Subcutaneous injection is one of the common drug administration methods in clinics. However, it can be painful and invasive, can produce sharp and biodangerous waste, and needs to be performed [1,2] by trained healthcare personnel [3]. In recent years, the strategy of transdermal drug delivery has become an important therapeutic option to complement oral, subcutaneous, and intravenous injections [4,5]. Transdermal administration avoids first-pass liver metabolism compared to subcutaneous administration; it is painless compared to injection; and it is a needle-free device that avoids the risk of disease transmission associated with the re-use of needles, thus reducing the unsafe factors commonly caused by medical waste, especially in developing countries [6,7]. However, the stratum corneum (SC), a layer of skin formed by dead corner cells and located in the outermost layer of the skin, with a thickness of 10–15 μm, is the main barrier to transdermal drug delivery, and can severely reduce the efficiency of drug delivery and limit the types of drugs that can be delivered transdermally [8,9,10].

In 1976, a new technology called microneedles was first introduced to overcome the limitations of traditional transdermal drug delivery [11]. MNs consist of micron-scale needle arrays with heights of approximately 50–900 μm. MNs can be manufactured with microfabrication techniques using a variety of materials and geometrics, because they disrupt the SC and epidermal layer and form microscale drug delivery channels without touching the nerve fibers or blood vessels located in the dermis of the epidermis and epidermal layer. Microneedle technology can significantly improve the efficiency of drug delivery [12,13]. It is also possible to increase the types of drugs transported in a painless and minimally invasive manner [14]. As a result, the device is easy to use and painless compared to traditional invasive injection and/or oral strategies with functional advantages [15,16]. MNs can not only help drug molecules bypass first-pass metabolism and gastrointestinal degradation, but also broaden the application range of their drug types, regardless of their molecular weight or hydrophilicity [17,18]. There are currently no confirmed reports that MNs cause or increase the chance of skin infections [19], nor that they affect normal skin function [20]. Numerous preclinical studies and a limited number of clinical trials have now shown that MNs can be used to deliver DNA, vaccines, insulin, and human growth hormone. In addition, MNs have been extensively studied for blood sampling, signal monitoring, and biosensors. This means that MNs have a broad market for transdermal drug delivery, vaccine preparation and biologics. Large amounts of MN have entered clinical trials for the treatment of various diseases, showing its universal effectiveness [12,21,22].

Over the past few decades, dissolving microneedles have been used in various fields, including anti-cancer, dermatology treatment, vaccine delivery, insulin delivery, and biomarker detection. Furthermore, in addition to single drug delivery, the design of microneedles allows for customizable special structures and intelligent response systems, such as controlled drug release; multi-therapy; and targeted delivery to specific sites, such as the heart [36], blood vessels [38], brain [39], etc.

As one of the main diseases threatening human health, the treatment of cancer has been the focus of attention all over the world. In addition to commonly used chemotherapy drugs, with the development of nanotechnology, more and more treatment methods are constantly improving the survival rates of cancer patients and reducing the recurrence of cancer. In addition, the on-demand delivery and responsive release of photothermal and photodynamic, gene, and immunotherapy drugs can also be achieved through microneedles. In the past decade, MN has been widely used for the transdermal delivery of anticancer drugs, showing improved drug utilization, reduced toxic side effects, enhanced tumor targeting ability, and low off-target toxicity [40]. Initially, microneedles were used as delivery tools for some first-line anticancer drugs to improve drug availability and target cancer cells [41,42]. Photothermal therapy is also often combined with microneedles for the treatment of cancer [44].

In addition to photothermal therapy, in recent years, combining the microneedle system with photodynamic therapy to allow for greater uniformity and depth in photosensitizer delivery to tumor sites, thus providing better treatment outcomes and an easier pathway, offers a promising strategy for the clinical application of PDT [47,48]. Tham et al. [49] developed a mesoporous nanocarrier with dual loading of photosensitizers and clinically relevant drugs for combination therapy while utilizing microneedle technology to promote its penetration into deep skin tissues. Skin fluorescence imaging shows that microneedles can encourage the nanocarrier to penetrate the skin epidermis and reach deep melanoma sites. The combination of PDT and targeted therapy with nanocarriers has been proven to have a superior therapeutic effect in the xenotransplantation of melanoma mice.

The use of MN broadens the delivery mode of cancer therapy, with various hydrophilic and hydrophobic drugs being able to penetrate the skin and reach the lesion area through microneedles, greatly improving drug utilization. In addition, MNs improve drug stability, making synergies of multiple therapies possible and reducing the need for trained operators, thus making them promising tools.

3. Dissolving Microneedles in Wound Healing