Evaluation of geometrical effects of microneedles on skin penetration by CT scan and finite element analysis

Fabrication and mechanical modelling of dissolvable PVA/PVP composite microneedles with biocompatibility for efficient transdermal delivery of ibuprofen

Microneedles offer minimally invasive, user-friendly, and subcutaneously accessible transdermal drug delivery and have been widely investigated as an effective transdermal delivery system. Ibuprofen is a common anti-inflammatory drug to treat chronic inflammation. It is crucial to develop microneedle patches capable of efficiently delivering ibuprofen through the skin for the effective treatment of arthritis patients requiring repeated medication. In this study, the mechanical properties of a new type of polymer microneedle were studied by finite element analysis, and the experimental results showed that the microneedle could effectively deliver drugs through the skin. In addition, a high ibuprofen-loaded microneedle patch was successfully prepared by micromolding and subjected to evaluation of its infrared spectrum morphology and dissolve degree. The morphology of microneedles was characterized by scanning electron microscopy, and the mechanical properties were assessed using a built linear stretching system. In the in-vitro diffusion cell drug release test, the microneedle released 85.2 ± 1.52% (210 ± 3.7 μg) ibuprofen in the modified Franz diffusion within 4 h, exhibiting a higher drug release compared to other drug delivery methods. This study provides a portable, safe and efficient treatment approach for arthritis patients requiring daily repeated medication.

Unravelling the role of microneedles in drug delivery: Principle, perspectives, and practices

In recent year, the research of transdermal drug delivery systems has got substantial attention towards the development of microneedles (MNs). This shift has occurred due to multifaceted advantages of MNs as they can be utilized to deliver the drug deeper to the skin with minimal invasion, offer successful delivery of drugs and biomolecules that are susceptible to degradation in gastrointestinal tract (GIT), act as biosensors, and help in monitoring the level of biomarkers in the body. These can be fabricated into different types based on their applications as well as material for fabrication. Some of their types include solid MNs, hollow MNs, coated MNs, hydrogel forming MNs, and dissolving MNs. These MNs deliver the therapeutics via microchannels deeper into the skin. The coated and hollow MNs have been found successful. However, they suffer from poor drug loading and blocking of pores. In contrast, dissolving MNs offer high drug loading. These MNs have also been utilized to deliver vaccines and biologicals. They have also been used in cosmetics. The current review covers the different types of MNs, materials used in their fabrication, properties of MNs, and various case studies related to their role in delivering therapeutics, monitoring level of biomarkers/hormones in body such as insulin. Various patents and clinical trials related to MNs are also covered. Covered are the major bottlenecks associated with their clinical translation and potential future perspectives.

A Hollow Microneedle Equipped with a Micropillar for Improved Needle Insertion and Injection of Drug Solution

PURPOSE

Hollow-type microneedles (hMNs) are a promising device for the effective administration of drugs into intradermal sites. Complete insertion of the needle into the skin and administration of the drug solution without leakage must be achieved to obtain bioavailability or a constant effect. In the present study, several types of hMN with or without a rounded blunt tip micropillar, which suppresses skin deformation, around a hollow needle, and the effect on successful needle insertion and administration of a drug solution was investigated. Six different types of hMNs with needle lengths of 1000, 1300, and 1500 µm with or without a micropillar were used.

METHODS

Needle insertion and the disposition of a drug in rat skin were investigated. In addition, the displacement-force profile during application of hMNs was also investigated using a texture analyzer with an artificial membrane to examine needle factors affecting successful insertion and administration of a drug solution by comparing with in vivo results.

RESULTS

According to the results with the drug distribution of iodine, hMN1300 with a micropillar was able to successfully inject drug solution into an intradermal site with a high success rate. In addition, the results of displacement-force profiles with an artificial membrane showed that a micropillar can be effective for depth control of the injected solution as well as the prevention of contact between the hMN pedestal and the deformed membrane.

