Novel in Situ Forming, Degradable Dextran Hydrogels by Michael Addition Chemistry:  Synthesis, Rheology, and Degradation

Various vinyl sulfone functionalized dextrans (dex-VS) (Mn,dextran = 14K or 31K) with degrees of substitution (DS) ranging from 2 to 22 were conveniently prepared by a one-pot synthesis procedure at room temperature. This procedure involved reaction of a mercaptoalkanoic acid with an excess amount of divinyl sulfone yielding vinyl sulfone alkanoic acid, followed by conjugation to dextran using N,N‘-dicyclohexylcarbodiimide (DCC)/4-(dimethylamino)pyridinium 4-toluenesulfonate (DPTS) as a catalyst system. By using two different mercaptoalkanoic acids, 3-mercaptopropionic acid (1a) and 4-mercaptobutyric acid (1b), dex-VS conjugates with either an ethyl spacer (denoted as dex-Et-VS) or a propyl spacer (denoted as dex-Pr-VS) between the thioether and ester groups were obtained. Linear and four-arm mercaptopoly(ethylene glycol) (Mn = 2.1K) with two or four thiol groups (denoted as PEG-2-SH and PEG-4-SH, respectively) were also prepared. Hydrogels were rapidly formed in situ under physiological conditions by Michael type addition upon mixing aqueous solutions of dex-VS and multifunctional PEG-SH at a concentration of 10−20% w/v. The gelation time ranged from 0.5 to 7.5 min, depending on the DS, concentration, dextran molecular weight, and PEG-SH functionality. Rheological studies showed that these dextran hydrogels are highly elastic. The storage modulus increased with increasing DS, concentration, and dextran molecular weight, and hydrogels with a broad range of storage moduli from 3 to 46 kPa were obtained. Swelling/degradation studies revealed that these dextran hydrogels have a low initial swelling and are degradable under physiological conditions. The degradation time varied from 3 to 21 days depending on the DS, concentration, dextran molecular weight, and PEG-SH functionality. Interestingly, dex-Pr-VS hydrogels showed prolonged degradation times, but otherwise similar properties compared to dex-Et-VS hydrogels. The hydrolysis of the linker ester bonds of the dex-VS conjugates under physiological conditions was confirmed by 1H NMR. The results showed that the hydrolysis kinetics were independent of the DS and the dextran molecular weight. Therefore, the degradation rate of these hydrogels can be precisely controlled.