Solid Microneedle Technology

The solid microneedle itself contains no drug. After insertion and removal of the microneedle, micron-sized pores are formed on the skin surface, and the drug formulation is applied to the skin surface above the pores to allow slow diffusion of the drug through the pores, both locally in the skin and systemically when absorbed by capillaries.

Fig. 1 Silicon Microneedles array. (Chandbadshah et al., 2022)

Advantages of Solid Microneedle

Solid microneedles are designed to penetrate the stratum corneum to enhance drug delivery to the dermis, thereby improving bioavailability and kinetic transmission through the skin. Solid microneedles have a longer duration and more robust antibody response than intramuscular injections, making them suitable for vaccine delivery. Solid microneedles are easy to manufacture, have superior mechanical properties and sharper tips.

Solid Microneedle Preparation

Microneedles made from polymers are not only mechanically strong enough to penetrate the human skin stratum corneum, but also have superior biocompatibility.

The microneedle production process consists of three main parts: the production of the original microneedle mold, the production of the microneedle mold, and the casting process to replicate the microneedle.

1. Production of the original microneedle mold

The production of the original microneedle mold can be divided into two processes: the first is the anisotropic etching to obtain the positive quadrilateral cone groove, that is, the needle tip mold; the second is the alignment lithography SU8 glue to remove the SU8 glue corresponding to the needle tip groove to obtain the needle body mold part.

2. Transferring the mold to make the polymer mold

Firstly, the prepolymer and curing agent are mixed in a certain ratio by mass, and then fully stirred to remove the air bubbles; then a certain amount of prepolymer is poured into the original mold. Next, the prepolymer is cured on the hot plate, and then the cured prepolymer is removed from the mold to obtain the prepolymer micro-needles. Finally, the equipped prepolymer is poured onto the polymer microneedle sputtered with metal film, cured and poured, and then removed from the polymer microneedle to obtain the polymer microneedle counter-mold, which is the final polymer mold for replicating the microneedle.

3. Replicating polymer microneedles

The resulting final polymer mold can be used for the casting process, which can be used to make polymer microneedles in bulk.

First, the prepolymer is mixed with the curing agent in a certain mass ratio, stirred thoroughly and then evacuated to remove any air bubbles. Secondly, a certain amount of potting adhesive is poured into the final polymer mold. Finally, the mold with the potting adhesive poured is vacuumed until no air bubbles are generated, and then the potting adhesive is cured on a hot plate, and then the cured potting adhesive is removed from the mold to obtain the final microneedle product.

Solid Microneedle Drug Delivery

Solid microneedle drug delivery can be divided into two steps: the first step is to make the microneedle pierce the skin surface to form microporous channels; the second step is to remove the solid microneedle and apply the drug to the microneedle piercing site, and the drug passively penetrates into the skin through the porous channels.

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