Critical behaviour in the nonlinear elastic response of hydrogels

In this paper we study the elastic response of synthetic hydrogels to an applied shear stress. The hydrogels studied here have previously been shown to mimic the behaviour of biopolymer networks when they are sufficiently far above the gel point. We show that near the gel point they exhibit an elastic response that is consistent with the predicted critical behaviour of networks near or below the isostatic point of marginal stability. This point separates rigid and floppy states, distinguished by the presence or absence of finite linear elastic moduli. Recent theoretical work has also focused on the response of such networks to finite or large deformations, both near and below the isostatic point. Despite this interest, experimental evidence for the existence of criticality in such networks has been lacking. Using computer simulations, we identify critical signatures in the mechanical response of sub-isostatic networks as a function of applied shear stress. We also present experimental evidence consistent with these predictions. Furthermore, our results show the existence of two distinct critical regimes, one of which arises from the nonlinear stretch response of semi-flexible polymers.

Polymerization-Induced Self-Assembly of Block Copolymer Nano-objects via RAFT Aqueous Dispersion Polymerization

In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells

Thermo-responsive Diblock Copolymer Worm Gels in Non-polar Solvents

Benzyl methacrylate (BzMA) is polymerized using a poly(lauryl methacrylate) macromolecular chain transfer agent (PLMA macro-CTA) using reversible addition–fragmentation chain transfer (RAFT) polymerization at 70 °C in n-dodecane. This choice of solvent leads to an efficient dispersion polymerization, with polymerization-induced self-assembly (PISA) occurring via the growing PBzMA block to produce a range of PLMA–PBzMA diblock copolymer nano-objects, including spheres, worms, and vesicles. In the present study, particular attention is paid to the worm phase, which forms soft free-standing gels at 20 °C due to multiple inter-worm contacts. Such worm gels exhibit thermo-responsive behavior: heating above 50 °C causes degelation due to the onset of a worm-to-sphere transition. Degelation occurs because isotropic spheres interact with each other much less efficiently than the highly anisotropic worms. This worm-to-sphere thermal transition is essentially irreversible on heating a dilute solution (0.10% w/w) but is more or less reversible on heating a more concentrated dispersion (20% w/w). The relatively low volatility of n-dodecane facilitates variable-temperature rheological studies, which are consistent with eventual reconstitution of the worm phase on cooling to 20 °C. Variable-temperature 1H NMR studies conducted in d26-dodecane confirm partial solvation of the PBzMA block at elevated temperature: surface plasticization of the worm cores is invoked to account for the observed change in morphology, because this is sufficient to increase the copolymer curvature and hence induce a worm-to-sphere transition. Small-angle X-ray scattering and TEM are used to investigate the structural changes that occur during the worm-to-sphere-to-worm thermal cycle; experiments conducted at 1.0 and 5.0% w/w demonstrate the concentration-dependent (ir)reversibility of these morphological transitions

The ADMORPH approach for Adaptively Morphing Embedded Systems

peer reviewedDue to the increasing performance demands of missionand safety-critical Cyber-Physical Systems (of Systems), these systems exhibit a rapidly growing complexity, manifested by an increasing number of (distributed) computational cores and application components connected via complex networks. However, with these systems’ growing complexity and interconnectivity, the chances of hardware failures and disruptions due to cyber-attacks will also quickly increase. In the ADMORPH project we explore system adaptivity, in terms of dynamically remapping application components to processing cores, to fuse fault- and intrusion tolerance with the increasing performance requirements of mission- and safety-critical CPS(oS). This paper describes the overall ADMORPH architecture and provides an overview of the developed methodologies, methods and tools for the specification, design, analysis and runtime deployment of adaptive mission- and safetycritical CPS(oS) that are robust against both component failures and cyber-attacks

Fully Stable And Homogeneous Lyotropic Liquid Crystal Alignment On Anisotropic Surfaces

Lyotropic chromonic liquid crystals have great potential in both biosensing and optical devices due to their biocompatible nature and strong optical characteristics. These applications, however, demand a homogeneous and stable alignment on anisotropic surfaces, a challenge that, so far, has not been solved adequately. In this work, it is shown how to drastically increase the quality of in-plane alignment and stability of these liquid crystals on conventional rubbed polyimide substrates by the addition of a small amount of a nonionic surfactant. Samples with surfactant show excellent alignment that is stable for months, while control samples without surfactant show much poorer alignment that further deteriorates in days. Also, well-aligned dry films of chromonics can be prepared following this approach. It is demonstrated how to obtain high-quality alignment by controlling the concentration and the nature of the surfactant, in particular its molecular structure and hydrophilic/lipophilic balance (HLB value) and other critical parameters are discussed. It is believed that this approach may very well be essential for advancing the applicability of these water-based, biocompatible, and often highly dichroic materials for a wide range of uses

Anchoring Strength Measurements Of A Lyotropic Chromonic Liquid Crystal On Rubbed Polyimide Surfaces

Techniques to achieve planar anchoring of the director of a lyotropic chromonic liquid crystal (LCLC) have been developed only recently. These techniques range from the very weak anchoring achieved by simply scratching the glass with a very fine abrasive to the very strong anchoring achieved with lithographic techniques. A possible alignment technique is to use rubbed polyimide (PI) alignment layers as is routinely done to align thermotropic liquid crystals (TLCs). The anchoring strength of the LCLC disodium cromoglycate (DSCG) on different rubbed PI alignment layers is measured using a pi/2 twist cell and optical polarisation techniques. Although the anchoring strength is not as large as can be achieved with lithographic techniques, it is larger than scratched glass for one PI. Just as important, rubbed PI provides a much more uniform surface than is possible with scratched glass. Interestingly, addition of a small amount of surfactant to the LCLC, which results in improved alignment, does not increase the anchoring strength. Weak anchoring of LCLCs can also be achieved with a photo-patterned dye surface layer. Finally, atomic force microscopy measurements of the surface combined with prior theoretical work linking surface features and anchoring strength predict anchoring strengths consistent with experiment

Structure and Dynamics of a Temperature-Sensitive Hydrogel

Contains fulltext : 236284.pdf (Publisher’s version ) (Open Access

Quantification of cell-mediated stiffening of synthetic extracellular matrices

Strain stiffening of extracellular matrices is increasingly recognized as a mechanical mechanism to tune cell behaviors and maintain tissue functions. Routinely, the mechanical properties of a matrix are determined using shear rheology in the absence of cells. However, this method lacks the cell-induced stiffening and consistently underestimate the (macroscopic) stiffness of biogels. We introduce a method using shear rheology in combination with cells and synthetic matrices, polyisocyanide hydrogels that possess nonlinear mechanics. Human adipose-derived stem cells (hASCs) were encapsulated in gels from polymers with different contour lengths but a constant peptide density and polymer concentration to study the cell-mediated nonlinear mechanical change of ECM quantitatively. Other parameters such as cell line, cell density, type of cell adhesion peptides were also changed systematically. Rheology results reveal that cells can adjust matrix stiffness to a similar value via contraction regardless of the cell-free elasticity of gels. Fluorescence imaging further shows the local physical matrix remodeling by cells. Our results suggest that biogels or their mimics with nonlinear mechanics possess dynamic mechanical properties that are highly responsive to cellular intervention. Our approach offers a new platform with synthetic semi-flexible polymer networks for research on the cell-matrix interface