Abstract
Silk fibroin (SF) has attracted much attention due to its high, tunable mechanical strength and excellent biocompatibility. Imparting the ability to respond to external stimuli can further enhance its scope of application. In order to imbue stimuli-responsive behavior in silk fibroin, we propose a new conjugated material, namely cationic SF (CSF) obtained by chemical modification of silk fibroin with ε-Poly-(L-lysine) (ε-PLL). This pH-responsive CSF hydrogel was prepared by enzymatic crosslinking using horseradish peroxidase and H2O2. Zeta potential measurements and SDS-PAGE gel electrophoresis show successful synthesis, with an increase in isoelectric point from 4.1 to 8.6. Fourier transform infrared (FTIR) and X-ray diffraction (XRD) results show that the modification does not affect the crystalline structure of SF. Most importantly, the synthesized CSF hydrogel has an excellent pH response. At 10 wt.% ε-PLL, a significant change in swelling with pH is observed. We further demonstrate that the hydrogel can be glucose-responsive by the addition of glucose oxidase (GOx). At high glucose concentration (400 mg/dL), the swelling of CSF/GOx hydrogel is as high as 345 ± 16%, while swelling in 200 mg/dL, 100 mg/dL and 0 mg/dL glucose solutions is 237 ± 12%, 163 ± 12% and 98 ± 15%, respectively. This shows the responsive swelling of CSF/GOx hydrogels to glucose, thus providing sufficient conditions for rapid drug release. Together with the versatility and biological properties of fibroin, such stimuli-responsive silk hydrogels have great potential in intelligent drug delivery, as soft matter substrates for enzymatic reactions and in other biomedical applications.
Highlights
Diabetes is a non-communicable, chronic metabolic disease often leading to serious long-term complications such as retinopathy, renal failure, neuropathy and cardiovascular disease
We further explored the response of the cationic silk fibroin (CSF) hydrogels to glucose concentration via incorporation of the enzyme glucose oxidase
The results showed that the CSF/glucose oxidase (GOx) hydrogel could be used as a drying agent
Summary
Diabetes is a non-communicable, chronic metabolic disease often leading to serious long-term complications such as retinopathy, renal failure, neuropathy and cardiovascular disease. During the current global pandemic, it has been observed that high blood glucose concentrations in diabetic patients may make them more susceptible to COVID-19 infection, and enhance the replication and expression of the virus. This has led to more serious clinical symptoms among diabetic patients with COVID-19 infection [2,3,4,5]. The control and management of blood glucose levels in diabetic patients is, of great importance This process involves real-time monitoring of blood glucose levels, and, often, multiple injections of insulin to ensure effective treatment [6,7,8]. Chemically-controlled closed-loop insulin delivery has been suggested as a treatment alternative. This can include delivery of insulin via a glucose-responsive material, or by modifying insulin with glucose-sensitive functional groups to trigger active release [9,10,11]
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