Introduction

Microneedle-based drug delivery has emerged as a transformative transdermal technology​ that uniquely combines the simplicity of a patch​ with the efficacy of subcutaneous injection. By creating microscopic channels across the stratum corneum, microneedles (MNs) enable efficient penetration of therapeutic agents into the skin while maintaining minimal invasiveness, ease of use, and excellent biocompatibility. These advantages have attracted enormous attention from the global research community.

This article focuses on the classification and functional principles of microneedles, drawing primarily from the work of Xiaowen Liu and Fei Xiao (Jinan University)​ and Bin Du (Tongji University), titled "Microneedle-mediated drug delivery for cutaneous diseases,"published in Frontiers in Bioengineering and Biotechnology.

Figure 1.​ Classification of microneedles and their respective transdermal delivery mechanisms.

1. Solid Microneedles (SMNs)

Overview

Solid microneedles are typically used for skin pre-treatment​ prior to the application of active pharmaceutical ingredients. They operate on the classic poke-and-patchprinciple: creating transient microchannels that enhance the permeability of subsequently applied topical formulations.

Materials

Common fabrication materials include:

Mechanism & Advantages

Create instantaneous, uniform microchannels​ in the stratum corneum

Significantly improve local drug penetration efficiency

High mechanical stiffness ensures reliable skin insertion

Relatively simple and low-cost to manufacture

Limitations

Channels close within hours due to natural skin healing, limiting sustained release

Risk of needle fracture and residual fragments​ in the skin

No intrinsic drug-carrying capacity-requires a separate drug reservoir

Figure 2.​ Schematic and representative SEM image of solid microneedles.

2. Hollow Microneedles (HMNs)

Overview

Unlike solid structures, hollow microneedles feature an internal lumen or bore​ (diameter: ~50–70 μm) that runs through the needle shaft-essentially functioning as microscale hypodermic syringes.

Materials

Silicon (via MEMS fabrication)

Glass (via draw lithography)

Polymers (via micro-molding or 3D printing)

Mechanism & Advantages

The internal cavity can be pre-loaded with therapeutic agents​ and delivers them via pressure-driven flow​ upon insertion

Capable of carrying larger fluid volumes​ than any other MN type

Enables precise, controllable dosing-ideal for macromolecules, vaccines, and even interstitial fluid extraction​ for diagnostics

Suitable for a wide range of active molecules

Limitations

Fabrication is significantly more complex​ (requires digital micro-electro-mechanical systems)

Dermal tissue tightness may cause lumen clogging​ during insertion

Higher manufacturing cost and longer production time

Figure 3.​ Schematic and working principle of hollow microneedles.

3. Dissolving Microneedles (DMNs)

Overview

Dissolving microneedles are fabricated entirely from water-soluble, biodegradable materials​ embedded with therapeutic agents. Upon insertion, the needle matrix dissolves or degrades in situ, releasing the payload directly into the skin-no removal or disposal step required.

Materials

• Sugars: Maltose, trehalose
• Natural polymers: Hyaluronic acid (HA), dextran, gelatin
• Synthetic biodegradable polymers: PLGA, PVA, PVP

• Mechanism & Advantages

• Excellent biocompatibility​ and safety profile
• No sharp medical waste-entire device dissolves, eliminating biohazard disposal

Significantly improves patient compliance; suitable for self-administration

Provides a new paradigm for long-term, chronic disease management

Extensively researched for diverse conditions: cancer, diabetes, psoriasis, vaccine delivery, and more

Limitations

Mechanical strength must be carefully balanced with dissolution rate

Drug stability during storage and the fabrication process requires optimization

Drug loading capacity is finite and depends on matrix composition

Figure 4.​ Dissolving microneedle structure and in-skin degradation process.

4. Coated Microneedles (CMNs)

Overview

Coated microneedles are solid MNs with a therapeutic coating​ applied to the needle surface. The drug is released as the coating dissolves or detaches upon skin contact.

Coating Methods

Mechanism & Advantages

Rapid drug release​ immediately upon insertion

Precise dose control-the amount of drug delivered is determined by coating thickness and surface area

Simple to operate; well-suited for high-potency, low-dose drugs​ (e.g., vaccines, peptides)

Cost-effective and scalable

Limitations

Drug loading is limited by the surface area​ of the microneedle

Coating may detach during insertion​ or degrade during storage

Ensuring uniform coating thickness and long-term stability remains a formulation challenge

Figure 5.​ Coated microneedle fabrication via dip-coating and spray-coating methods.

Summary Comparison

Concluding Remarks

Each MN architecture offers a distinct set of trade-offs​ in terms of drug capacity, release kinetics, manufacturing complexity, and clinical suitability. Understanding these differences is essential for matching the right MN platform to the right therapeutic application-particularly in the rapidly growing field of microneedle-mediated drug delivery for cutaneous diseases.

As materials science and microfabrication continue to advance, we can expect even more sophisticated hybrid designs that combine the best features of all four types-bringing us closer to truly personalized, painless, and precision transdermal therapy.