Characterization of the crosslinking kinetics of multi-arm poly(ethylene glycol) hydrogels formed via Michael-type addition

Abstract

Tunable properties of multi-arm poly(ethylene glycol) (PEG) hydrogel, crosslinked by Michael-type addition, support diverse applications in tissue engineering. Bioactive modification of PEG is achieved by incorporating integrin binding sequences, like RGD, and crosslinking with tri-functional protease sensitive crosslinking peptide (GCYKNRGCYKNRCG), which compete for the same reactive groups in PEG. This competition leads to a narrow range of conditions that support sufficient crosslinking density to provide structural control. Kinetics of hydrogel formation plays an important role in defining the conditions to form hydrogels with desired mechanical and biological properties, which have not been fully characterized. In this study, we explored how increasing PEG functionality from 4 to 8-arms and the concentration of biological moieties, ranging from 0.5mM to 3.75mM, affected the kinetics of hydrogel formation, storage modulus, and swelling after the hydrogels were allowed to form for 15 or 60minutes. Next, human bone marrow stromal cells were encapsulated and cultured in these modified hydrogels to investigate the combined effect of mechano-biological properties on phenotypes of encapsulated cells. While the molar concentration of the reactive functional groups (-vinyl sulfone) was identical in the conditions comparing 4 and 8-arm PEG, the 8-arm PEG formed faster, allowed a greater degree of modification, and was superior in three-dimensional culture. The degrees of swelling and storage modulus of 8-arm PEG were less affected by the modification compared to 4-arm PEG. These findings suggest that 8-arm PEG allows a more precise control of mechanical properties that could lead to a larger spectrum of tissue engineering applications.

Poly(HDDA)-Based Polymers for Microfabrication and Mechanobiology

“…These composite polymers fabricated with HDDA as a crosslinker, and starPEG, a precursor to compliant hydrogels [19,20], are two orders of magnitude more compliant than poly(HDDA) (78.15 ± 1.5 kPa and 2.59 ± 0.11 MPa, respectively) as measured by AFM-enabled nanoindentation (Figure 5a). Previously reported E of poly(HDDA) are two orders of magnitude higher, likely due to differences in fitting models (e.g.…”

Injectable Hydrogels for Spinal Cord Repair: A Focus on Swelling and Intraspinal Pressure

“…Recent reports indicate that increasing numbers of PEG arms (e.g. 4-vs. 8-arm PEG macromers) increases the gelation rate, reduces swelling ratios by ∼ 40%, and improves overall predictability and reproducibility of hydrogel networks [Kim et al, 2016]. This reduction of variability is particularly important for injection into the spinal cord, where a relatively narrow range of mechanical and/or swelling properties may be required to create good apposition to host tissue and prevent swelling-induced damage.…”

“…Using this strategy, the network properties (including swelling and mechanics) can be controlled by the molecular weight of the PEG arms and the stoichiometry of click functionalities [Elbert and Hubbell, 2001;Lutolf and Hubbell, 2003;Shikanov et al, 2011;Kim et al, 2016]. Recent reports indicate that increasing numbers of PEG arms (e.g.…”

“…However, more work needs to be done to identify optimum gelation times for different in situ-forming, injectable hydrogels. Recently, Kim et al [2016] demonstrated that faster network formation correlates to decreased swelling ratios in vitro, independently of the total macromer content. However, it remains unclear how the gelation time affects the pressure exerted during swelling in vivo.…”

“…Commonly used conditions to trigger in situ crosslinking include: (1) exposure to a specific wavelength of light [Piantino et al, 2006;Khaing et al, 2011], temperature [Gupta et al, 2006;Bearat et al, 2012], or pH [Prang et al, 2006;Pawar et al, 2015b], and (2) combining two separate components that spontaneously react upon mixing [Johnson et al, 2010a;Nimmo et al, 2011;Liang et al, 2013;Führmann et al, 2015;Kim et al, 2016]. In situ crosslinking of hydrogels in response to various triggers has been achieved by covalent chemistries (e.g.…”