A critical review of advances and challenges in microinjection moulding of polymeric microneedles

Microneedles are an innovative alternative to hypodermic needles for delivering vaccines and therapeutic agents and sampling biofluids for point-of-care diagnostics. Two key advantages of microneedle patches over hypodermic needles are their painless application and ease of use without requiring skilled personnel. Polymers stand out as an ideal material for the mass production of microneedles, offering a combination of low cost, good mechanical properties, biocompatibility, biodegradability, and high chemical stability. Among different polymer fabrication techniques, microinjection moulding is a well-established industrial process with high potential to address the challenge of mass-producing microneedle devices cost-effectively and efficiently. Despite the significant potential of the technique, research in this area is limited. This article comprehensively reviews the recent advancement in the fabrication of polymeric microneedles via microinjection moulding. It covers injection moulding steps, mould fabrication, and quality assessment of the final product, with a focus on the challenges associated with the mass manufacturing of microneedles. The article investigates the laser micromachining process and its challenges in fabricating negative metal moulds for microneedles, elaborating on the critical role of mould manufacturing in precisely shaping mould-injected microneedles. The physical and rheological properties of thermoplastic polymers that affect the quality of mould-injected microneedles are also reviewed. Finally, opportunities and future directions for advancing research in this field are discussed.