Microneedles for drug and vaccine delivery

Microneedles were first conceptualized for drug delivery many decades ago, but only became the subject of significant research starting in the mid-1990's when microfabrication technology enabled their manufacture as (i) solid microneedles for skin pretreatment to increase skin permeability, (ii) microneedles coated with drug that dissolves off in the skin, (iii) polymer microneedles that encapsulate drug and fully dissolve in the skin and (iv) hollow microneedles for drug infusion into the skin. As shown in more than 350 papers now published in the field, microneedles have been used to deliver a broad range of different low molecular weight drugs, biotherapeutics and vaccines, including published human studies with a number of small-molecule and protein drugs and vaccines. Influenza vaccination using a hollow microneedle is in widespread clinical use and a number of solid microneedle products are sold for cosmetic purposes. In addition to applications in the skin, microneedles have also been adapted for delivery of bioactives into the eye and into cells. Successful application of microneedles depends on device function that facilitates microneedle insertion and possible infusion into skin, skin recovery after microneedle removal, and drug stability during manufacturing, storage and delivery, and on patient outcomes, including lack of pain, skin irritation and skin infection, in addition to drug efficacy and safety. Building off a strong technology base and multiple demonstrations of successful drug delivery, microneedles are poised to advance further into clinical practice to enable better pharmaceutical therapies, vaccination and other applications.

Microneedles for transdermal drug delivery

The success of transdermal drug delivery has been severely limited by the inability of most drugs to enter the skin at therapeutically useful rates. Recently, the use of micron-scale needles in increasing skin permeability has been proposed and shown to dramatically increase transdermal delivery, especially for macromolecules. Using the tools of the microelectronics industry, microneedles have been fabricated with a range of sizes, shapes and materials. Most drug delivery studies have emphasized solid microneedles, which have been shown to increase skin permeability to a broad range of molecules and nanoparticles in vitro. In vivo studies have demonstrated delivery of oligonucleotides, reduction of blood glucose level by insulin, and induction of immune responses from protein and DNA vaccines. For these studies, needle arrays have been used to pierce holes into skin to increase transport by diffusion or iontophoresis or as drug carriers that release drug into the skin from a microneedle surface coating. Hollow microneedles have also been developed and shown to microinject insulin to diabetic rats. To address practical applications of microneedles, the ratio of microneedle fracture force to skin insertion force (i.e. margin of safety) was found to be optimal for needles with small tip radius and large wall thickness. Microneedles inserted into the skin of human subjects were reported as painless. Together, these results suggest that microneedles represent a promising technology to deliver therapeutic compounds into the skin for a range of possible applications.

Current status and future potential of transdermal drug delivery

The past twenty five years have seen an explosion in the creation and discovery of new medicinal agents. Related innovations in drug delivery systems have not only enabled the successful implementation of many of these novel pharmaceuticals, but have also permitted the development of new medical treatments with existing drugs. The creation of transdermal delivery systems has been one of the most important of these innovations, offering a number of advantages over the oral route. In this article, we discuss the already significant impact this field has made on the administration of various pharmaceuticals; explore limitations of the current technology; and discuss methods under exploration for overcoming these limitations and the challenges ahead.

Microfabricated microneedles: a novel approach to transdermal drug delivery

Although modern biotechnology has produced extremely sophisticated and potent drugs, many of these compounds cannot be effectively delivered using current drug delivery techniques (e.g., pills and injections). Transdermal delivery is an attractive alternative, but it is limited by the extremely low permeability of skin. Because the primary barrier to transport is located in the upper 10-15 micron of skin and nerves are found only in deeper tissue, we used a reactive ion etching microfabrication technique to make arrays of microneedles long enough to cross the permeability barrier but not so long that they stimulate nerves, thereby potentially causing no pain. These microneedle arrays could be easily inserted into skin without breaking and were shown to increase permeability of human skin in vitro to a model drug, calcein, by up to 4 orders of magnitude. Limited tests on human subjects indicated that microneedles were reported as painless. This paper describes the first published study on the use of microfabricated microneedles to enhance drug delivery across skin.

