Fabrication of porous polymeric microneedles: A concise overview

Abstract

Recent advancements in microneedle (MN) technology have increasingly focused on porous polymeric microneedles (PPMNs), which are, among various types of MN, emerging as a promising platform for diverse biomedical applications, including transdermal drug delivery, interstitial fluid (ISF) extraction, and biosensing. This growing interest stems from their distinctive internal architecture, characterized by continuous nano- or micro-scale pores that enable the efficient transport of drugs and biofluids, primarily through capillary action. The optimal selection of polymeric materials, combined with appropriate fabrication techniques, plays a critical role in enhancing the functional performance of PPMNs while ensuring sufficient mechanical strength. This concise review summarizes recent research progress in the fabrication methods of PPMNs, emphasizing the interplay between polymer(s) choice, manufacturing technique, intended biomedical application, and the resulting structural and functional properties of the microneedles. It also addresses key challenges in the fabrication field and discusses future development.

Introduction

By definition, the microneedle (MN) system consists of integrally formed micron-scale needles on a patch substrate.1–3 These tiny needles, which are 25 to 2000 μm long,4,5 can go through the outerlayer of skin to effectively transport a plethora of diverse bioactive materials while avoiding skin injury.1,5,6 This system neatly integrates the benefits of skin injection with the safety of a transdermal patch.1 Because of their promising clinical results, tissue tolerability, patient acceptance, and capacity to self-administer, MN systems offer easy-to-use tool for controlled transdermal drug release in a variety of settings.1

MNs are usually fabricated from metal, silicon, glass, ceramic,7,8 or polymer (e.g., carbohydrates and hydrogels).9 These materials have been used to fabricate microneedles for diverse purposes, based on their mechanical and degrading qualities.9 Silicon, ceramics, and metals have stiffness values over 10 GPa and are nondegradable under typical circumstances.9 Metal MNs are cost-effective to produce and exhibit superior mechanical and physical properties; yet, they are non-degradable and not flexible.10 There are currently six main categories of MNs: solid, coated, dissolving, hollow, hydrogel,11–15 and porous.11,13,16 Porous microneedles (PMNs) are channel-based devices1 made up of arrays with a network of linked channels or pores capable of delivering medications17–19 or capturing biological fluids via the epidermis or other tissues. Furthermore, PMNs facilitate therapeutic monitoring11 or biosensing applications by periodically and selectively capturing (when functionalized) and detecting biological molecules11,17via capillary action11,18,19 (Fig. 1). They are also attracting interest for their capacity to encapsulate larger volumes of fluid and for providing superior isotropic fluid directionality compared to hollow MNs, enabling both fluid injection and extraction at significantly greater volumes.20 PMNs are usually fabricated using inorganic substances, biocompatible metals, or polymers.11,19,21

Fig. 1 (A) Microneedle structures and the delivery mechanisms, and microneedle structures and the sampling mechanisms. (B) Microneedle dimensions. Reproduced from26. (C) Cargos including the small molecular drug, peptide, protein, nucleic acid, nanoparticle, microparticle, virus, exosome, and cell could be loaded in microneedle for delivery. The cargo in microneedle maybe linear increase,1 first increase and then decrease,2 not affect,3 or linear decrease4 the mechanical strength of microneedle in response to the increase of cargo concentration. Reproduced with permission from15 [Copyright© 2023, Elsevier]. — **Fig. 1** (A) Microneedle structures and the delivery mechanisms, and microneedle structures and the sampling mechanisms. (B) Microneedle dimensions. Reproduced from26. (C) Cargos including the small molecular drug, peptide, protein, nucleic acid, nanoparticle, microparticle, virus, exosome, and cell could be loaded in microneedle for delivery. The cargo in microneedle maybe linear increase,1 first increase and then decrease,2 not affect,3 or linear decrease4 the mechanical strength of microneedle in response to the increase of cargo concentration. Reproduced with permission from15 [Copyright© 2023, Elsevier].

Over the past twenty years, great progress has been made in developing MN-based drug delivery systems.3 Among the various types of MNs, polymeric MNs have garnered significant interest in drug delivery research due to limitations associated with other materials—such as high cost of raw materials, complex fabrication techniques, fragility, limited biocompatibility, low drug-loading capacity, and the risk of MN facture in the skin upon insertion.22 Polymeric porous microneedles (PPMNs), in particular, have drawn substantial attention for their unique features, as they are fabricated using biocompatible and biodegradable polymers, which can be customized to control the release profile of the encapsulated active compounds.23Fig. 2 depicts the research trends for each type of PPMN over the last two decades (2001–2024) based on data from the Web of Science (∼109 articles), excluding review articles.

The use of PPMNs for transdermal drug delivery (TDD) offers several advantages. First, the minimally invasive nature of PPMNs can considerably reduce pain and discomfort experienced by patients during application, making them more patient-friendly. Additionally, the ability to engineer the MN geometry and porosity allows for tailored drug release profiles, enabling both burst and sustained release, depending on the specific therapeutic needs.24,25 Furthermore, PPMNs exhibit greater flexibility in applications compared to other MN types due to their distinctive drug-loading method, which enables the separation of the MN preparation process from the drug loading process, therefore minimizing drug loss and inactivation during preparation, especially for large-molecule drugs like vaccines, and improving mass production of MNs. Moreover, PPMNs offer excellent detection capabilities because of their large specific surface area and their flexibility to be integrated with other detection platforms, making detection easier and faster. This type is an adaptable and useful choice when designing and manufacturing wearable products and point-of-care (POC) testing devices.11

In this review paper, we considered recent studies involving the fabrication of PPMNs. These strategies were discussed in detail, together with examples, challenges, limitations, and factors to be considered.

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Eyman M. Eltayib, Fabrication of porous polymeric microneedles: a concise overview, Open Access Article. Published on 04 June 2025. Downloaded on 6/10/2025 9:56:13 AM, RSC Adv.,2025,15, 18697, Received 9th May 2025, Accepted 26th May 2025, DOI: 10.1039/d5ra03274a, rsc.li/rsc-advances