Bioinspired microneedle patches: Biomimetic designs, fabrication, and biomedical applications

Introduction
Microneedles (MNs) are microscale needles that are utilized in the biomedical sector for applications such as transdermal drug delivery and tissue engineering.1, 2, 3 They are used to achieve optimal tissue contact in a minimally invasive, easy-to-use, and painless manner via penetration into the skin without touching blood capillaries and nerve endings.4, 5, 6
To achieve optimal MN performance, researchers have developed a novel class of MN patches that are known as biomimetic or bioinspired MNs (BMNs). These BMNS have diverse applications.7, 8, 9 Over the ages, nature has created optimized solutions for many problems through the process of evolution.10 The fascinating properties observed in various natural species offer excellent opportunities to develop BMNs with functionalities that approximate their natural counterparts. These biomimetic simulations are achieved via manipulation of morphology, structure, and chemistry.7,8
Representative examples are used in the present review to highlight how properties originating from living organisms (e.g., teeth of limpets,11 stingers of honeybees,12 mosquito/endoparasite Pomphorhynchus laevis's proboscis,13,14 clawed toes of eagles,8 snake fangs7) may be exploited with mimicking tactics to design MNs with specific functionalities to overcome challenges that are faced by conventional MNs. Bioinspired designs as well as their structures and medical significance are highlighted. The techniques for fabricating BMNs, ranging from traditional to sophisticated approaches, are introduced. The applications of BMNs, including drug delivery, tissue adhesion, and regenerative medicine, as well as interstitial fluid extraction are discussed to provide a backdrop for future research in this fascinating field of development.