Dissolving polymer microneedle patches for influenza vaccination

Influenza prophylaxis would benefit from a vaccination method enabling simplified logistics and improved immunogenicity without the dangers posed by hypodermic needles. Here we introduce dissolving microneedle patches for influenza vaccination using a simple patch-based system that targets delivery to skin's antigen-presenting cells. Microneedles were fabricated using a biocompatible polymer encapsulating inactivated influenza virus vaccine for insertion and dissolution in the skin within minutes. Microneedle vaccination generated robust antibody and cellular immune responses in mice that provided complete protection against lethal challenge. Compared to conventional intramuscular injection, microneedle vaccination resulted in more efficient lung virus clearance and enhanced cellular recall responses after challenge. These results suggest that dissolving microneedle patches can provide a new technology for simpler and safer vaccination with improved immunogenicity that could facilitate increased vaccination coverage.

Dissolving microneedles for transdermal drug delivery

Microfabrication technology has been adapted to produce micron-scale needles as a safer and painless alternative to hypodermic needle injection, especially for protein biotherapeutics and vaccines. This study presents a design that encapsulates molecules within microneedles that dissolve within the skin for bolus or sustained delivery and leave behind no biohazardous sharp medical waste. A fabrication process was developed based on casting a viscous aqueous solution during centrifugation to fill a micro-fabricated mold with biocompatible carboxymethylcellulose or amylopectin formulations. This process encapsulated sulforhodamine B, bovine serum albumin, and lysozyme; lysozyme was shown to retain full enzymatic activity after encapsulation and to remain 96% active after storage for 2 months at room temperature. Microneedles were also shown to be strong enough to insert into cadaver skin and then to dissolve within minutes. Bolus delivery was achieved by encapsulating molecules just within microneedle shafts. For the first time, sustained delivery over hours to days was achieved by encapsulating molecules within the microneedle backing, which served as a controlled release reservoir that delivered molecules by a combination of swelling the backing with interstitial fluid drawn out of the skin and molecule diffusion into the skin via channels formed by dissolved microneedles. We conclude that dissolving microneedles can be designed to gently encapsulate molecules, insert into skin, and enable bolus or sustained release delivery.

Biodegradable polymer microneedles: fabrication, mechanics and transdermal drug delivery

To overcome the skin's barrier properties that block transdermal delivery of most drugs, arrays of microscopic needles have been microfabricated primarily out of silicon or metal. This study addresses microneedles made of biocompatible and biodegradable polymers, which are expected to improve safety and manufacturability. To make biodegradable polymer microneedles with sharp tips, micro-electromechanical masking and etching were adapted to produce beveled- and chisel-tip microneedles and a new fabrication method was developed to produce tapered-cone microneedles using an in situ lens-based lithographic approach. To replicate microfabricated master structures, PDMS micromolds were generated and a novel vacuum-based method was developed to fill the molds with polylactic acid, polyglycolic acid, and their co-polymers. Mechanical testing of the resulting needles measured the force at which needles broke during axial loading and found that this failure force increased with Young's modulus of the material and needle base diameter and decreased with needle length. Failure forces were generally much larger than the forces needed to insert microneedles into skin, indicating that biodegradable polymers can have satisfactory mechanical properties for microneedles. Finally, arrays of polymer microneedles were shown to increase permeability of human cadaver skin to a low-molecular weight tracer, calcein, and a macromolecular protein, bovine serum albumin, by up to three orders of magnitude. Altogether, these results indicate that biodegradable polymer microneedles can be fabricated with an appropriate geometry and sufficient strength to insert into skin, and thereby dramatically increase transdermal transport of molecules.

