Flexible PEGDA-based microneedle patches with detachable PVP–CD arrowheads for transdermal drug delivery

Abstract

Traditional drug administration using hypodermic needles not only causes severe pain and bleeding to the patients, but also requires professional operation by trained healthcare practitioners. Recently, various types of microneedle patches have been developed for pain-free subcutaneous delivery of drugs and vaccines. However, for most existing microneedle devices, the backing layer where the microneedles grow remains attached to the skin until complete release of the drug. In this work, a novel microneedle patch is designed and fabricated by assembling drug-loaded polyvinylpyrrolidone–cyclodextrin (PVP–CD) arrowheads that are detachable from polyethylene glycol diacrylate (PEGDA)-based needle shafts. When applying this patch onto the skin, the drug-loaded polymer arrowheads with sharp tips can effectively penetrate through the epidermis. Then the backing substrate with blunt PEGDA shafts can be peeled off and discarded shortly after administration, while the drug-loaded polymer arrowheads remain embedded in the dermis for further dissolution and drug release. This simple and convenient drug administration using detachable microneedle patches is very user-friendly and also can effectively address the cosmetic concerns of the patients. Additionally, this patch device features a flexible substrate and thus can adhere to the lesion surface with any curvature. The drug delivery based on this versatile microneedle patch is expected to create new therapeutic modalities for treating various local and systemic diseases with the advantages of convenience, safety and self-administration.

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Introduction

Pharmaceutical molecules, such as peptides, nucleic acids and antibodies, are important candidates for current therapeutics. However, most of these molecules are associated with issues of solubility and permeability when they are inducted through traditional routes. Therefore, large doses of drugs are usually required to compensate for inefficient delivery in order to achieve a remedy of desired efficacy, which further increases the medical cost and results in potential side effects due to toxicity.^1–3 Although a huge number of drugs have been commercialized to date, the options of drug delivery are rather poorly limited. Oral formulations have been developed as the oldest and still the most popular method to deliver drugs nowadays. However, not all types of drugs could maintain their structural stability in the harsh environment of gastrointestinal tract.⁴ Alternatively, transdermal drug delivery has become a standard way for clinical therapeutics in modern medicine, through which the drugs are free from gastrointestinal degradation and thus able to maintain relatively higher plasma concentrations, and the patient compliance is also improved.⁵ However, conventional injection using hypodermic needles not only induces severe pain, bleeding and psychological stress to the patients, but also produces sharp and bio-hazardous wastes that are not ecofriendly.^6–8 In addition, this precise medical operation needs to be administered by qualified healthcare professionals, which is not convenient and raises extra costs to the patients.⁹

To address these disadvantages of hypodermic needles, a new approach using microneedle patches (MNPs) has appeared in the field for minimally-invasive drug delivery.¹⁰ A typical MNP includes arrays of sharp projections in microscale size that are designed to penetrate the stratum corneum forming conduits for drugs to diffuse into the dermal circulation without stimulating the nerve endings, which is a pain-free experience for patients.^11–13 MNPs can be used to deliver various types of drugs with a wide size range from small molecules to nanoparticles.¹⁴ Furthermore, MNPs have much lower manufacturing cost due to less complexity, and also enable self-administration by the patients themselves. A large variety of materials, such as metals, silicon and polymers, have be used for fabrication of MNPs with various configurations.¹⁵ However, MNPs made of metal or silicon still generate non-degradable and bio-hazardous sharps after administration, which requires special treatment and disposal of medical wastes with additional cost. Moreover, these metal- or silicon-based MNPs usually do not contain drugs in the needle materials, while each microneedle encloses a hollow conduit for inducting liquid drug from an attached or external source. Therefore, such mode of delivery is rather passive and still requires human intervention. On the other hand, MNPs made from biodegradable polymers can avoid the issues related to non-degradable bio-hazardous waste. More importantly, many drugs are able to be encapsulated in the polymer matrix of the inserted microneedles and can be released spontaneously via dissolution and polymer degradation in the physiological condition, leaving only the external MNP back layer to be disposed.^12,16,17 However, the backing layer usually needs to remain attached on the skin until complete release of drugs, bringing discomfort and cosmetic concerns to the patients.

