Mechanical characterization of sequentially layered photo-clickable thiol-ene hydrogels

Abstract

Multi-layer hydrogels are promising for tissue engineering due to the ability to control the local properties within each layer. However, the interface that forms between each layer has the potential to affect the performance of the hydrogel. The goals of this study were to characterize how the interface forms via its thickness and mechanical properties, identify its impact on the overall hydrogel properties, and provide new insights into how to control the interface. A photo-clickable poly(ethylene glycol) hydrogel was used to form bilayer hydrogels that were sequentially polymerized in a step-and-repeat process. Different processing conditions were studied: the time (0-20min) before initiating polymerization of the second layer (soak time, t) and the hydrogel crosslink density (the same, less crosslinked, or more crosslinked) of the first layer as compared to the second layer. Interface thickness was characterized by confocal microscopy, monomer transport by Fickian diffusion, single and bilayer hydrogel mechanics by bulk moduli measurements, and interface moduli measurements using AFM, nanoindentation, and strain mapping. The interface thickness ranged from ~70 to 600μm (1-10% of total height) depending on processing conditions, but did not affect the bulk hydrogel modulus. Analysis of monomer transport revealed that convection, due to changes in hydrogel swelling, and diffusion contribute to interface thickness. Nanomechanical analysis of bilayer hydrogels formed from soft (75kPa) and stiff (250kPa) layers showed a gradient in elastic modulus across the interface, which corresponded to strain maps. In summary, this work identifies that diffusive and convective transport of monomers across the interface controls its thickness and that a mechanically robust interface forms, which does not affect the hydrogel modulus. By controlling the processing conditions, the thickness of the interface can be tuned without affecting the mechanical properties of the bulk hydrogel.

The effects of dynamic compressive loading on human mesenchymal stem cell osteogenesis in the stiff layer of a bilayer hydrogel

“…Because, the hydrogels are formed under nonequilibrium conditions, they undergo differential swelling. In this study, the stiff hydrogel exhibited a higher degree of swelling, a finding consistent with our previous study (Aziz et al, ). Residual stresses were visible around the interface and at the bottom of the stiff layer.…”

Stereolithographic 3D Printing for Deterministic Control over Integration in Dual‐Material Composites

“…For example, the hydrogel mechanical properties for the soft hydrogel are easily tunable by varying the monomer concentration, molecular weight, and architecture (four‐arm vs. eight‐arm PEG) and /or by changing the solvent content. In the thiol‐ene hydrogel formulation used herein, changing the solvent content can result in moduli that span ≈5 to ≈250 kPa . Under this scenario, the monomers themselves would not change and therefore their transport properties into the material #1 would be unchanged.…”

Enhanced mechanical properties of photo-clickable thiol–ene PEG hydrogels through repeated photopolymerization of in-swollen macromer

“…However, diffusion of the PEG-NB macromer will be more limited due to its size and thus its concentration may be low. 35 Consequently, any reactions to the initial network may not, on its own, contribute significantly to the macroscopic properties. However, swelling the hydrogels with PEG-dithiol and photoinitiator in PBS led to increases in the compressive modulus by a factor of two (ESI 5†).…”

“…35 To understand the impact of in-swelling on the level of polymer chain stretching at each SE cycle, the ratio of linear deformation of the hydrogel at equilibrium ($Q_{n,f}^{1/3}$) to the linear deformation of hydrogel immediately after the gel is formed but before swelling ($Q_{n,i}^{1/3}$) was evaluated for cycle n . This ratio is defined in terms of ϕ by 34 $${\lambda }_n={true(\frac{{\varphi }_{n,i}}{{\varphi }_{n,f}}true)}^{1/3}.$$ …”