CONCLUSION

In the present study, hMN1300S showed effective solution delivery into an intradermal site. In particular, a micropillar can be effective for depth control of the injected solution as well as preventing contact between the hMN pedestal and the deformed membrane. The obtained results will help in the design and development of hMNs that ensure successful injection of an administered drug.

Impedance-based polymer microneedle patch sensor for continuous interstitial fluid glucose monitoring

Early detection and effective blood glucose control are critical for preventing and managing diabetes-related complications. Conventional glucometers provide point-in-time measurements but are painful and cannot facilitate continuous monitoring. Continuous glucose monitoring systems are comfortable but face challenges in terms of accuracy, cost, and sensor lifespan. This study aimed to develop a microneedle-based sensor patch for minimally invasive, painless, and continuous glucose monitoring in the interstitial fluid to address these limitations. Experimental results confirm painless and minimally invasive penetration of the skin tissue with cylindrical microneedles (3 × 3 array) to a depth of approximately 520 μm with minimal loading. The microneedle sensors fabricated with precision using the complementary metal-oxide semiconductor process were immobilized with glucose oxidase, as confirmed through phase angle analysis. Long-term tests confirmed the effective operation of the sensor for up to seven days. Glucose concentrations determined from the fitted concentration-impedance curves correlated well with those measured using commercial glucometers, indicating the reliability and precision of the microneedle sensor. The flexible and minimally invasive sensor developed in this study facilitates painless and continuous glucose monitoring.

Botulinum toxin A dissolving microneedles for hyperhidrosis treatment: design, formulation and in vivo evaluation

Multiple periodic injections of botulinum toxin A (BTX-A) are the standard treatment of hyperhidrosis which causes excessive sweating. However, BTX-A injections can create problems, including incorrect and painful injections, the risk of drug entry into the bloodstream, the need for medical expertise, and waste disposal problems. New drug delivery systems can substantially reduce these problems. Transdermal delivery is an effective alternative to conventional BTX-A injections. However, BTX-A's large molecular size and susceptibility to degradation complicate transdermal delivery. Dissolving microneedle patches (DMNPs) encapsulated with BTX-A (BTX-A/DMNPs) are a promising solution that can penetrate the dermis painlessly and provide localized translocation of BTX-A. In this study, using high-precision 3D laser lithography and subsequent molding, DMNPs were prepared based on a combination of biocompatible polyvinylpyrrolidone and hyaluronic acid polymers to deliver BTX-A with ultra-sharp needle tips of 1.5 ± 0.5 µm. Mechanical, morphological and histological assessments of the prepared DMNPs were performed to optimize their physicochemical properties. Furthermore, the BTX-A release and diffusion kinetics across the skin layers were investigated. A COMSOL simulation was conducted to study the diffusion process. The primary stability analysis reported significant stability for three months. Finally, the functionality of the BTX-A/DMNPs for the suppression of sweat glands was confirmed on the hyperhidrosis mouse footpad, which drastically reduced sweat gland activity. The results demonstrate that these engineered DMNPs can be an effective, painless, inexpensive alternative to hypodermic injections when treating hyperhidrosis.

Dermal tissue penetration of in-plane silicon microneedles evaluated in skin-simulating hydrogel, rat skin and porcine skin

Recently, microneedle-based sensors have been introduced as novel strategy for in situ monitoring of biomarkers in the skin. Here, in-plane silicon microneedles with different dimensions and shapes are fabricated and their ability to penetrate skin is evaluated. Arrays with flat, triangular, hypodermic, lancet and pencil-shaped microneedles, with lengths of 500-1000 μm, widths of 200-400 μm and thickness of 180-500 μm are considered. Fracture force is higher than 20 N for all microneedle arrays (MNA) confirming a high mechanical stability of the microneedles. Penetration force in skin-simulating hydrogels, excised rat abdominal skin and porcine ear skin is at least five times lower than the fracture force for all MNA designs. The lowest force for skin penetration is required for triangular microneedles with a low width and thickness. Skin tissue staining and histological analysis of rat abdominal skin and porcine ear skin confirm successful penetration of the epidermis for all MNA designs. However, the penetration depth is between 100 and 300 μm, which is considerably lower than the microneedle length. Tissue damage estimated by visual analysis of the penetration hole is smallest for triangular microneedles. Penetration ability and tissue damage are compared to the skin prick test (SPT) needle applied in allergy testing.

