Fabrication of Photocurable Hyaluronic Acid Coated Microneedle Sensor for Glucose Monitoring

Abstract

Introduction Microneedle (MN) has been developed to diagnose and detect the biomarkers which are related to various diseases such as diabetes mellitus (DB) and caner. The biomarkers could be extracted from interstitial fluid (ISF) in the human body by using MN with a minimally-invasive and pain-free manner. In particular, glucose sensing has widely been investigated due to increase of DB patients who suffered inconvenience of frequent monitoring of glucose levels. MN-based sensors targeting ISF glucose monitoring was introduced and showed the possibility to replace the existing blood-based glucose sensing technologies [1-3]. We hypothesized that the coating of a MN surface with swellable material such as hydrogel could increase detection sensitivity and time for stabilization. Thus, we developed a swellable hydrogel coated MN sensor which could extract the glucose from ISF with swelling for a short time. Methacrylated hyaluronic acid (MeHA), photo-crosslinkable hydrogel, was utilized to absorb ISF containing biomarkers for electrochemical reaction with the metallized surface of a MN. Cyclic voltammetry (CV) with a model redox species, ferricyanide, were performed to characterize the MeHA-coated MN sensors for the detection of glucose. Experimental Methods To fabricate the MeHA coated MN sensor, 3×3 SU-8 MN array was transfer molded on the 1 cm × 2 cm of the glass wafer. The female PDMS mold which was patterned MN shapes were filled with SU-8 in a vacuum condition and the mold was gently transferred on the glass wafer. After UV curing, the MN electrode was prepared by the Ti-gold sputtering. MeHA was synthesized by modifying from the hydroxyl groups to the methyl group in basic condition. Dimethylformamide, methacrylic anhydride, and sodium chloride were mixed in the 2% of hyaluronic acid solution while a solution was controlled in basic condition. The precursor was washed three times by ethanol and was freeze-dried to obtain the MeHA sponges. Then, the MeHA sponges were dissolved into the deionized water which was contained 0.05% of the photoinitiator for preparing the MeHA solution. The MeHA solution was poured on the surface of gold-sputtered SU-8 MN arrays and the swellable MN sensor was fabricated after drying. Before measuring the glucose, potassium ferricyanide (K3[Fe(CN)6]) which was the standard redox solution to conduct CV was used as an alternative of the biomarker. The MeHA coated MN electrode was used as a working electrode and a Pt mesh electrode was used as a counter electrode. The MN sensor was immersed into the potassium ferricyanide solution and the CV was conducted while the concentration chose 10 mM, 10 μM, and 1 μM to confirm sensitivity and detection limit of the three-electrode system with MeHA coated MN sensor. Results and Discussion The 3×3 array of SU-8 MNs fabricated on a glass wafer had the height of 640 μm and the aspect ratio of 1.6. The MeHA was successfully synthesized and the MeHA film had a swelling ratio of 450% after 10 sec swelling in the phosphate buffered saline. By using the three-electrode system including the MeHA uniform coated MN electrode, in vitro sensing tests were performed with various concentrations of potassium ferricyanide. As the results of the CV analysis, when the concentration of potassium ferricyanide was 10 mM, the anodic and cathodic peak currents of the measured I-V curve were 135 and 144 μA, respectively. Although the peaks were observed from the CV plots, it was hard to measure the exact value of peak currents when the concentration of potassium ferricyanide, 10 μM, and 1 μM. Nevertheless, the current peaks of 10 μM was slightly higher than the peaks of the 1 μM sample. Since the peak current was increased in proportion to the level of potassium ferricyanide, it was concluded that the MeHA coated MN sensor system could be used to measure the biomarker. These results also support that the biomarkers could be extracted from the ISF by the swelling behavior of the MeHA coated on the surface of MNs. Summary The MeHA coated MN sensor was fabricated with simple molding techniques and various photocurable materials such as SU-8 2010 and MeHA. The ferricyanide could be extracted by using the swelling behavior of the MeHA. As the results of in vitro test, the various concentration of potassium ferricyanide solution was measured with the three-electrode system including MeHA coated MN sensor and the detection limit was observed at least 1 μM. Reference 1. Sharma, Sanjiv, et al. "A pilot study in humans of microneedle sensor arrays for continuous glucose monitoring." (2018).2. Caliò, A., et al. "Polymeric microneedles based enzymatic electrodes for electrochemical biosensing of glucose and lactic acid." Sensors and Actuators B: Chemical 236 (2016): 343-349.3. Ribet, Federico, Göran Stemme, and Niclas Roxhed. "Real-time intradermal continuous glucose monitoring using a minimally invasive microneedle-based system." Biomedical microdevices 20.4 (2018): 101.