Insertion of microneedles into skin: measurement and prediction of insertion force and needle fracture force

As a hybrid between a hypodermic needle and transdermal patch, we have used microfabrication technology to make arrays of micron-scale needles that transport drugs and other compounds across the skin without causing pain. However, not all microneedle geometries are able to insert into skin at reasonable forces and without breaking. In this study, we experimentally measured and theoretically modeled two critical mechanical events associated with microneedles: the force required to insert microneedles into living skin and the force needles can withstand before fracturing. Over the range of microneedle geometries investigated, insertion force was found to vary linearly with the interfacial area of the needle tip. Measured insertion forces ranged from approximately 0.1-3N, which is sufficiently low to permit insertion by hand. The force required to fracture microneedles was found to increase with increasing wall thickness, wall angle, and possibly tip radius, in agreement with finite element simulations and a thin shell analytical model. For almost all geometries considered, the margin of safety, or the ratio of fracture force to insertion force, was much greater than one and was found to increase with increasing wall thickness and decreasing tip radius. Together, these results provide the ability to predict insertion and fracture forces, which facilitates rational design of microneedles with robust mechanical properties.

Microfabricated needles for transdermal delivery of macromolecules and nanoparticles: fabrication methods and transport studies

Arrays of micrometer-scale needles could be used to deliver drugs, proteins, and particles across skin in a minimally invasive manner. We therefore developed microfabrication techniques for silicon, metal, and biodegradable polymer microneedle arrays having solid and hollow bores with tapered and beveled tips and feature sizes from 1 to 1,000 microm. When solid microneedles were used, skin permeability was increased in vitro by orders of magnitude for macromolecules and particles up to 50 nm in radius. Intracellular delivery of molecules into viable cells was also achieved with high efficiency. Hollow microneedles permitted flow of microliter quantities into skin in vivo, including microinjection of insulin to reduce blood glucose levels in diabetic rats.

Coated microneedles for transdermal delivery

Coated microneedles have been shown to deliver proteins and DNA into the skin in a minimally invasive manner. However, detailed studies examining coating methods and their breadth of applicability are lacking. This study's goal was to develop a simple, versatile and controlled microneedle coating process to make uniform coatings on microneedles and establish the breadth of molecules and particles that can be coated onto microneedles. First, microneedles were fabricated from stainless steel sheets as single microneedles or arrays of microneedles. Next, a novel micron-scale dip-coating process and a GRAS coating formulation were designed to reliably produce uniform coatings on both individual and arrays of microneedles. This process was used to coat compounds including calcein, vitamin B, bovine serum albumin and plasmid DNA. Modified vaccinia virus and microparticles of 1 to 20 micro m diameter were also coated. Coatings could be localized just to the needle shafts and formulated to dissolve within 20 s in porcine cadaver skin. Histological examination validated that microneedle coatings were delivered into the skin and did not wipe off during insertion. In conclusion, this study presents a simple, versatile, and controllable method to coat microneedles with proteins, DNA, viruses and microparticles for rapid delivery into the skin.

Effect of microneedle design on pain in human volunteers

OBJECTIVES

To design microneedles that minimize pain, this study tested the hypothesis that microneedles cause significantly less pain than a 26-gauge hypodermic needle, and that decreasing microneedle length and the number of microneedles reduces pain in normal human volunteers.

METHODS

Single microneedles with lengths ranging from 480 to 1450 microm, widths from 160 to 465 microm, thicknesses from 30 to 100 microm, and tip angles from 20 to 90 degrees; and arrays containing 5 or 50 microneedles were inserted into the volar forearms of 10 healthy, human volunteers in a double-blinded, randomized study. Visual analog scale pain scores were recorded and compared with each other and to the pain from a 26-gauge hypodermic needle.

RESULTS

All microneedles investigated were significantly less painful than the hypodermic needle with microneedle pain scores varying from 5% to 40% of the hypodermic needle. Microneedle length had the strongest effect on pain, where a 3-fold increase in length increased the pain score by 7-fold. The number of microneedles also affected the pain score, where a 10-fold increase in the number of microneedles increased pain just over 2-fold. Microneedle tip angle, thickness, and width did not significantly influence pain.