Microneedles for advanced ocular drug delivery

In the field of ocular drug delivery, topical delivery remains the most common treatment option for managing anterior segment diseases, whileintraocular injectionsare the current gold standard treatment option for treating posterior segment diseases. Nonetheless, topical eye drops are associated with low bioavailability (<5%), and theintravitreal administration procedure is highly invasive, yielding poor patient acceptability. In both cases, frequent administration is currently required. As a result, there is a clear unmet need for sustained drug delivery to the eye, particularly in a manner that can be localised. Microneedles, which are patches containing an array of micron-scale needles (<1 mm), have the potential to meet this need. These platforms can enable localised drug delivery to the eye while enhancing penetration of drug molecules through key ocular barriers, thereby improving overall therapeutic outcomes. Moreover, the minimally invasive manner in which microneedles are applied could provide significant advantages over traditional intravitreal injections regarding patient acceptability. Considering the benefitsofthis novel ocular delivery system, this review provides an in-depth overviewofthe microneedle systems for ocular drug delivery, including the types of microneedles used and therapeutics delivered. Notably, we outline and discuss the current challenges associated with the clinical translation of these platforms and offer opinions on factors which should be considered to improve such transition from lab to clinic.

Decohesion of a polyolefin overlay from a substrate high density polyethylene layer by impact induced stress waves

High-density polythene (HDPE) is difficult to separate from food packaging waste for recycling because the packaging occasionally has multilayer plastic labels attached. Solvents are employed in the current separation techniques to remove undesirable layers from HDPE substrates. The possibility of separating HDPE via the impact-delamination phenomenon was explored both theoretically and experimentally. Using the cohesive zone model (CZM), the decohesion of layers in a model two-layer laminate made of HDPE and LDPE layers was studied theoretically. According to this study, stress waves emerge and severely damage the adhesion between the layers as a cutting blade strikes the laminate at speeds greater than 40 m/s. The damage can be enhanced by increasing the strike velocity and the apex radius of the blade. These findings show that a novel plastic delaminator that can cut and delaminate the laminates simultaneously can be designed. The proposed machine will feature two sets of blades with varying edge apex radii. One set of blades can be designed to cause the most adhesion damage while the other blades cut the laminate. This unique combination of cutting and delamination operations has several benefits, including less solvent waste and downstream processes, greater environmental friendliness, and faster HDPE separation. Laminates from HDPE milk bottles were cut using a high-speed cutter-blender with six blades to test the predicted results. The cut HDPE flakes were separated pneumatically. According to FTIR analysis and SEM, only a trace of adhesive was present on the cut and separated HDPE flakes.

Assessing the risk of a clinically significant infection from a Microneedle Array Patch (MAP) product

