1. Background
2. Preparation of microneedles
3. Future Work and Integration
4. Conclusion
Reference
Allergen immunotherapy (AIT) is a cornerstone treatment for allergic diseases, aiming to induce long-term immune tolerance. However, conventional AIT, often administered via subcutaneous injections, faces significant challenges including lengthy treatment duration, potential side effects (e.g., risk of anaphylaxis), and patient non-compliance due to needle phobia and the inconvenience of frequent clinic visits(Chaaban, Mansi, Tripple, & Wise, 2019).
Soluble microneedle (MN) technology presents a revolutionary approach to transdermal drug delivery, effectively addressing these limitations. Soluble MNs are micron-scale needle arrays fabricated from biocompatible, water-soluble materials. They painlessly penetrate the skin's outermost barrier, the stratum corneum, and subsequently dissolve within the skin's interstitial fluid, releasing their payload(Al-Zahrani et al., 2012). This method offers numerous advantages:
- Enhanced Compliance: Nearly painless administration and potential for self-administration improve patient adherence.
- Improved Safety: Reduced risk of systemic side effects and needle-stick injuries; no sharp biological waste.
- Efficient Delivery: Facilitates direct intradermal delivery of biomolecules (e.g., allergens, vaccines), enhancing immunogenicity and bioavailability while protecting payloads from degradation.
- Thermostability: MN-based vaccines often exhibit improved stability, reducing cold chain reliance.
Figure 1 Comparison of Different Administration Methods (Dawn, 2021)
To establish a reliable and scalable process for fabricating soluble microneedles with high structural integrity, minimal defects, and rapid dissolution properties, using biocompatible materials:
Sodium hyaluronate (HA, 50,000 MW)- Polyvinyl Alcohol (PVA)
- Polydimethylsiloxane (PDMS) molds
- Deionized water
- Pigment
- Design & Build:
Based on initial protocols, the backing layer gel was prepared by dissolving PVA powder in water to a concentration of 250 mg/mL at room temperature with stirring. The needle layer was formulated using sodium hyaluronate (0.3g sequentially added to 1mL of aqueous solution).
- Test:
After drying at room temperature for 3-4 days, the PVA backing layer in the fabricated MNs demonstrated incomplete dissolution, remaining partially gel-like and lacking the desired structural integrity and mechanical strength for easy handling and application.
- Learn:
PVA's dissolution is highly dependent on its degree of hydrolysis (DH), polymerization degree (DP), and dissolution protocol. Higher DH and DP PVA grades require elevated temperatures for complete dissolution. Room temperature dissolution was insufficient for the chosen PVA grade.
- Design & Build:
The PVA dissolution protocol was modified to a heated water bath-assisted method. The PVA-water mixture was heated to 85-90°C with constant stirring (70-100 rpm) for 0.5 hours until a clear solution was obtained, ensuring complete polymer dissolution before casting. After dissolution, the degassed gels were pipetted into pre-warmed, cleaned PDMS molds. Centrifugation was applied after filling the mold cavities.
- Test:
The fabricated MN patches exhibited structural defects. We found that air bubbles trapped at the interface between the needle layer and the backing layer, as well as within the needle shafts themselves (Figure 2). These defects will compromise mechanical strength and payload uniformity.
Figure 2 Soluble microneedles prepared by Cycle 2.
- Learn:
The high viscosity of the polymer gels (especially HA) makes them prone to entrapping air during pipetting and handling. A single centrifugation step after completely filling the mold was ineffective at removing all bubbles, particularly from the intricate needle cavities.
- Design & Build:
The filling and centrifugation strategy was revised to incorporate degassing and incremental filling. Each gel component was centrifuged at 3000 rpm for 3 minutes separately before application to the mold to pre-remove bulk bubbles (Figure 3).
Figure 3 Preparation Process of Soluble Microneedles.
- HA solution before centrifugation, (B) HA solution after centrifugation, (C) HA solution added to the mold.
- Test:
The resulting MN showed complete needle structures, sharp tips, excellent morphological integrity, and no visible bubbles at the interface or within the needles (Figure 4).
Figure 4 Soluble microneedles prepared by Cycle 3.
To validate functionality, MNs were immersed in pre-warmed PBS (37°C). The needle tips demonstrated rapid dissolution, typically within 5 minutes, confirming their ability to release payloads effectively upon application (Figure 5).
Figure 5 Water solubility test of soluble microneedles.
