Decision letter for "A Self‐adhesive Microneedle Patch with Drug loading Capability through Swelling Effect"

Microneedles (MNs) offer a rapid method of transdermal drug delivery through penetration of the stratum corneum. However, commercial translation has been limited by fabrication techniques unique to each drug. Herein, a broadly applicable platform is explored by drug-loading via swelling effect of a hydrogel MN patch. A range of small molecule hydrophilic, hydrophobic, and biomacromolecule therapeutics demonstrate successful loading and burst release from hydrogel MNs fabricated from methacrylated hyaluronic acid (MeHA). The post-fabrication drug loading process allows MeHA MN patches with drug loadings of 10 μ g cm − 2 . Additional post-fabrication processes are explored with dendrimer bioadhesives that increase work of adhesion, ensuring stable fixation on skin, and allow for additional drug loading strategies.


| INTRODUCTION
Transdermal delivery via microneedle (MN) patches enhances skin penetration of therapeutics in a minimally invasive manner. [1][2][3] It opens the possibility of pill-free drug delivery of both small molecule and biomacromolecule therapeutics. Conventional drug loading strategies employ layer-by-layer, 4 spray, 5 or embedded coatings on the surface of MNs. The latter incorporates the drug directly into the MN matrix during fabrication. 6,7 However, surface coating is limited to the concentration of drugs that can be coated, and often requires the addition of excipients to improve the viscosity in order to achieve uniform coating. 8 Pre-loading of drugs into the MN matrix is constrained by the need of new fabrication procedures for different drugs.
Hydrogels as controlled-release vehicles offer a variety of porous structures and chemical environments for tuning release kinetics and aqueous swelling. 9,10 Drugs can be incorporated either by gradients or pre-mixing the hydrogel monomer(s) with therapeutics prior or during the fabrication. 9 The tunable swelling property of hydrogels inspired us to develop poly(ethylene glycol) diacrylate (PEGDA)-based hydrogel MNs for the delivery therapeutic peptides-which is especially important in prevention of scar tissues. For example, Gap 26 is a connexin mimetic peptide that inhibits cellular gap-junction for the treatment and prevention of keloids. 11 However, PEGDA MNs were restricted by swelling ratios below 20%. This leads to day long incubation period preventing therapeutic loading within minutes. To solve this issue, there is a need to devise a MN platform with high swelling ability for rapid drug loading by swelling effect.
Hyaluronic acid (HA) is known to hold up to 100 times its weight in water. 12 Crosslinking can be exploited to control swelling.
Recently, we demonstrated that methacrylated HA (MeHA)-based hydrogel MNs could swell up to nine times its original weight in within minutes. 13 The higher swelling ratio and shorter duration of  [14][15][16] Thus, the MeHA MN patch eliminates the need for adhesive films for fixation during application, reducing the possibility of skin irritations such as local inflammation (e.g., redness) or erythema that is observed with usage of commercial adhesives. 17,18 Additionally, it may offer an additional platform for long term drug loading and biosensing.

| Development and characterization of swellable MeHA MNs
MeHA polymer was synthesized by modifying HA with methacrylate anhydride according to the protocol reported previously. 13 The degree of methacrylation was 70.5% according to 1 H NMR map ( Figure S1). The MNs were fabricated from MeHA polymer using template molding approach. 13 13 Longer UV exposure time would result in smaller swelling ratio, thereby decreasing the drug loading capacity. We examined the swelling behavior of MeHA MNs crosslinked with 3, 4, 5, 10, and 15 min of UV exposure (namely CL3, CL4, CL5, CL10, and CL15, respectively) by measuring the mass change before and after incubation in 1X phosphate buffered saline (PBS). As shown in Figure S3a   Histology study demonstrated successful disruption of stratum corneum (~25 μm thickness) (Figure 3b). 25 The penetration depth was approximately 600 μm for the 800 μm CL5-MeHA MNs. Elastic nature of skin prevents 100% MN insertion due to deformation of porcine skin during MN insertion. 26 CL5-MeHA MN patches also penetrate across viable epidermis (50-120 μm) and reach the superficial dermis. This allows the released drug to diffuse and reach the capillary blood supply in the dermal layer for circulation in the body. 27 We examined the release profiles of molecules from the CL5-MeHA MN patches after incubation in 1X PBS at 37 C to mimic physiological conditions. The release kinetics was assessed until >90% release of the molecules was recorded from the MN patches. As Slower release kinetics observed in Dox-loaded MN patches could be due to the poor hydrophilicity nature of the drug that also resulted in lower amount loaded into the patches through swelling effect. Over 95% of the molecules were released after 72 hr (Figure 4b). To further enhance the peel adhesion strength for high friction or wet tissue environments (e.g., foot soles and perspiration), the surface of the MN patch was coated with PAMAM-g-diazirine (PDz) tissue adhesives that impart contact sterility and conformal wetting. 14

| Preparation of PDz bioadhesive
Polyamidoamine-g-diazirine 15% grafted (PDz) was synthesized according to the reported protocol. 15 Set amount of PDz, PEG 400, and tertiary PEGs (10 kDa; 5% w/w in MeOH) were suspended in a 2 ml Eppendorf and diluted with methanol to obtain a homogeneous solution. The adhesive was formulated to have the following weight ratios PDz (30%): PEG 400 (60%): PEG 10K (10%) as this formulation has shown to high storage modulus (~270 kPa) with welldefined porous structure. 14 Before use, the MeOH suspended adhesive was evaporated (vacuum oven at 37 C for 96 hr) to obtain a pale yellow viscous liquid and stored in dark at 4 C.

| Statistical analysis
All data were obtained from at least three independent experiments with at least three parallel samples per condition in each experiment and were expressed as means ± SDs. Significance was determined by Student's ttest. A probability value of p < .05 was considered significant (p ≤ .01).

CONFLICT OF INTERESTS
The authors declare no conflict of interest.