stem cell osteogenesis.

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Microenvironments such as protein composition, physical features, geometry, and elasticity play important roles in stem cell lineage specification. The components of the extracellular matrix are known to subsequently assemble into fibrillar networks in vivo with defined periodicity. However, the effect of the most critical parameter, which involves the periodicity of these fibrillar networks, on the stem cell fate is not yet investigated. Here, we show the effect of synthetic fibrillar networks patterned with nanometric periodicities, using bottom-up approaches, on the response of stem cells. We have used helical organic nanoribbons based on self-assemblies of Gemini-type amphiphiles to access chiral silica nanoribbons with two different shapes and periodicities (twisted ribbons and helical ribbons) from the same native self-assembled organic nanostructure. We demonstrate the covalent grafting of these silica nanoribbons onto activated glass substrates and the influence of this programmed isotropically oriented matrix to direct the commitment of human mesenchymal stem cells (hMSCs) into osteoblast lineage in vitro, free of osteogenic-inducing media. The specific periodicity of 63 nm (±5 nm) with helical ribbon shape induces specific cell adhesion through the fibrillar focal adhesion formation and leads to stem cell commitment into osteoblast lineage. In contrast, the matrix of periodicity 100 nm (±15 nm) with twisted ribbon shape does not lead to osteoblast commitment. The inhibition of non-muscle myosin II with blebbistatin is sufficient to block this osteoblast commitment on helical nanoribbon matrix, demonstrating that stem cells interpret the nanohelical shape and periodicity environment physically. These results indicate that hMSCs could interpret nanohelical shape and periodicity in the same way they sense microenvironment elasticity. This provides a promising tool to promote hMSC osteogenic capacity, which can be exploited in a 3D scaffold for bone tissue engineering.