Microneedles (MNs) are microscale needle structures designed for transdermal drug delivery, interstitial fluid sampling, signal detection, biosensing, and other biomedical applications. Benefiting from their painless, minimally invasive, and user-friendly​ nature, MNs have attracted extensive research attention. A typical MN ranges from 25 to 2000 μm in length, with a tip size of 10–20 μm-significantly sharper than conventional hypodermic needles. This geometry enables MNs to easily penetrate the stratum corneum​ (the skin's outermost barrier) and create microchannels, while deliberately avoiding contact with nerve fibers and blood vessels located in the epidermis and dermis.

The concept of MN fabrication was first proposed in 1976​ to overcome the limitations of traditional transdermal drug delivery. Since the 1990s, driven by advances in modern microfabrication technologies, a wide variety of MNs have been successfully developed and widely applied in the biomedical field.

Following our previous discussion on microneedle patches and the latest advances in transdermal delivery, Engineering for Life (EFL) presents this comprehensive classification of MNs based on their structural and functional characteristics, along with representative fabrication methods and case studies.

Five Major Types of Microneedles

(Reference: Advanced Drug Delivery Reviews, 2019 - https://doi.org/10.1016/j.addr.2019.10.004)

1. Solid Microneedles (SMNs)

Solid microneedles are fabricated on a planar substrate, typically arranged in arrays ranging from 5×5 to 20×20, resembling a spiked plate.

📌 Case Study: Fabrication and characterization of gold-coated solid silicon microneedles with improved biocompatibility- https://doi.org/10.1007/s00170-018-2596-3

2. Hollow Microneedles (HMNs)

Hollow microneedles are essentially conventional hypodermic needles scaled down to micrometer dimensions. They can be configured as a single needle or an array mounted on a base, which may be connected to an external device providing a driving force to inject liquid or drugs through the internal lumen.

📌 Case Study: https://doi.org/10.1063/5.0008983

3. Coated Microneedles (CMNs)

Coated microneedles are fabricated using biodegradable and biocompatible materials, capable of rapidly delivering high-molecular-weight drugs into the skin. Drugs are stored in a solid phase as a coating on the needle surface, which enhances long-term stability.

📌 Case Study: A novel method for fabrication of coated microneedles with homogeneous and controllable drug dosage for transdermal drug delivery- https://doi.org/10.1007/s13346-022-01123-8

4. Dissolving Microneedles (DMNs)

Dissolving microneedles are fabricated from biocompatible, water-soluble matrix materials, eliminating concerns about needle fracture residue and biocompatibility. Upon insertion, the needle tips dissolve in the interstitial fluid, naturally releasing the encapsulated drug.

📌 Case Study: https://doi.org/10.1016/j.ijpharm.2021.120749

5. Hydrogel-Forming Microneedles (HFMNs)

Hydrogel-forming microneedles are prepared from hydrogel-forming polymeric matrices. Upon insertion, these arrays rapidly absorb interstitial fluid, causing the hydrogel to swell and form continuous, unobstructed conduits​ for sustained drug permeation.

Summary Comparison Table

Note: This classification and technical summary is compiled by Engineering for Life (EFL). For further resources or collaboration opportunities, please contact EFL directly.