DISCUSSION

Microneedles are significantly less painful than a 26-gauge hypodermic needle over the range of dimensions investigated. Decreasing microneedle length and number of microneedles reduces pain.

Permeability of cornea, sclera, and conjunctiva: a literature analysis for drug delivery to the eye

The objective of this study was to collect a comprehensive database of ocular tissue permeability measurements found in a review of the literature to guide models for drug transport in the eye. Well over 300 permeability measurements of cornea, sclera, and conjunctiva, as well as corneal epithelium, stroma, and endothelium, were obtained for almost 150 different compounds from more than 40 different studies. In agreement with previous work, the corneal epithelium was shown generally to control transcorneal transport, where corneal stroma and endothelium contribute significantly only to the barrier for small, lipophilic compounds. In addition, other quantitative comparisons between ocular tissues are presented. This study provides an extensive database of ocular tissue permeabilities, which should be useful for future development and validation of models to predict rates of drug delivery to the eye.

Electroporation of mammalian skin: a mechanism to enhance transdermal drug delivery

Mammalian skin owes its remarkable barrier function to its outermost and dead layer, the stratum corneum. Transdermal transport through this region occurs predominantly through intercellular lipids, organized largely in bilayers. Electroporation is the creation of aqueous pores in lipid bilayers by the application of a short (microseconds to milliseconds) electric pulse. Our measurements suggest that electroporation occurs in the intercellular lipid bilayers of the stratum corneum by a mechanism involving transient structural changes. Flux increases up to 4 orders of magnitude were observed with human skin in vitro for three polar molecules having charges between -1 and -4 and molecular weights up to slightly more than 1000. Similar flux increases were observed in vivo with animal skin. These results may have significance for drug delivery and other medical applications.

Transdermal delivery of insulin using microneedles in vivo

PURPOSE

The purpose of this study was to design and fabricate arrays of solid microneedles and insert them into the skin of diabetic hairless rats for transdermal delivery of insulin to lower blood glucose level.

METHODS

Arrays containing 105 microneedles were laser-cut from stainless steel metal sheets and inserted into the skin of anesthetized hairless rats with streptozotocin-induced diabetes. During and after microneedle treatment, an insulin solution (100 or 500 U/ml) was placed in contact with the skin for 4 h. Microneedles were removed 10 s, 10 min, or 4 h after initiating transdermal insulin delivery. Blood glucose levels were measured electrochemically every 30 min. Plasma insulin concentration was determined by radioimmunoassay at the end of most experiments.

RESULTS

Arrays of microneedles were fabricated and demonstrated to insert fully into hairless rat skin in vivo. Microneedles increased skin permeability to insulin, which rapidly and steadily reduced blood glucose levels to an extent similar to 0.05-0.5 U insulin injected subcutaneously. Plasma insulin concentrations were directly measured to be 0.5-7.4 ng/ml. Higher donor solution insulin concentration, shorter insertion time, and fewer repeated insertions resulted in larger drops in blood glucose level and larger plasma insulin concentrations.

CONCLUSIONS

Solid metal microneedles are capable of increasing transdermal insulin delivery and lowering blood glucose levels by as much as 80% in diabetic hairless rats in vivo.

Polymer Microneedles for Controlled-Release Drug Delivery

PURPOSE

As an alternative to hypodermic injection or implantation of controlled-release systems, this study designed and evaluated biodegradable polymer microneedles that encapsulate drug for controlled release in skin and are suitable for self-administration by patients.

METHODS

Arrays of microneedles were fabricated out of poly-lactide-co-glycolide using a mold-based technique to encapsulate model drugs--calcein and bovine serum albumin (BSA)--either as a single encapsulation within the needle matrix or as a double encapsulation, by first encapsulating the drug within carboxymethylcellulose or poly-L: -lactide microparticles and then encapsulating drug-loaded microparticles within needles.