In this study, we aim to revamp the polymer-based MNP devices with a novel design of detachable sharp tip on each microneedle, so that the backing layer and blunt needle shafts can be easily separated from the inserted tips after application. The shaft–substrate complex and the detachable arrowhead tips are made from polyethylene glycol diacrylate (PEGDA) and polyvinylpyrrolidone–cyclodextrin (PVP–CD) respectively, both of which are approved biocompatible materials.^18,19 PEGDA is a derivative polymer of polyethylene glycol (PEG), which can form hydrogel through photo-induced polymerization upon the appropriate option of photo-initiator.²⁰ The mechanical properties of PEGDA hydrogel can be controlled through polymer molecular weight, irradiation time and intensity during polymerization process.²¹ PVP is a water soluble polymer with excellent wetting capability and other advantages, such as biocompatibility, low toxicity and storage stability.²² PVP has been proposed as a structure material for fabricating microneedles due to its rapid dissolution in the skin.²³ CD is generated from starch and have been extensively used in pharmaceutical industry for drug coating and encapsulation.²⁴ CD can be integrated with PVP to create host–guest PVP–CD complexes. The hydroxyl group of CD cavity can form hydrogen bonding with PVP, which restrict the PVP movement and increase the mechanical strength of the entire complex.²⁵ In this study, the PEGDA-based shaft–substrate complex is fabricated by one-step photolithography as described in a previous report.²⁶ The drug-loaded and detachable PVP–CD arrowhead tips with pyramid shape are precisely assembled onto the PEGDA backing shafts. This assembled MNP device provides desired mechanical strength to insert the arrowhead tips during skin penetration. Moreover, the flexible PEGDA substrate allows seamless adherence between the MNP and the lesion surface with any curvature. As a prominent feature, the backing shafts together with the patch substrate can be easily detached from the skin shortly after application, while the arrowheads remain inside the skin and release preloaded drugs continuously as PVP–CD gradually degrade and dissolve into the tissue (Fig. 1). In the following, we demonstrate the detailed fabrication procedures of this MNP and a proof-of-concept application to delivery molecules of rhodamine B into a porcine skin.

Fig. 1 Schematic illustration of applying the detachable microneedle patch onto the skin.

Experimental

Materials

PEGDA (polyethylene glycol diacrylate, average M_n = 250), PVP (polyvinylpyrrolidone, average M_W = 10k), HMP (2-hydroxy-2-methyl-propiophenone), CD (hydroxypropyl-beta-cyclodextrin, average M_W = 1460) and rhodamine B (HPLC grade) were purchased from Sigma-Aldrich, Singapore. DI (deionized) water was prepared from Milli-Q water purification system (Synthesis A10, Molsheim, France).

Preparation of backing substrate and microneedle shaft

The microneedle backing substrate and shaft were fabricated using PEGDA by photopolymerization. To fabricate the backing substrate, two coverslips (2.2 cm × 2.2 cm, SPD Scientific Pte Ltd, Singapore) were juxtaposed on a glass slide (1 inch × 3 inch, Fisher Scientific, Singapore) to form a gap in between (Fig. 2). Another coverslip of the same size was precisely aligned on top of this gap to form a shallow cavity. A prepolymer mixture containing PEGDA and photoinitiator (HMP, 0.5% w/w) was carefully fed into the shallow cavity through capillary action, followed by UV-laser irradiation (wavelength = 360 nm, intensity = 17 mW cm⁻²) for 4 s. Thus the backing layer was fabricated after rapid photopolymerization and was carefully peeled off from the coverslip. To fabricate the microneedle shaft array, the set-up was similar to that for the backing substrate but the depth of the gap was increased to 1 mm by stacking more coverslips. Then, the backing substrate was reversely aligned on top of the gap, followed by filling the cavity with prepolymer solution. A film photomask designed with transparent circle array was firmly held against the top side of the backing substrate (Fig. 2). Each transparent circle has a diameter of 300 μm and the distance between the centres of two neighbouring circles is 500 μm. The subsequent UV irradiation for 12 s selectively exposed the prepolymer solution underneath the backing substrate, creating the needle shaft array. Finally, the shaft array sticking on the backing substrate was carefully peeled off from the glass substrate and rinsed with DI water to remove uncrosslinked polymer.

