**7. Types of microneedles**

#### **7.1 Solid microneedle**

Solid microneedles are arranged as an array in a two-part system; microscopic wells are created on the skin using this microneedle just deep enough to penetrate the outermost layer of the skin, and then a transdermal patch is used to apply the drug. Collagen induction therapy is a method in dermatology, which uses solid microneedles. This method involves the repeated puncturing of the skin with microneedles, wherein the expression and deposition of elastin and collagen proteins are induced [32].

#### **7.2 Hollow microneedle**

These are similar to solid microneedles with respect to the material. They act like a reservoir containing the drug to be delivered to the site directly. The flow rate of the microneedle influences the drug delivery; therefore, a flawed design or excessive swelling may cause clogging of the array. Hollow microneedles have a higher probability of collapsing under pressure, thus failing to deliver drugs [32].

#### **7.3 Coated microneedle**

Coated microneedles are designed using metals or polymers, similar to that of solid microneedles. Here the drug is directly applied to the microneedle, unlike in other cases where patches or applicators are used. To ensure the proper delivery of the drug, thickening agents or surfactants are used to cover coated microneedles. The chemicals that are used on coated microneedles are known as irritants. There is sometimes the risk of local inflammation in the area where the array was used. In such cases, the array can be removed immediately without harming the patient [32].

#### **7.4 Dissolvable microneedle**

In the case of dissolvable microneedles, they encapsulate the drug using a nontoxic polymer, which is dissolved completely when it enters the skin. Fibroin is a polymer that is derived from silk protein. This fibroin can be molded into structures of microneedles and also dissolves once into the body; therefore, in the recent past, researchers and pharmaceutical companies have started to study its mechanisms and potential [32].

#### **7.5 Hydrogel-forming microneedles**

This type of microneedles has no drug in itself. They follow the technique of swelling in the skin to allow the diffusion of the drug inside the reservoir layer attached to the microneedle for dermal microcirculation for systemic absorption [32].

The various types of microneedles explained above are shown in **Figure 19**.

*Minimally Invasive Microneedle: A Novel Approach for Drug Delivery System and Infected… DOI: http://dx.doi.org/10.5772/intechopen.105771*

#### **Figure 19.**

*A schematic representation of five different types of microneedles: solid removable, coated, dissolving, hollow, and hydrogel-forming microneedles [32].*
