Solid microneedles:

Solid microneedles are mainly used for skin pretreatment by pore formation. The pointed tips of the needles penetrate the skin; creating small channels, through which the drug enters directly into the skin layers when applying a drug patch, thereby increasing transdermal permeation

Coated microneedles:

They are solid microneedles whose surface is covered by a matrix that contains the drug in solution or suspension. When introduced into the skin, the drug molecules are transferred to the surrounding skin in a similar way to what happens with a classic transdermal but without the resistance offered by the stratum corneum.

Dissolving Microneedles:

Dissolution microneedles are made from biodegradable polymers by encapsulating the drug in the polymer. After inserting the microneedle into the skin, the dissolution that releases the drug is produced.

The microneedle should not be withdrawn after insertion as in other cases the polymer degrades within the skin.

The bioacceptability and dissolution of the polymer within the skin make it one of the best options for long-term therapy with better patient compliance.

Hollow microneedles:

Hollow microneedles have an empty space or reservoir inside with a hole that connects it to the external environment. A solution of the active ingredient is placed in the reservoir

When inserted into the skin, the drug is deposited directly in the epidermis or the upper layer of the dermis. It is mainly used for high molecular weight compounds such as proteins, vaccines and oligonucleotides. These microneedles are capable of delivering large doses of drug since more drug can be accommodated in the empty space inside the needle.

Hydrogel-forming microneedles:

This type of microneedle has been developed recently. Superswellable polymers are used to make microneedles. Polymers constitute the hydrophilic structure that makes them capable of absorbing a large amount of water in their three-dimensional polymeric network. These polymers swell when inserted into the skin due to the presence of interstitial fluid. This leads to the formation of channels between the capillary circulation and the drug patch. Before needling, these microneedles are only used to break the skin barrier. As they swell, they behave like a membrane that controls speed.

Based on the foregoing, it can be affirmed that microneedles are very appropriate systems for the painless release of drugs and with high bioavailability. However, despite the relatively long time that has elapsed since the appearance of the first invention patent in this field, there are no drugs available with this administration system. This apparent phenomenon is attributed by some authors to the technological difficulties for production. If they are available and authorized by offices such as the FDA for their direct use, instruments for non-medical treatments but cosmetics, the so-called microneedling that produce, with the use of microneedles, small perforations in the skin causing a positive cosmetic reaction.

In recent years, the interest in these systems has grown markedly judging by the profusion of publications on the subject and the initiation of clinical work studying the efficacy for the administration of relatively high molecular weight active ingredients such as insulin or for immunotherapy.