Drawbacks of Microneedles
Microneedles have been widely employed for safe and efficient drug and vaccine delivery through microchannels into the skin. However, many studies done related to the safety of microneedles show that the microchannels created by microneedles may infect the injection site by allowing the permeation of pathogens or other toxic materials. Also drug delivery into the skin by using microneedles may not be completely reliable and efficient due to some small deformation of the skin or the presence of a hair in the skin, which can affect microneedle insertion because of its micron-size length. Microneedle applicators increase the reliability of insertion but introduce their own limitations in terms of cost, size, and complexity .
Gupta et al. and Gill et al. conducted separate studies targeting the pain associated with the microneedle dimension and quantity. The results showed direct relationship between length or number of microneedles and pain [135, 184]. However, Bal et al. and Kaushik et al. established the safety of microneedles and reported it as a painless, non-invasive drug delivery approach [185, 186]. Although microneedles are safe to use, a few challenges still remain in their development and commercialization for successful transdermal drug delivery. With frequent application of the metallic microneedles, traces of metal may remain beneath the skin, leading to irritation, erythema, swelling, discoloration, etc.
A small amount of drugs can be loaded into microneedles for delivery. However, transfer of large and hydrophilic compounds is still a major challenge. To find a solution for this problem, the design of the system needs to be improved using the proper material with adequate mechanical force to pierce the skin.
After use, microneedle patches require safe disposal since they may have residual drug and account for sharp waste, making them biohazardous. Unless drug delivery is 100% efficient, the use of microneedles will result in residual drug retention in the patch. In some cases, this may not be of significant concern. However, in other cases, this may impose some danger of residual drug exposure to others, including children, animals, and the environment. Used microneedle patches are considered as biohazardous waste due to their contact with body fluids, even though in small amounts (i.e., interstitial fluid of the skin]. The used microneedle patches may be considered as sharp waste. Dissolving microneedles are likely not to be considered as sharp waste as no remnant microneedle part is present after usage. Again nondissolving (e.g., coated] microneedles fall in the category of sharp waste, although the hazard they pose is different compared to hypodermic needles or scalpel blades and they may therefore be handled differently. In all of these cases, safe disposal may need to be facilitated through suitable waste streams (e.g., placement in sharps containers or biohazard bags) or through suitable packaging after use (e.g., microneedles contained in a protective housing after use).
The future of drug delivery is significantly dependent on microfabrication technologies, which will help to realize the development of new and improved devices that will be smaller, cheaper, pain- free, and more convenient, with a wide range of biomedical and other applications. The drug delivery devices made by micro-/ nanofabricated technology can enable efficient drug delivery, not attainable with conventional drug delivery techniques, thus enhancing the therapeutic activity of a drug.
However, the ultimate measure of the impact of microneedles is their translation into clinical medicine. This translation requires significant funding from cautious pharmaceutical industries for product development and advanced clinical trials, along with advancement in science and technology. For microneedle patch development research, the greatest challenge is its translation into novel medical products that will improve patient compliance. For research and development, two important outstanding concerns are as follows: (i) the true benefits of microneedles to patients and (ii) the development of low-cost, scalable, and reliable microneedles. Application at different sites eveiy time or variation in skin thickness in individuals may result in variation in bioavailability, which needs to be considered while developing microneedles . Animal studies show that vaccines are generally more immunogenic and can be administered at lower doses. From studies it is proved that administration of drugs using microneedles helps in faster drug uptake by the bloodstream for prolonged duration without any skin irritation. Research is going on for further developments in microneedles that can be potentially valuable in future vaccine delivery. With the help of effective drug delivery, patient compliance, cost saving, and ease of storage, microneedle drug delivery systems will be a milestone in the development and dissemination of vaccines and will potentially contribute to human public health.