RESULTS

By measuring failure force over a range of conditions, poly-lactide-co-glycolide microneedles were shown to exhibit sufficient mechanical strength to insert into human skin. Microneedles were also shown to encapsulate drug at mass fractions up to 10% and to release encapsulated compounds within human cadaver skin. In vitro release of calcein and BSA from three different encapsulation formulations was measured over time and was shown to be controlled by the encapsulation method to achieve release kinetics ranging from hours to months. Release was modeled using the Higuchi equation with good agreement (r2 > or = 0.90). After microneedle fabrication at elevated temperature, up to 90% of encapsulated BSA remained in its native state, as determined by measuring effects on primary, secondary, and tertiary protein structure.

CONCLUSIONS

Biodegradable polymer microneedles can encapsulate drug to provide controlled-release delivery in skin for hours to months.

The safety, immunogenicity, and acceptability of inactivated influenza vaccine delivered by microneedle patch (TIV-MNP 2015): a randomised, partly blinded, placebo-controlled, phase 1 trial

BACKGROUND

Microneedle patches provide an alternative to conventional needle-and-syringe immunisation, and potentially offer improved immunogenicity, simplicity, cost-effectiveness, acceptability, and safety. We describe safety, immunogenicity, and acceptability of the first-in-man study on single, dissolvable microneedle patch vaccination against influenza.

METHODS

The TIV-MNP 2015 study was a randomised, partly blinded, placebo-controlled, phase 1, clinical trial at Emory University that enrolled non-pregnant, immunocompetent adults from Atlanta, GA, USA, who were aged 18-49 years, naive to the 2014-15 influenza vaccine, and did not have any significant dermatological disorders. Participants were randomly assigned (1:1:1:1) to four groups and received a single dose of inactivated influenza vaccine (fluvirin: 18 μg of haemagglutinin per H1N1 vaccine strain, 17 μg of haemagglutinin per H3N2 vaccine strain, and 15 μg of haemagglutinin per B vaccine strain) (1) by microneedle patch or (2) by intramuscular injection, or received (3) placebo by microneedle patch, all administered by an unmasked health-care worker; or received a single dose of (4) inactivated influenza vaccine by microneedle patch self-administered by study participants. A research pharmacist prepared the randomisation code using a computer-generated randomisation schedule with a block size of 4. Because of the nature of the study, participants were not masked to the type of vaccination method (ie, microneedle patch vs intramuscular injection). Primary safety outcome measures are the incidence of study product-related serious adverse events within 180 days, grade 3 solicited or unsolicited adverse events within 28 days, and solicited injection site and systemic reactogenicity on the day of study product administration through 7 days after administration, and secondary safety outcomes are new-onset chronic illnesses within 180 days and unsolicited adverse events within 28 days, all analysed by intention to treat. Secondary immunogenicity outcomes are antibody titres at day 28 and percentages of seroconversion and seroprotection, all determined by haemagglutination inhibition antibody assay. The trial is completed and registered with ClinicalTrials.gov, number NCT02438423.

FINDINGS

Between June 23, 2015, and Sept 25, 2015, 100 participants were enrolled and randomly assigned to a group. There were no treatment-related serious adverse events, no treatment-related unsolicited grade 3 or higher adverse events, and no new-onset chronic illnesses. Among vaccinated groups (vaccine via health-care worker administered microneedle patch or intramuscular injection, or self-administered microneedle patch), overall incidence of solicited adverse events (n=89 vs n=73 vs n=73) and unsolicited adverse events (n=18 vs n=12 vs n=14) were similar. Reactogenicity was mild, transient, and most commonly reported as tenderness (15 [60%] of 25 participants [95% CI 39-79]) and pain (11 [44%] of 25 [24-65]) after intramuscular injection; and as tenderness (33 [66%] of 50 [51-79]), erythema (20 [40%] of 50 [26-55]), and pruritus (41 [82%] of 50 [69-91]) after vaccination by microneedle patch application. The geometric mean titres were similar at day 28 between the microneedle patch administered by a health-care worker versus the intramuscular route for the H1N1 strain (1197 [95% CI 855-1675] vs 997 [703-1415]; p=0·5), the H3N2 strain (287 [192-430] vs 223 [160-312]; p=0·4), and the B strain (126 [86-184] vs 94 [73-122]; p=0·06). Similar geometric mean titres were reported in participants who self-administered the microneedle patch (all p>0·05). The seroconversion percentages were significantly higher at day 28 after microneedle patch vaccination compared with placebo (all p<0·0001) and were similar to intramuscular injection (all p>0·01).