Fig. 2 The fabrication procedure of PEGDA-based backing substrate and microneedle shafts.

Fabrication of final patch with detachable arrowheads

50 mg PVP, 50 mg CD and 0.1 mg rhodamine B were dissolved in 1 mL DI water, followed by sonication for 30 min and vortex stirring for 1 day. The prepared PVP–CD polymer solution containing rhodamine B was placed in a drying cabinet for 2 days before being used for molding. The PDMS mold was prepared by casting the PDMS prepolymer onto a customized steel master (Micropoint Technologies Pte Ltd, Singapore) with array of pyramid tips (Fig. 3). After degassing in a vacuum box and thermal curing at 70 °C for 2 h, the crosslinked PDMS mold was peeled off from the steel master and ready for use. To fabricate the microneedle arrowheads, 100 μL of PVP–CD polymer solution was firstly pipetted onto the PDMS mold. After being vacuumed at −91 kPa for 3 min, the system was centrifuged at 3000 rpm for 10 min at room temperature to force the polymer solution into the pyramid cavities and thus prevent the trapped air from forming void arrowheads. The residual PVP–CD polymer solution on the mold surface was carefully collected for future usage.

Fig. 3 The fabrication procedure of detachable PVP–CD arrowheads and assembly of the final microneedle patch.

To assemble the final MNP device, the microneedle shaft array was carefully aligned on top of the PDMS mold cavities filled with PVP–CD polymer solution under a stereomicroscope (Leica MZ6). A slight force was applied on the backing substrate to ensure all the shafts submerged in the base of PVP–CD arrowheads for a depth of about 200 μm. After drying in a desiccator at room temperature for 3 days, the arrowhead array comprising PVP–CD inclusion complexes²⁷ with superior mechanical strength was created and attached on the needle shafts, forming the final assembled MNP with detachable arrowheads. The entire patch was removed from the PDMS mold and ready for skin test.

Measurement of mechanical property

A NanoIndenter XP (MTS Systems, Oak Ridge, TN) was used to measure the hardness and elastic modulus of the arrowheads of the detachable microneedle patch.²⁸ Three independent locations with distance of at least 300 μm on each sample were indented using NanoIndenter XP. Based on the maximum loading level and fixed indentation depth of 20 μm, the hardness and elastic modulus of the arrowheads were calculated accordingly.

In vitro drug release

To assess the drug release profile in vitro, the prepared microneedle patch with drug-loaded arrowheads were completely immersed in a vial containing 25 mL of PBS (10 mM, pH = 7.4) and incubated at 37 °C for 30 min under magnetic stirring. Free drug encapsulated in a dialysis bag (MWCO = 12 313 Da, Spectrum Laboratories Inc.) was used as control. At each specific time point, 2 mL of release medium was removed from the vial and replenished with equal amount of fresh PBS to maintain a constant volume. The amount of model drugs (rhodamine B) released from the microneedle arrowheads was analysed using a fluorescence spectrometer (PerkinElmer, Waltham, MA, USA) under excitation at 540 nm and emission at 625 nm.²⁹

Microscopic imaging

All images of the microneedle patch and administration sites on the porcine skin were captured using a stereomicroscope (Leica MZ6) with a digital camera (Leica DFC 300FX). The morphology of arrowhead tips was examined using a scanning electron microscope (SEM, Hitachi 3500N) at an accelerating voltage of 5 kV. For histological assay, the same skin tissue was fixed in formalin (10%) and submerged in tissue freezing medium (Leica Biosystems Singapore) at −20 °C. Sagittally oriented cryo-sections were prepared at 8 μm intervals using a Leica CM1850 cryotome (Leica Biosystems, Singapore). The specific morphological features were examined using a fluorescence microscope (IX71, Olympus, Singapore) under bright field.

