Microneedles for Easier Fish Skin Penetration and Longer Attachment

Abstract

Aquaculture is the farming of aquatic animals for commercial purposes. This growing industry supplies around 50% of the world’s seafood and has reduced overfishing. However, it has also facilitated the spread of diseases between fish by growing them in close quarters, which results in poor growth and higher mortality levels. Injection vaccination is the most common way to combat this issue, but it is labor-intensive and stress-intensive on the fish. As an alternative to this method, the Marelli lab proposed using impermeable silk microneedle patches to encapsulate the medication and deliver it through diffusion to rainbow trout fry. When a microneedle patch was tested on a 7 g fry, it had difficulty penetrating the skin and only stayed attached for 10 min after injection. Consequently, it caused significant stress to the fish upon insertion and fell short of the 4 hrs required for complete payload diffusion into the animal. This work aimed to reduce the force necessary for the needle to pierce fish skin and augment the force needed to dislodge it, allowing for easier piercing and longer animal attachment time. Thus, the study intended to decrease the patch’s insertion force and increase its retraction force. The initial needles were cone-shaped and had an angle of 21º. To assess the effects of needle tip angle and overall shape on the forces, the new needles’ tip angle varied between 15º, 20º, and 25º, and a cylindrical base was added to them and varied between 0%, 33%, and 66% of the total needle height. The insertion and retraction forces of microneedle patches were quantified and revealed that sharper needles and needles with cylindrical bases amounting to 66 % of the total needle height reduced the insertion force. In contrast, the retraction force was independent of both factors. The 25º 66%, 15º 33%, and 15º 0% needles displayed the lowest insertion forces and were tested on zebrafish to quantify how long they could stay attached. Preliminary tests on the live animals demonstrated that the new needles stayed attached to the fish for up to 8 hrs. This improved upon the initial Marelli lab design, which remained attached for 30 min at most. Overall, pursuing live fish testing would allow for selecting the best-performing design and further developing it as a viable alternative to current vaccination methods.