We report the structural and mechanical behavior of multicomponent hydrogels comprising the commercial poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer F127 and alginate. Previous studies on this system have shown thermoreversible behavior in shear rheology. Here we explore the properties of these materials under compression and large deformations, relevant to applications such as wound dressings that require mechanical robustness. For gels with lower F127 concentration, we find that the stiffness of the gels can be ascribed to the alginate network, and that the Young's modulus and fracture stress do not strongly depend on temperatures. However, for gels with an F127 concentration of 30 wt %, the Young's modulus is enhanced at higher temperatures. Under large deformations, the fracture stress and fracture strain of the materials can be independently varied using the alginate and F127 concentrations, respectively; without the trade‐off in these properties that is often observed in rigid polymer networks. Small‐angle X‐ray scattering shows a power‐law dependence scattering intensity on
Normal stress anisotropy and marginal stability in athermal elastic networks
Hydrogels of semiflexible biopolymers such as collagen have been shown to contract axially under shear strain, in contrast to the axial dilation observed for most elastic materials. Recent work has shown that this behavior can be understood in terms of the porous, two-component nature and consequent time-dependent compressibility of hydrogels. The apparent normal stress measured by a torsional rheometer reflects only the tensile contribution of the axial component σ zz on long (compressible) timescales, crossing over to the first normal stress difference, N 1 = σ xx − σ zz at short (incompressible) times. While the behavior of N 1 is well understood for isotropic viscoelastic materials undergoing affine shear deformation, biopolymer networks are often anisotropic and deform nonaffinely. Here, we numerically study the normal stresses that arise under shear in subisostatic, athermal semiflexible polymer networks. We show that such systems exhibit strong deviations from affine behavior and that these anomalies are controlled by a rigidity transition as a function of strain.
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- Award ID(s):
- 1826623
- NSF-PAR ID:
- 10104380
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 15
- Issue:
- 7
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 1666 to 1675
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract q arising from the alginate network and scattering peaks consistent with rearranging micelles. For gels with lower F127 concentrations, we find a disordered–body‐centered cubic (BCC)‐face‐centered cubic (FCC) progression of states with temperature, and a BCC/FCC mixture for gels with higher F127 concentrations. -
Abstract Purpose . Laboratory models of human arterial tissues are advantageous to examine the mechanical response of blood vessels in a simplified and controllable manner. In the present study, we investigated three silicone-based materials for replicating the mechanical properties of human arteries documented in the literature. Methods . We performed uniaxial tensile tests up to rupture on Sylgard184, Sylgard170 and DowsilEE-3200 under different curing conditions and obtained their True (Cauchy) stress-strain behavior and Poisson’s ratios by means of digital image correlation (DIC). For each formulation, we derived the constitutive parameters of the 3-term Ogden model and designed numerical simulations of tubular models under a radial pressure of 250 mmHg. Results . Each material exhibits evident non-linear hyperelasticity and dependence on the curing condition. Sylgard184 is the stiffest formulation, with the highest shear moduli and ultimate stresses at relative low strains ( μ 184 = 0.52–0.88 MPa, σ 184 = 15.90–16.54 MPa, ε 184 = 0.72–0.96). Conversely, Sylgard170 and DowsilEE-3200 present significantly lower shear moduli and ultimate stresses that are closer to data reported for arterial tissues ( μ 170 = 0.33–0.7 MPa σ 170 = 2.61–3.67 MPa, ε 170 = 0.69–0.81; μ dow = 0.02–0.09 MPa σ dow = 0.83–2.05 MPa, ε dow = 0.91–1.05). Under radial pressure, all formulations except DowsilEE-3200 at 1:1 curing ratio undergo circumferential stresses that remain in the elastic region with values ranging from 0.1 to 0.18 MPa. Conclusion . Sylgard170 and DowsilEE-3200 appear to better reproduce the rupture behavior of vascular tissues within their typical ultimate stress and strain range. Numerical models demonstrate that all three materials achieve circumferential stresses similar to human common carotid arteries (Sommer et al 2010), making these formulations suited for cylindrical laboratory models under physiological and supraphysiological loading.more » « less
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Abstract When hydrogels are designed for biological applications, the mechanical properties are carefully chosen to match their precise application. However, traditional methodologies of mechanical characterization (simple shear or compression/extension) commonly ignore the multiaxiality of in vivo deformations. A recent study highlights that biopolymers and tissue indeed show a complex response to combined uniaxial and shear strains. In this study a synthetic yet biomimetic fibrous hydrogel is used, which is based on polyisocyanides and forms a self‐assembled network of branched semiflexible chains, similar in architecture networks of structural biopolymers like actin, collagen, and fibrin. Its synthetic nature allows to decouple key parameters of these networks and individually understand their impact on the mechanical response under multiaxial deformation. Experimentally, it is found that the persistence length is a key parameter of biological networks, which tunes softening of gels under compression: The stiffer the polymer, the more the network softens in compression. This study provides insights into tissue behavior that likely is only obtainable from synthetic model systems and is able to direct further the design of new synthetic biomimetic soft materials that are in high demand as tunable bio‐free extracellular matrix materials.
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null (Ed.)Networks formed by crosslinking bottlebrush polymers are a class of soft materials with stiffnesses matching that of ‘watery’ hydrogels and biological tissues but contain no solvents. Because of their extreme softness, bottlebrush polymer networks are often subject to large deformations. However, it is poorly understood how molecular architecture determines the extensibility of the networks. Using a combination of experimental and theoretical approaches, we discover that the yield strain γ y of the network equals the ratio of the contour length L max to the end-to-end distance R of the bottlebrush between two neighboring crosslinks: γ y = L max / R − 1. This relation suggests two regimes: (1) for stiff bottlebrush polymers, γ y is inversely proportional to the network shear modulus G , γ y ∼ G −1 , which represents a previously unrecognized regime; (2) for flexible bottlebrush polymers, γ y ∼ G −1/2 , which recovers the behavior of conventional polymer networks. Our findings provide a new molecular understanding of the nonlinear mechanics for soft bottlebrush polymer networks.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c8sm02192a
