Hyaluronic acid (HA) is a vital, functional component of the extracellular matrix (ECM). Following a synthetic biology approach, we designed polyelectrolyte hydrogels composed of HA as a polymeric backbone interconnected by short covalent crosslinkers. Utilizing a thiol‐Michael addition, defined network structures are created, which can be modulated by the aromatic core and charge of the crosslinkers. With increasing negative network charge, varied both through the charge of the crosslinker and the degree of functionalization of the HA chain, the Young's modulus of the hydrogels decreases linearly as swelling ratios increase. Interestingly, secondary interactions with aromatic crosslinkers are stronger for elastin inspired crosslinkers with a pyridinium core compared to analogous triazolium crosslinkers. Based on the defined covalent interconnectivity of these HA‐hydrogels, we were able to demonstrate the specific electrostatic and aromatic interactions induced by different short crosslinkers.
The extracellular matrix (ECM) represents a highly charged and hydrated network in which different cells in vertebrate tissues are embedded. Hydrogels as minimal ECM mimetics with a controlled chemistry offer the opportunity to vary material properties by varying the negative network charge. In this paper, a synthetic biology model of the ECM based on natural and highly negatively charged polyelectrolyte hyaluronic acid (HA) is characterized with specific emphasis on its charge-related bioactivity. Therefore, the thiol-Michael addition click reaction is used to produce HA hydrogels with defined network structure and charge density. The presented hydrogels show enzymatic degradability and cell attachment. These properties depend on both covalent and electrostatic interactions within the hydrogel network. Furthermore, no unspecific or specific attachment of proteins to the presented hydrogels is observed. In addition, these fundamental insights into charge-related ECM behavior and the influence of electrostatic properties could also lead to innovations in existing biomedical products.
A small library of cross-linkers for hydrogels was synthesized. The cross-linkers consisted of 2,6-and 3,5-diacylpyridine or 2,4,6-triacylpyridine as the core unit, which were tethered via ethylene glycol, amino ethanol, and 1,n-diamine spacers to terminal acrylate or acrylamide moieties. Esterification and amide formation of the terminal acryl units were found to be dependent on the ratio of NH/O in the spacer, the constitution pattern of the pyridine ring, and the total number of acryl groups. Thus, esters generally gave higher yields than amides decreasing with increasing number of NH in the spacer and with increasing number of acryl units. In the case of 3,5-diacylpyridine derivatives, these trends were less prominent as compared to the 2,6-diacylpyridine series, indicating that steric hindrance and unfavorable hydrogen bonding interaction of the spacers might influence the observed reactivity differences. The 3,5-diacylpyridines were converted to the N-methylpyridinium salts and selected members of both neutral and cationic 3,5-diacylpyridinium derivatives were submitted to hydrogelations with synthetic polymer poly(1-glycidylpiperazine) via aza-Michael addition and thiolated natural hyaluronan via thio-Michael reaction, respectively. Rheological properties of the resulting hydrogels were studied, revealing that both spacer type as well as charge affected elastic moduli and degree of swelling.
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