Coating Formulations Suitable for Biodegradable Microneedles with Immediate Release

Introduction: The administration of hypodermic needles can be associated with issues like physical and psychological pain. A novel method to administrate compounds such as vaccines and other drugs is therefore warranted. Microneedles (MNs) that consist of multiple needles in the microscopic region causes little to no pain and have successfully delivered drugs both locally and systemically. The MNs are often divided into subgroups like biodegradable MNs that can either be coated for immediate release or have the drug substance incorporated in the material resulting in an extended release. The biodegradable MNs in this study were made from calcium sulfate, commonly known as gypsum, which have been used previously in for example craniofacial surgeries. Objective: The purpose of the study was to find coating formulations that displayed good coating uniformity without compromising the needle sharpness. The formulation should be evenly distributed on top of the MN array without diffusing into the material. It should also be miscible with sodium salicylate and present an immediate release according to release studies. Method: MN arrays were constructed of calcium sulfate with two different sieve fractions (≤125 μm and ≤63μm) to determine the potential difference. After determining the optimal sieve fraction the MNs were coated with different combinations of PEG 3350, sodium alginate, Kollicoat® IR and distilled water which acted as a reference. Evaluation of the formulations concerned coating uniformity and the extent of diffusion through the material. Release studies were then performed on MN arrays coated with sodium salicylate combined with the above-mentioned excipients. A novel release method was used over a 24 h period where samples were taken at specified time intervals. The results were statistically analyzed using standard deviations, confidence intervals and analysis of variance. Results and discussion: The MNs displayed variation concerning the overall appearance and this could affect the coating properties and release profiles since they differ in e.g. porosity. The different sieve fractions did not demonstrate a significant difference in appearance and the sieve fraction of ≤125 μm was therefore chosen for further studies due to material assets. PEG 3350 demonstrated a uniform coating in addition to a modest absorption into the calcium sulfate. It was therefore in contrast to the other excipients that presented various issues. The coatings that contained Kollicoat® IR showed the fastest release although the results were often not statistical significant which could be attributed to the irregular diffusion of the formulation into the MN array. Other factors that contributed to the variability of the results were the release methods variability and the human factor. Conclusion: The biodegradable MNs showed variation between manufactured batches concerning the overall appearance which could affect the results. PEG 3350 demonstrated the most favorable coating properties and Kollicoat® IR presented the fastest release. Further studies are needed to continue to develop the manufacturing process of the MNs, the drug release method and to address the issue with the diffusion of the formulations into calcium sulfate.