Controlled degradation of low-fouling poly(oligo(ethylene glycol)methyl ether methacrylate) hydrogels

Degradable low-fouling hydrogels are ideal vehicles for drug and cell delivery. For each application, hydrogel degradation rate must be re-optimized for maximum therapeutic benefit. We developed a method to rapidly and predictably tune degradation rates of low-fouling poly(oligo(ethylene glycol)methyl ether methacrylate) (P(EG)_xMA) hydrogels by modifying two interdependent variables: (1) base-catalysed crosslink degradation kinetics, dependent on crosslinker electronics (electron withdrawing groups (EWGs)); and, (2) polymer hydration, dependent on the molecular weight (M_W) of poly(ethylene glycol) (PEG) pendant groups. By controlling PEG M_W and EWG strength, P(EG)_xMA hydrogels were tuned to degrade over 6 to 52 d. A 6-member P(EG)_xMA copolymer library yielded slow and fast degrading low-fouling hydrogels suitable for short- and long-term delivery applications. The degradation mechanism was also applied to RGD-functionalized poly(carboxybetaine methacrylamide) (PCBMAA) hydrogels to achieve slow (∼50 d) and fast (∼13 d) degrading low-fouling, bioactive hydrogels.

To tune degradation rates of low-fouling hydrogels, a 6-member P(EG)_xMA copolymer library with different electronics and hydration levels was developed.

Influence of Hydrophobic Cross-Linkers on Carboxybetaine Copolymer Stimuli Response and Hydrogel Biological Properties

Poly(carboxybetaine) (pCB) hydrogels do not elicit a foreign body response due to their low-fouling properties, making them ideal implantable materials for in vivo drug and cell delivery. Current reported pCB hydrogels are cross-linked using cytotoxic UV-initiated radical polymerization limiting clinical and in vivo translation. For clinical translation, we require in situ and biorthogonal cross-linking of pCB hydrogels that are both low-fouling and low-swelling to limit nonspecific interactions and minimize tissue damage, respectively. To this end, we synthesized carboxybetaine (CB) random copolymers (molecular weight (MW): ∼7-33 kDa; Đ: 1.1-1.36) containing azide (pCB-azide) or strained alkyne (Dibenzocyclooctyne (DBCO); pCB-DBCO) that rapidly cross-link upon mixing. Unlike CB homopolymers and other CB copolymers studied, high DBCO content pCB-DBCO₃₀ (30% DBCO mole fraction) is thermoresponsive with a upper critical solution temperature (UCST; cloud point of ∼20 °C at 50 g/L) in water due to electrostatic associations. Due to the antipolyelectrolyte effect, pCB-DBCO₃₀ is salt-responsive and is soluble even at low temperatures in 5 M NaCl, which prevents zwitterion electrostatic associations. pCB-azide and pCB-DBCO with 0.05 to 0.16 cross-linker mole fractions rapidly formed 10 wt % hydrogels upon mixing that were low-swelling (increase of ∼10% in wet weight) while remaining low-fouling to proteins (∼10-20 μg cm^-2) and cells, making them suitable for in vivo applications. pCB-X₃₁ hydrogels composed of pCB-azide₃₂ and pCB-DBCO₃₀ formed opaque gels in water and physiological conditions that shrunk to ∼70% of their original wet weight due to pCB-DBCO₃₀'s greater hydrophobicity and interchain electrostatic interactions, which promotes nonspecific protein adsorption (∼35 μg cm^-2) and cell binding. Once formed, the electrostatic interactions in pCB-X₃₁ hydrogels are not fully reversible with heat or salt. Although, pCB-X₃₁ hydrogels are transparent when initially prepared in 5 M NaCl. This is the first demonstration of a thermo- and salt-responsive CB copolymer that can tune hydrogel protein and cell fouling properties.