(C-86) Optimization of a hydrogel-forming microneedle platform as a biofilm therapy

Biofilm-associated infections are a serious global threat, constituting nearly 80% of chronic wounds and costing $280 billion annually. Biofilms are communities of bacterial cells embedded into three-dimensional matrices of extracellular polymeric substances (EPS), and they are associated with a wide range of conditions including tooth caries, diabetic foot ulcers, medical device-related infections, and other chronic infections. Bacterial biofilm infections are especially difficult to treat due to their antibiotic resistance mechanisms and their ability to evade immune clearance. Current treatments for biofilms on skin microbiota, including debridement, topical ointments, and systemic antibiotics, have significant limitations: debridement of infected tissue may be highly uncomfortable for patients, topical ointments exhibit limited biofilm penetration, and systemic antibiotics can exacerbate antibiotic resistance.

Microneedle (MN) patches have gained traction in recent years as minimally invasive biomedical devices typically used for transdermal drug delivery. MN patches, consisting of an array of small needles (< 1 mm in height) may be used to penetrate biofilms to deliver therapeutics without discomfort, tissue damage, inflammation, or triggering needle phobia. Although MNs have previously been made with materials like glass, metals, and ceramics, hydrogel-based MNs are of significant interest in treating biofilms, as they can allow for swelling and passive diffusion of encapsulated drugs upon exposure to the hydrated biofilm microenvironment post-puncture. Here, we developed hydrogel-based MNs consisting of a hybrid of poly(ethylene) glycol methacrylate (PEGMA) and poly(ethylene) glycol diacrylate (PEGDA). We then investigated the morphology and mechanical properties of the MN patches post-fabrication.

The MN patches consist of two distinct layers, a MN and a backing layer. Table 1 details the components and compositions for each patch formulation. Both poly(vinyl alcohol) (PVA) and PEGDA-based solutions were dissolved in 1x phosphate buffered saline; the latter also incorporated the photoinitiators triethanolamine (1.5% v/v), eosin Y (1% v/v), and 1-vinyl-2-pyrrolidone (0.5% v/v) for photocrosslinking. The MN hydrogel prepolymer solution was added to polydimethylsiloxane molds (Micropoint Technologies Pte Ltd., Singapore; 15x15 array with needle height: 600 μm, base: 200 μm, and pitch: 500 μm) and was vacuumed for 10 minutes to fill the mold cavities. Excess MN precrosslinked layer was removed, and PEGDA-based MNs were exposed to white light (λ = 514 nm) for 30 minutes. The backing material was then added, followed by 5 minutes of vacuuming and photocrosslinking as necessary. The patch was desiccated for 12 hours at 20 °C and subsequently removed from the mold.  

The compressive response of the MN patch was evaluated using the Instron 6800 Series universal testing system (Norwood, MA). The patch was placed on the base plate under a preload of 0.1 N, confirming MN contact. The MNs were compressed at a constant rate of 0.02 mm/s until the desired displacement of 0.6 mm was reached.

A Thermo Scientific Quattro Environmental Scanning Electron Microscope was used to image the MN patches before and after compression; patches were carbon fiber coated via low vacuum evaporation prior to imaging.

Although PVA-based solutions sufficiently hardened during desiccation, free radical polymerization of PEGMA-co-PEGDA gels was highly dependent on exposure to white light, requiring at least 30 minutes of white light exposure for their shape to hold. SEM imaging of the patches prior to mechanical compression revealed MN heights ranging from ~595 to 625 μm in iteration 1, ~490 to 520 μm in iteration 2, and ~380 to 400 μm in iteration 3; no deformities were observed in iterations 1 and 2, while bubbles in the form of circular aberrations were seen in iteration 3 (Figure 1a).

The force-displacement curves for all iterations of fabricated MNs are shown in Figure 1b below. Compressive strengths, the maximum capacity for materials to withstand compressive loads were extracted as local peaks in the force-displacement curves; breaking points of the MN patches were subsequently defined as the displacements at these peaks. Iterations 1 and 2 showed breaking points of 0.26 mm and 0.47 mm, respectively, while a breaking point was not observed in 3, suggesting the need for a greater preload to obtain a more representative force-displacement response. There is limited data available on MN puncture through biofilms, and therefore we examined MN response with respect to previously reported MNs used for skin puncture. The force-displacement responses are comparable to previously reported MNs required for puncturing skin.

Compression testing resulted in complete destruction of patch 1, while post-compression SEM imaging revealed partial deformation of patch 2 to MN heights ranging from ~340 to 500 μm, and no deformation of patch 3, resulting in MN heights ranging from ~380 to 400 μm, reflecting the breaking points noted from compression testing (Figure 1c).

Overall, MN patches composed of PVA or PEGDA-based solutions as MNs and backing layers were fabricated. A patch of PEGMA-co-PEGDA MNs and PEGDA backing showed promising compression testing results, demonstrating its potential to puncture biofilm with future confirmation from in vitro puncture studies. The polymeric nature of the fabricated MNs leads them to serve as an invaluable platform in future studies to load antibiotics to be used as inexpensive and efficacious drug delivery systems.

I would like to thank the Brown Undergraduate Teaching and Research Awards for their financial support. I would also like to acknowledge Anthony McCormick, Microscopist at the Electron Microscopy Facility, and Gerald Zani, Senior Technical Assistant at the Brown Design Workshop.

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