Simultaneously physically and chemically gelling polymer system utilizing a poly(NIPAAm-co-cysteamine)-based copolymer

Abstract

The objective of this work was to create an in situ physically and chemically cross-linking hydrogel for in vivo applications. N-Isopropylacrylamide (NIPAAm) was copolymerized with N-acryloxysuccinimide (NASI) via free radical polymerization. Poly(NIPAAm-co-NASI) was further modified to obtain poly(NIPAAm-co-cysteamine) through a nucleophilic attack on the carbonyl group of the NASI by the amine group of the cysteamine. Modification was verified by nuclear magnetic resonance. In addition to thermoresponsive physical gelling due to the presence of NIPAAm, this system also chemically gels via a Michael-type addition reaction when mixed with poly(ethylene glycol) diacrylate. The presence of both physical and chemical gelation resulted in material properties that are much improved compared to purely physical gels. The chemical gelation time of the copolymers was not significantly affected by the amount of thiol present due to the increased pKa of the copolymer containing more thiols. In addition, the swelling of the copolymers was highly dependent on the temperature and thiol content. Last, the rate of nucleophilic attack in the Michael-type addition reaction was shown to be highly dependent on pH and on the mole ratio of thiol to acrylate. Due to the improved mechanical properties, this material may be better suited for long-term functional replacement applications than other thermosensitive physical gels. With further development and biocompatibility testing, this material could potentially be applied as a temperature-responsive injectable biomaterial for functional embolization.

Figure 1

¹H-NMR spectra of poly(NIPAAm-co-NASI) and poly(NIPAAm-co-cysteamine).

Figure 2

(A) Temperature sweeps showing G′ and G″ for each copolymer (copolymer 1, G′ (●) and G″ (○); copolymer 2, G′ (■) and G″ (□)). (B) DSC scans indicating the transition of each copolymer.

Figure 3

(A) Absorbance at 233 nm as a function of the pH for copolymer 1, (n=3). (B) -log[(A_max – A_i)/A_i vs pH, where the pK_a value corresponds to the intercept with the y-axis. The linear approximation was only used in the buffer region, as indicated by the dotted line (B) .

Figure 4

(A) Kinetic study of disulfide bond formation for copolymer 1 at various pH (pH 6.4, ▲; pH 7.4, □; pH 8.4, ●). (B) Kinetic study of the Michael-Type addition reaction between thiols and acrylates (ratio 1:1) for copolymer 1 at various pH. (C) Kinetic study of the Michael-Type addition reaction between thiols and acrylates (ratio 1:10) for copolymer 1 at various pH. (n=4 for all).

Figure 5

Time sweeps of copolymer 1 (G′ (■) and G″ (□)) and copolymer 2 (G′ (▲) and G″ (△)). The gel point for each is indicated by an arrow, and is defined as the point where G′ is greater than G″.

Figure 6

Frequency sweeps of purely physical gelation (●), purely chemical gelation (■), and physical-chemical gelation (▲) for copolymer 1 (A) and copolymer 2 (B). Modulus of physical gel in (A) is elevated due to the presence of disulfide bonding.

Figure 7

(A) Percent swelling vs. Time for copolymer 1 at various temperatures (5 °C, ●; 20 °C , ■; 37 °C ,▲ (B) Percent swelling vs. Time for copolymer 2 at various temperatures (5 °C, ●; 20 °C , ■; 37 °C ,▲. (C) Equilibrium percent swelling for both copolymers at various temperatures (copolymer 1, white bars; copolymer 2, black bars). (n=3 for all).

Scheme 1

(A) Synthesis scheme of poly(NIPAAm-co-NASI) and poly(NIPAAm-co-cysteamine). (B) Physical and Chemical Gel formation of poly(NIPAAm-co-cysteamine) with PEGDA.