Tissue engineers have explored a set of materials cues that can allow control of cell viability and guide cell fate and functions. Although the effect of substrate stiffness on cell fate has been extensively studied and established, the role of substrate stress relaxation, the ability of a substrate to dissipate cell-induced forces, is only emerging. Recently, several studies have demonstrated that substrate stress relaxation is an important mechanical cue for cell spreading, proliferation and differentiation in vitro. In this mini-review, we highlight the influence of substrate stress relaxation on cell behavior and function as well as provide future perspectives. Firstly, we describe the methods used for characterizing the stress relaxation/creep responses of hydrogels along with the molecular origin of viscoelastic properties. Then, we highlight the most recent studies elucidating the stress relaxation effect on cellular behavior using physically cross-linked hydrogels. Finally, we report on an emerging alternative design of tunable viscoelastic hydrogels: chemically cross-linked (reversible linkages) adaptable hydrogels that have been used as stable 3D cell culture platforms for a few years in the era of hydrogel systems.

A nanoGraphite (nanoG) having a high surface area and a high shape anisotropy, defined as the ratio between the crystallite dimensions in a direction orthogonal and parallel to structural layers, was used to prepare nanocomposites based on poly(1,4-cis-isoprene) (IR), in the neat polymer matrix and in the presence of carbon black (CB). Tensile and dynamic-mechanical measurements showed that nanoG forms a filler network at a relatively low concentration in neat IR and a hybrid filler network at a lower nanoG concentration in the presence of CB. A synergistic effect between the two carbon allotropes was found: composites containing both fillers present initial modulus values much higher than those calculated through the simple addition of the initial moduli of the composites containing only CB or nanoG.

Nanocomposites were prepared via melt blending, based on organically modified clays (OC), carbon nanotubes (CNT), and graphitic nanofillers made by a few layers of graphene (nanoG). In particular, nanocomposites based on a hybrid filler system, with a nanostructured filler such as carbon black (CB), are examined. It is shown that low crystalline order in the interlayer space of a layered nanofiller (such as OC and nanoG) leads to easier delamination. Nanofillers give rise to filler networking at low concentration, particularly in the presence of CB. Hybrid filler systems lead to nanocomposites' having initial moduli that are much higher than those calculated through the sum of the initial modulus of composites containing either only CB or only the nanofiller. Nanofillers enhance the matrix modulus by a multiplication factor that depends only on the nanofiller type and content, regardless of whether the matrix is a neat or a CB-filled polymer. Furthermore, the filler–polymer interfacial area is shown to be a parameter able to correlate the mechanical behavior of both nano-CNT and nanostructured (CB) fillers. By plotting values of the composite initial modulus versus the filler–polymer interfacial area, points due to CB, CNT, and the hybrid CB-CNT system lie on the same curve.

Nowadays sustainability is a key topic and waste should be considered a resource. For this reason, a large body of literature is being developed on the management of the large quantity of post-consumer tires with sustainable approaches. The ongoing research activity on tires is focused on providing eco-sustainable solutions aimed, on one hand, at reducing the amount of waste and, on the other hand, at optimizing and improving the technologies and methodologies to obtain ‘green’ products to be recycled on large scale. This review examines the solutions for the reuse of waste from post-consumer tires proposed in the literature during the last five years (2015–2019) with a focus on advantages, drawbacks and future research developments of each end-of-life tires (ELTs) recovery technology. The final aim is to provide a comparison of different fields of application for the recovery of post- consumer tires, particularly for those researchers who want to start an activity in this field. The results show that the most investigated field of application is the civil engineering area. However, it could be environmentally and socially useful to further investigate solutions suitable for recovery in sanitary applications such as water purification. Porous carbon, obtained from ELTs by pyrolysis and subsequent activation, can be successfully used as adsorbent to remove contaminants that could be harmful to the environment and human health. The latter application seems to be particularly advisable in less economically developed areas as it would represent a possible solution to the problem of environmental reclamation of water resources. GRAPHICAL ABSTRACT

The mechanical milieu of the extracellular matrix (ECM) plays a key role in modulating the cellular responses. The native ECM exhibits viscoelasticity with stress relaxation behavior. Here, we reported the preparation of degradation-mediated stress relaxing semi-interpenetrating (semi-IPN) polymeric networks of hydroxyethyl cellulose in the crosslinked gelatin-polyethylene glycol (PEG) architecture, leveraging a newly developed synthesis protocol which successively includes one-pot gelation under physiological conditions, freeze-drying and a post-curing process. Fourier transform infrared (FTIR) confirmed the formation of the semi-IPN blend mixture. A surface morphology analysis revealed an open pore porous structure with a compact skin on the surface. The hydrogel showed a high water-absorption ability (720.00 ± 32.0%) indicating the ability of retaining a hydrophilic nature even after covalent crosslinking with functionalized PEG. Detailed mechanical properties such as tensile, compressive, cyclic compression and stress relaxation tests were conducted at different intervals over 28 days of hydrolytic degradation. Overall, the collective mechanical properties of the hydrogel resembled the mechanics of cartilage tissue. The rate of stress relaxation gradually increased with an increasing swelling ratio. Hydrolytic degradation led to a marked increase in the percentage dissipation energy and stress relaxation response, indicating the degradation-dependent viscoelasticity of the hydrogel. Strikingly, the hydrogel maintained the structural stability even after degrading two-thirds of its initial mass after a month-long hydrolytic degradation. This study demonstrates that this semi-IPN G-PEG-HEC hydrogel possesses bright prospects as a potential scaffolding material in tissue engineering.