In vitro test of MNP insertion

The microneedle patches were tested on a porcine skin in vitro. Fresh skin samples were obtained according to Institution-approved protocols. Firstly, the sample was stabilized and flattened on a corkboard. The microneedle patch was then pressed onto the skin for 3 min manually using thumb pressure. To facilitate the detachment of arrowheads, the patch was slightly agitated against the skin before being peeled off from the application site. Images of the administered skin with released rhodamine B in the needle insertion sites were taken under standard light microscopy.

Results and discussion

It is essential that the microneedle shaft needs to have excellent mechanical strength to facilitate the arrowhead tip to pierce into the skin. Metal material, such as steel, is commonly used to fabricate the shaft and the backing substrate of microneedle patches.^30,31 However, the biosafety of many metal materials is still unclear and metal-based backing layer is usually not flexible and adaptive to the arbitrary skin curvature.^4,32 Meanwhile, PEGDA is a FDA-approved polymer material with acceptable biosafety for clinical use. Therefore, PEGDA-based microneedle shafts and backing substrate were fabricated following a simple photo-initiated polymerization method (Fig. 2) as described in our previous study.²⁶ This PEGDA shaft–substrate assembly was transparent and flexible. The microneedle patch could remain intact after being bent with radius of curvature of 1.21 cm, which allows the patch to be fitted on any curved surface of body skin, such as nose, ear and articular surfaces that have small radius of curvature. Moreover, the PEGDA microstructure has been proved to have excellent mechanical strength with a hardness of 45 ± 11 MPa that is 20 times greater compared to a regular skin (2.0 ± 0.5 MPa),²⁶ which ensures efficient penetration through stratum corneum the toughest barrier of skin. Meanwhile, the arrowheads had excellent mechanical property with hardness of 67 ± 4.0 MPa and elastic modulus of 0.93 ± 0.07 GPa, which further facilitated skin penetration during drug administration. In addition, the packing density and the base diameter of needle shafts can be easily adjusted by photomask patterning. The shaft length can be also tuned by varying the cavity height and UV irradiation time during fabrication.

The entire dosage of drug was encapsulated inside the arrowhead tips made of PVP–CD complex. PVP is a biocompatible and biodegradable material that has been approved for using as pharmaceutical excipient.³³ In this study, PVP with molecular weight of about 10 kDa was used since it can be more efficiently cleared in the kidney during blood circulation compared to that of ∼20 kDa.¹ CD derived from starch has been commercialized in the pharmaceutical industry for surface coating of drugs.³⁴ CD was added into PVP forming PVP–CD complex in order to reduce the water absorption by the arrowhead tips and thus maintain their mechanical strength.²⁷ Rhodamine B was pre-mixed in the PVP–CD complex as drug surrogate to demonstrate the drug encapsulation capability.

Fig. 4a shows the image of a final assembled MNP with detachable arrowhead tips. Each microneedle comprises a PEGDA shaft (base circle diameter = 300 μm, height = 800 μm) capped with a PVP–CD arrowhead (base square = 300 μm × 300 μm, height = 600 μm). The red colour of the arrowhead tips is due to the encapsulated rhodamine B. All the arrowhead tips have a shape of rectangular pyramid and are uniformly distributed on the base shafts. The density and the patterning of arrowheads can be adjusted by the designs of photomask and steel master. The PEGDA backing substrate is transparent and flexible, which facilitates the MNP administration. The scanning electron microscopy (SEM) revealed the detailed sharp morphology of each arrowhead tip (Fig. 4b). It was observed that there were minor defects on some tip surface and some dried PVP–CD flakes at the base of the tips, which implied that the fabrication procedure needed to be further optimized. Despite of these imperfections, these PVP–CD microneedle tips were much sharper than those we previously fabricated using PEGDA only.²⁶

Fig. 4 (a) The image of a final assembled microneedle patch with detachable arrowhead tips encapsulating rhodamine B; (b) a SEM image of the PVP–CD arrowhead tips.