INTERPRETATION

Use of dissolvable microneedle patches for influenza vaccination was well tolerated and generated robust antibody responses.

FUNDING

National Institutes of Health.

Microfabricated microneedles for gene and drug delivery

By incorporating techniques adapted from the microelectronics industry, the field of microfabrication has allowed the creation of microneedles, which have the potential to improve existing biological-laboratory and medical devices and to enable novel devices for gene and drug delivery. Dense arrays of microneedles have been used to deliver DNA into cells. Many cells are treated at once, which is much more efficient than current microinjection techniques. Microneedles have also been used to deliver drugs into local regions of tissue. Microfabricated neural probes have delivered drugs into neural tissue while simultaneously stimulating and recording neuronal activity, and microneedles have been inserted into arterial vessel walls to deliver anti-restenosis drugs. Finally, microhypodermic needles and microneedles for transdermal drug delivery have been developed to reduce needle insertion pain and tissue trauma and to provide controlled delivery across the skin. These needles have been shown to be robust enough to penetrate skin and dramatically increase skin permeability to macromolecules.

Micro-scale devices for transdermal drug delivery

Skin makes an excellent site for drug and vaccine delivery due to easy accessibility, immuno-surveillance functions, avoidance of macromolecular degradation in the gastrointestinal tract and possibility of self-administration. However, macromolecular drug delivery across the skin is primarily accomplished using hypodermic needles, which have several disadvantages including accidental needle-sticks, pain and needle phobia. These limitations have led to extensive research and development of alternative methods for drug and vaccine delivery across the skin. This review focuses on the recent trends and developments in this field of micro-scale devices for transdermal macromolecular delivery. These include liquid jet injectors, powder injectors, microneedles and thermal microablation. The historical perspective, mechanisms of action, important design parameters, applications and challenges are discussed for each method.

Engineering Microneedle Patches for Vaccination and Drug Delivery to Skin

Microneedle patches (MNPs) contain arrays of solid needles measuring hundreds of microns in length that deliver drugs and vaccines into skin in a painless, easy-to-use manner. Optimal MNP design balances multiple interdependent parameters that determine mechanical strength, skin-insertion reliability, drug delivery efficiency, painlessness, manufacturability, and other features of MNPs that affect their performance. MNPs can be made by adapting various microfabrication technologies for delivery of small-molecule drugs, biologics, and vaccines targeted to the skin, which can have pharmacokinetic and immunologic advantages. A small number of human clinical trials, as well as a large and growing market for MNP products for cosmetics, indicate that MNPs can be used safely, efficaciously, and with strong patient acceptance. More advanced clinical trials and commercial-scale manufacturing will facilitate development of MNPs to realize their potential to dramatically increase patient access to otherwise-injectable drugs and to improve drug performance via skin delivery.

Mechanism of intracellular delivery by acoustic cavitation

Using conditions different from conventional medical imaging or laboratory cell lysis, ultrasound has recently been shown to reversibly increase plasma membrane permeability to drugs, proteins and DNA in living cells and animals independently of cell or drug type, suggesting a ubiquitous mechanism of action. To determine the mechanism of these effects, we examined cells exposed to ultrasound by flow cytometry coupled with electron and fluorescence microscopies. The results show that cavitation generated by ultrasound facilitates cellular incorporation of macromolecules up to 28 nm in radius through repairable micron-scale disruptions in the plasma membrane with lifetimes >1 min, which is a period similar to the kinetics of membrane repair after mechanical wounding. Further data suggest that cells actively reseal these holes using a native healing response involving endogenous vesicle-based membrane resealing. In this way, noninvasively focused ultrasound could deliver drugs and genes to targeted tissues, thereby minimizing side effects, lowering drug dosages, and improving efficacy.

