To pierce through the skin and interact with the first biofluid available, microneedles should be mechanically strong. However, some polymers used to fabricate microneedles yield insufficient strength for the fabrication of arrays (PDMS, highly porous structures, etc.). To enhance mechanical properties, piercing materials can be used. They aim to pierce the skin evenly and dissolve quickly, clearing the way for underlying microneedles to interact with the interstitial fluid (ISF). Three materials-carboxymethyl cellulose (CMC), alginate, and hyaluronic acid (HA)-are discussed in this article. Low concentrations, for a quick dissolution while keeping enhancing effect, are used ranging from 1-5%(w/w) in deionized water. Their overall aspects, such as geometrical parameters (tip width, height, and width), piercing capabilities, and dissolution time, are measured and discussed. For breaking the skin barrier, two key parameters-a sharp tip and overall mechanical strength-are highlighted. Each material fails the piercing test at a concentration of 1%(w/w). Concentrations of 3%(w/w) and of 5%(w/w) are giving strong arrays able to pierce the skin. For the purpose of this study, HA at a concentration of 3%(w/w) results in arrays composed of microneedles with a tip width of 48 ± 8 μm and pierced through the foil with a dissolution time of less than 2 min.

In the past two decades, microneedles (MNs), as a painless and simple drug delivery system, have received increasing attention for various biomedical applications such as transdermal drug delivery, interstitial fluid (ISF) extraction, and biosensing. Among the various types of MNs, porous MNs have been recently researched owing to their distinctive and unique characteristics, where porous structures inside MNs with continuous nano- or micro-sized pores can transport drugs or biofluids by capillary action. In addition, a wide range of materials, including non-polymers and polymers, were researched and used to form the porous structures of porous MNs. Adjustable porosity by different fabrication methods enables the achievement of sufficient mechanical strength by optimising fluid flows inside MNs. Moreover, biocompatible porous MNs integrated with biosensors can offer portable detection and rapid measurement of biomarkers in a minimally invasive manner. This review focuses on several aspects of current porous MN technology, including material selection, fabrication processes, biomedical applications, primarily covering transdermal drug delivery, ISF extraction, and biosensing, along with future prospects as well as challenges. Graphical abstract

Porous microneedles are expected to have a variety of potential for applications in diagnostics owing to their ability to penetrate human skin painlessly and extract bio‐fluid by capillary action. In this paper, a ‘Porous Microneedle on a Paper substrate’ (PMP) is proposed as a novel platform for direct integration of sensors. A microneedle array with height of approximately 840 μm was prepared on a paper. The fabrication process consists of salt leaching and heat press moulding. In this method, the salt particles are utilized as porogen materials. Mixture of the biodegradable polymer and the salt was shaped into microneedles by moulding. Furthermore, the polymer penetrated the paper matrix owing to heating during the pressing process. Subsequently, the salt particles are removed to develop the porous structure. Various ratios of salt to polymer were investigated to adjust the porosity of microneedles as well as their sample absorption property. A paper‐based glucose sensor was integrated into the platform to demonstrate the absorption property of the microneedles, and showed successful sample extraction and glucose concentration analysis from agarose gel‐based skin mimic. The developed platform has the potential to integrate various paper‐based bio‐sensors to painless, disposal and fast screening and diagnostic test for patient, as well as those with prediabetes.