Rhodamine B-loaded (0.1% w/w) microneedle patch was used to investigate the drug release kinetics in PBS medium in vitro (Fig. 5). Free rhodamine B (100 μg mL⁻¹) loaded in a dialysis bag was used as control. The results showed that 92.1% of free drug was eluted from the dialysis bag within 30 min. Meanwhile, the drug-loaded microneedle patch exhibited slightly higher drug elution rate. Specifically, the cumulative drug release reached up to 80% in 10 min and 100% in 30 min, indicating rapid drug elution from the arrowhead tips in physiological conditions through diffusion, swelling and dissolving-mediated process.³⁵

Fig. 5 In vitro drug release kinetics of microneedle patch in PBS medium for 30 min in comparison with free drug encapsulated in a dialysis bag.

To simulate the transdermal drug administration on a real skin, we applied a rhodamine B-loaded MNP with detachable arrowheads to a fresh porcine skin simply using thumb pressure. The patch substrate and needle shafts (Fig. 6a) were peeled off shortly after application, leaving all the red arrowheads embedded inside the insertion sites (red dots in Fig. 6b). It was found that the inserted PVP–CD tips quickly dissolved within a few minutes in the humid environment of the fresh skin sample due to the superior dissolvability of PVP.³⁶ And these coloured dots could not be wiped off or removed by washing with DI water, indicating that rhodamine B were successfully released into the dermis. The histological assay of the pierced skin (Fig. 6c) further confirmed that the arrowhead tips successfully penetrated the epidermis layer with an insertion depth of about 500 μm. This penetration depth was much shorter than the length of the entire microneedle, which was quite common due to the deformation of highly elastic skin as found in other similar studies.^12,37

Fig. 6 (a) The peeled-off PEGDA substrate and shafts after application of MNP. Scale bar = 500 μm. (b) The insertion sites by the detachable arrowheads loaded with rhodamine B on a porcine skin. Scale bar = 500 μm. (c) Histological assay of the pierced skin after the PVP–CD tips were dissolved.

This self-administered microneedle patch provides a simple way of transdermal drug delivery for patients requiring hypodermic injection. The method is rapid, efficient and free from pain, bleeding and generation of bio-hazardous sharp wastes. This MNP device can be obtained with an extremely low cost for convenient self-administration onsite or at home without the assistance from healthcare professionals. This polymeric patch can adapt to any lesion site owing to the good flexibility of the backing substrate. Additionally, the backing layer and the needle shafts can be easily peeled off and discarded shortly after application without causing any cosmetic concern to the patients. Moreover, the microneedle fabrication process is conducted under aqueous conditions at room temperature, which causes no damage to the fragile biomolecules during encapsulation. Therefore, many vaccines or other bioactive molecules could be incorporated into the polymer matrix for transdermal delivery, which has a great potential to treat aggressive skin and breast cancers.³⁸ For further studies, the detailed release kinetics against the type and concentration of various drugs from the PVP–CD complex can be investigated in vivo, which may elucidate the releasing mechanisms and further optimize the actual therapeutic effect of this MNP device.

Conclusions

This study demonstrated the design and fabrication of a polymeric microneedle patch device with detachable and drug-loaded arrowhead tips for potential delivery of bioactive molecules into skin. This microneedle patch features good flexibility and convenient self-administration by patients without generating bio-hazardous sharp wastes. Shortly after skin piercing, the PVP–CD arrowhead tips are detached from the PEGDA shaft–substrate assembly and embedded into the skin for instant dissolution and release of the loaded molecules. This MNP device is expected to be a promising and eco-friendly tool for rapid, reliable, pain-free and convenient transdermal delivery of diverse bioactive molecules in self-administered or clinical applications in the future.

Acknowledgements

This work was supported by a Tier 2 Academic Research Fund (ARC 22/13) and a Tier 1 Academic Research Fund (RG 37/14) from the Ministry of Education of Singapore awarded to Y. K. The Ph.D. scholarship from Nanyang Technological University awarded to P. X. is gratefully acknowledged.

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