- Learn
After three rounds of testing and refinement, we ultimately obtained the optimal preparation protocol for soluble microneedles.
Place the molds in a beaker filled with water for a short period of ultrasonic treatment; remove excess water (using absorbent paper), and then place them in a 60°C oven.
-
Weigh 0.3 g of hyaluronic acid sodium (50,000 MW), and add 1 mL of the drug solution each time (a total of 3
times). After each addition, use the tip of a 1 mL pipette to stir thoroughly until completely dissolved
(forming a needle-layer gel).
-
Weigh polyvinyl alcohol (PVA) and dissolve it in water to prepare a backing layer gel solution with a
concentration of 250 mg/mL. Stir continuously at 85~90°C and 70~100 rpm for 0.5 hours until a transparent
solution forms.
- Centrifuge the needle-layer gel and backing-layer gel at 3000 rpm for 3 min to remove gas.
-
Use the tip of a pipette to transfer the gel to the mold. Cover the mold cavity with an impermeable sealing
film, then cover it with another mold, and place the mixture in a centrifuge tube for centrifugation.
- Repeat the gel addition and centrifugation operation (at 3000 rpm, 3 min) at least twice.
- Scrape off the surface gel.
Add an appropriate amount of backing adhesive solution and fill the mold completely. Dry it at room temperature for 3~4 days.
The successful development of this robust MN fabrication platform enables advanced applications:
-
Therapeutic Integration and Biocompatibility Testing:
- The next critical step is integrating our previously engineered hypoallergenic derivatives into the needle layer. Subsequent studies will focus on in vitro and in vivo biocompatibility assessment using assays like CCK-8 to ensure cell viability.
- Effect evaluation of the hypoallergenic derivatives-loaded MNs in animal models, monitoring IgG blocking antibody production, T-cell response modulation, and reduction in IgE-mediated basophil activation compared to traditional subcutaneous administration.
-
Smart Closed-Loop System Development:
- We plan to advance this technology by integrating MN patches with miniaturized electronic sensors to create a closed-loop therapeutic monitoring system (García-Guzmán, Pérez-Ràfols, Cuartero, & Crespo, 2021).
- Sensor Integration: Explore embedding ultra-thin, flexible electrochemical or optical biosensors within the patch backing or adjacent to it for real-time, in situ monitoring of biomarkers (e.g., histamine, cytokines) in the interstitial fluid.
- Feedback Mechanism: The goal is to develop a system where sensor data can inform and potentially trigger controlled release from the MNs or provide clinicians and patients with objective, real-time data on treatment response and inflammation status, enabling personalized therapy adjustments.
Figure 6 Microneedle based electrochemical sensing (García-Guzmán et al., 2021).
Through three iterations of the DBTL process, we successfully optimized the manufacturing process of soluble microneedles and solved the key problems related to polymer dissolution and bubble-induced defects. The final solution adopted the heating dissolution method for PVA and used stepwise centrifugation to separate the microneedle sheets. It was able to consistently produce microneedle sheets with high structural fidelity and ideal dissolution properties. This hardware platform is the foundation for achieving our future goals. If the project can be fully developed, we will evaluate new low-allergenic substances and develop intelligent, sensor-integrated microneedle systems to achieve more effective treatment monitoring and patient compliance in allergen immunotherapy and other fields.
Al-Zahrani, S., Zaric, M., McCrudden, C., Scott, C., Kissenpfennig, A., & Donnelly, R. F. (2012). Microneedle-mediated vaccine delivery: harnessing cutaneous immunobiology to improve efficacy. Expert Opin Drug Deliv, 9(5), 541-550. doi:10.1517/17425247.2012.676038
Chaaban, M. R., Mansi, A., Tripple, J. W., & Wise, S. K. (2019). SCIT Versus SLIT: Which One Do You Recommend, Doc? Am J Med Sci, 357(5), 442-447. doi:10.1016/j.amjms.2019.02.004
García-Guzmán, J. J., Pérez-Ràfols, C., Cuartero, M., & Crespo, G. A. (2021). Microneedle based electrochemical (Bio)Sensing: Towards decentralized and continuous health status monitoring. TRAC Trends in Analytical Chemistry, 135, 116148. doi:https://doi.org/10.1016/j.trac.2020.116148
Dawn Connelly. (2021). Microneedles: a new way to deliver vaccines. The Pharmaceutical Journal, PJ, September 2021, Vol 307, No 7953;307(7953). DOI:10.1211/PJ.2021.1.100134
