(T1130-04-19) A Novel Self-Crosslinked Matrix Microneedle Patch for “Smart” Delivery of Insulin at Hyperglycemia

Purpose: All people with type 1 diabetes (T1D) and advanced type 2 diabetes require insulin therapy to control their blood glucose levels (BGLs). Improved glycemic control currently relies on hybrid closed-loop insulin delivery systems that combine a continuous glucose monitoring device connected to an insulin pump for delivering insulin at hyperglycemia. However, patients need to wear such devices 24 hours a day, which poses risks and implications such as skin infections, device malfunction, and the high cost of the device and accessories. To overcome these problems, there is an increasing interest in developing glucose-responsive or “smart” polymeric microneedle (MN) systems that can deliver insulin automatically without the need for patient or caregiver intervention. However, most of reported hydrogel-based MN patches are fabricated using a multistep procedure that requires thermal, photo- or chemical crosslinking, which may cause insulin denaturation presenting a challenge for clinical translation. Herein, we designed a novel self-crosslinkable hydrogel-based matrix MN (mMN) patch, fabricated using a simple and protein-friendly method that enabled encapsulation of highly concentrated insulin, preservation of insulin structure and bioactivity, and delivering insulin in response to changes in glucose level.

Methods: The glucose-responsive polymers were synthesized by conjugating functional groups to the polymer backbone of hyaluronic acid. The degree of conjugated functional groups of synthesized polymers was analyzed using nuclear magnetic resonance (NMR) and verified by ultraviolet-visible (UV-vis) spectroscopy. Insulin-loaded mMN was prepared by self-crosslinking in a single step under physiological pH. Mechanical strength of the mMN was analyzed by compression testing and evaluated on rat skin. Glucose-responsive insulin release was assessed in vitro and assayed by reverse-phase high-performance liquid chromatography (RP-HPLC) and circular dichroism (CD) spectropolarimetry. The plasma insulin delivered by mMN was determined using an ELISA kit. The bioactivity and biocompatibility of mMN were evaluated in a streptozotocin-induced T1D rat model.

Results: The synthesized and purified polymers were characterized using 1H-NMR and UV-vis spectroscopy to confirm the successful conjugation of the functional groups. A distinctive brown color of the patch was observed, which was attributed to the novel crosslinking mechanism of the mMN under mild fabrication conditions. The mMN provided adequate mechanical strength for skin penetration as evidenced by the delivery of trypan blue dye into the rat skin. In vitro insulin release study and swelling test demonstrated the glucose-responsiveness of mMN. Both CD and HPLC results confirmed the native insulin stability was preserved after the patch fabrication and subsequent release process. In vivo efficacy of the mMN against hyperglycemia in a T1D rat model showed that the rats treated with the insulin mMN maintained their BGLs within a tight glycemic range over 7 hr and were able to tolerate high doses of glucose injection 2 hr post-patch application compared to subcutaneous injection of insulin. This glucose suppression effect was ascribed to the insulin release from the mMN patch triggered by a glucose challenge as evidenced by a significant increase in plasma insulin concentration. Histological analysis of rat skin treated with mMN indicated minimal evidence of cell inflammation and tissue damage comparable to healthy rat skin.

Conclusion: The mMN patch is capable of loading highly concentrated insulin, preserving its stability, and delivering insulin at hyperglycemia. The mMN prepared by simple pH adjustment of the polymer solution eliminates multistep and/or harsh fabrication processes and brings opportunities for easy scale up of manufacturing. The excellent properties and performance of the mMN and the simple design and fabrication method impart the proposed mMN great potential for clinical translation.

Acknowledgments: The work was supported through funding from Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery (RGPIN 170460-13) and Equipment grants (EQPEQ 374799-09; EQPEQ 440689-13) to X.Y.W. The authors would also like to thank the staff of the Analytical Laboratory for Environmental Science Research (ANALEST), Centre for Research and Applications in Fluidic Technologies (CRAFT), and Microscopy Imaging Laboratory (MIL) facilities at the University of Toronto for helping with analytical experiments.

Conflict of Interest: The authors declare no conflict of interest.