Effect of microneedles shape on skin penetration and minimally invasive continuous glucose monitoring in vivo

The effect of microneedle shape on skin penetration and minimally invasive continuous glucose monitoring (CGM) was successfully investigated. Straight silicon microneedle arrays (SSMA) and tapered silicon microneedle arrays (TSMA) were fabricated using standard silicon micromachining techniques. Microneedle heights were ∼325 μm and ∼350 μm and pitch was 400 μm and 450 μm, respectively. Mode-I planar crack and mode-II ring crack propagation were assumed for TSMA and SSMA penetration mechanisms, respectively. Skin penetration was first qualitatively evaluated via methylene blue staining. TSMA and SSMA were also mounted on CGM system prototypes and applied on human subjects with an impact velocity of 10 m/s and worn for 6.5 h. The hollow microneedles allowed the glucose from the interstitial fluid in the epidermis to diffuse into the sensor chamber of the CGM system prototype. Sensor currents from the applied CGM system prototypes ranged from 100 nA to 400 nA and they appeared to follow the trend of the reference blood glucose sensor that ranged from ∼90 to 220 mg/dL. The CGM system prototype with SSMA yielded lower sensor current than that with TSMA. This may be due to the SSMA preferential penetration of skin via mode-II ring crack propagation, resulting in possible lumen occlusion from displaced skin material.