Dissolving microneedle patch for transdermal delivery of human growth hormone

The clinical impact of biotechnology has been constrained by the limitations of traditional hypodermic injection of biopharmaceuticals. Microneedle patches have been proposed as a minimally invasive alternative. In this study, the translation of a dissolving microneedle patch designed for simple, painless self-administration of biopharmacetucials that generates no sharp biohazardous waste is assessed. To study the pharmacokinetics and safety of this approach, human growth hormone (hGH) was encapsulated in 600 μm-long dissolving microneedles composed of carboxymethylcellulose and trehalose using an aqueous, moderate-temperature process that maintained complete hGH activity after encapsulation and retained most activity after storage for up to 15 months at room temperature and humidity. After manual insertion into the skin of hairless rats, hGH pharmacokinetics were similar to conventional subcutaneous injection. After patch removal, the microneedles had almost completely dissolved, leaving behind only blunt stubs. The dissolving microneedle patch was well tolerated, causing only slight, transient erythema. This study suggests that a dissolving microneedle patch can deliver hGH and other biopharmaceuticals in a manner suitable for self-administration without sharp biohazardous waste.

Kinetics of skin resealing after insertion of microneedles in human subjects

Over the past decade, microneedles have been shown to dramatically increase skin permeability to a broad range of compounds by creating reversible microchannels in the skin. However, in order to achieve sustained transdermal drug delivery, the extent and duration of skin's increased permeability needs to be determined. In this study, we used electrical impedance spectroscopy to perform the first experiments in human subjects to analyze the resealing of skin's barrier properties after insertion of microneedles. Microneedles having a range of geometries were studied in conjunction with the effect of occlusion to test the hypothesis that increasing microneedle length, number, and cross-sectional area together with occlusion leads to an increase in skin resealing time that can exceed one day. Results indicated that in the absence of occlusion, all microneedle treated sites recovered barrier properties within 2 h, while occluded sites resealed more slowly, with resealing windows ranging from 3 to 40 h depending on microneedle geometry. Upon subsequent removal of occlusion, the skin barrier resealed rapidly. Longer microneedles, increased number of needles, and larger cross-sectional area demonstrated slower resealing kinetics indicating that microneedle geometry played a significant role in the barrier resealing process. Overall, this study showed that pre-treatment of skin with microneedles before applying an occlusive transdermal patch can increase skin permeability for more than one day, but nonetheless allow skin to reseal rapidly after patch removal.

Mechanisms of sampling interstitial fluid from skin using a microneedle patch

Although interstitial fluid (ISF) contains biomarkers of physiological significance and medical interest, sampling of ISF for clinical applications has made limited impact due to a lack of simple, clinically useful techniques that collect more than nanoliter volumes of ISF. This study describes experimental and theoretical analysis of ISF transport from skin using microneedle (MN) patches and demonstrates collection of >1 µL of ISF within 20 min in pig cadaver skin and living human subjects using an optimized system. MN patches containing arrays of submillimeter solid, porous, or hollow needles were used to penetrate superficial skin layers and access ISF through micropores (µpores) formed upon insertion. Experimental studies in pig skin found that ISF collection depended on transport mechanism according to the rank order diffusion < capillary action < osmosis < pressure-driven convection, under the conditions studied. These findings were in agreement with independent theoretical modeling that considered transport within skin, across the interface between skin and µpores, and within µpores to the skin surface. This analysis indicated that the rate-limiting step for ISF sampling is transport through the dermis. Based on these studies and other considerations like safety and convenience for future clinical use, we designed an MN patch prototype to sample ISF using suction as the driving force. Using this approach, we collected ISF from human volunteers and identified the presence of biomarkers in the collected ISF. In this way, sampling ISF from skin using an MN patch could enable collection of ISF for use in research and medicine.