Microneedle Array Patches (MAPs) are an emerging dosage form that creates transient micron-sized disruptions in the outermost physical skin barrier, the stratum corneum, to facilitate delivery of active pharmaceutical ingredients to the underlying tissue. Numerous MAP products are proposed and there is significant clinical potential in priority areas such as vaccination. However, since their inception scientists have hypothesized about the risk of a clinically significant MAP-induced infection. Safety data from two major Phase 3 clinical trials involving hundreds of participants, who in total received tens of thousands of MAP applications, does not identify any clinically significant infections. However, the incumbent data set is not extensive enough to make definitive generalizable conclusions. A comprehensive assessment of the infection risk is therefore advised for MAP products, and this should be informed by clinical and pre-clinical data, theoretical analysis and informed opinions. In this article, a group of key stakeholders identify some of the key product- and patient-specific factors that may contribute to the risk of infection from a MAP product and provide expert opinions in the context of guidance from regulatory authorities. Considerations that are particularly pertinent to the MAP dosage form include the specifications of the finished product (e.g. microbial specification), it's design features, the setting for administration, the skill of the administrator, the anatomical application site, the target population and the clinical context. These factors, and others discussed in this article, provide a platform for the development of MAP risk assessments and a stimulus for early and open dialogue between developers, regulatory authorities and other key stakeholders, to expedite and promote development of safe and effective MAP products.

Biodegradable Microneedles Array with Dual-Release Behavior and Parameter Optimization by Finite Element Analysis

Microneedles (MNs) are particularly attractive for transdermal administration because of the improved safety, patient compliance and convenience. Dissolving MNs could provide rapid transdermal delivery, but with relatively low mechanical strength and almost no sustainability. On the other hand, hydrogel MNs are complicated to fabricate and have risk concerns. Herein, we developed a biodegradable MNs array composed of biocompatible silk fibroin and poly(vinyl alcohol) to overcome these limitations. Finite element analysis was employed for parameter optimization. The MNs array fabricated by the optimal parameters and material displayed sufficient mechanical strength to disrupt stratum corneum and formed microchannels for transdermal delivery. Dual-release profile was observed in the MNs array, with rapid release in the beginning, and prolonged release afterward. This release behavior fits Weibull release model and is favorable for topical application. The initial immediate release can quickly deliver active compounds to reach the therapeutic effective concentration and facilitate skin penetration, and the sustained release may supply the skin with active compounds over a prolonged period. This biodegradable MNs array is easy to fabricate, mechanically robust, could eliminate safety concerns, and provide the sustainability and advantage for large-scale production.

Advances and Prospects for Hydrogel-Forming Microneedles in Transdermal Drug Delivery

Transdermal drug delivery (TDD) is one of the key approaches for treating diseases, avoiding first-pass effects, reducing systemic adverse drug reactions and improving patient compliance. Microneedling, iontophoresis, electroporation, laser ablation and ultrasound facilitation are often used to improve the efficiency of TDD. Among them, microneedling is a relatively simple and efficient means of drug delivery. Microneedles usually consist of micron-sized needles (50-900 μm in length) in arrays that can successfully penetrate the stratum corneum and deliver drugs in a minimally invasive manner below the stratum corneum without touching the blood vessels and nerves in the dermis, improving patient compliance. Hydrogel-forming microneedles (HFMs) are safe and non-toxic, with no residual matrix material, high drug loading capacity, and controlled drug release, and they are suitable for long-term, multiple drug delivery. This work reviewed the characteristics of the skin structure and TDD, introduced TDD strategies based on HFMs, and summarized the characteristics of HFM TDD systems and the evaluation methods of HFMs as well as the application of HFM drug delivery systems in disease treatment. The HFM drug delivery system has a wide scope for development, but the translation to clinical application still has more challenges.

Recent advancements in the expedition of microneedles: from lab worktops to diagnostic care centers

Microneedle (MN) technology plays a significant role in bioengineering as it allows for minimally invasive exposure to the skin via the non-invasive procedure, increased drug permeability, and improved biological molecule detectability in the epidermal layers, all while improving therapeutic safety and effectiveness. However, MNs have several significant drawbacks, including difficulty scaling up, variability in drug delivery pattern regarding the skin's external environment, blockage of dermal tissues, induction of inflammatory response at the administration site, and limitation of dosing based on the molecular weight of drug and size. Despite these drawbacks, MNs have emerged as a special transdermal theranostics instrument in clinical research to assess physiological parameters. Bioimaging technology relies on microneedles that can measure particular analytes in the extracellular fluid effectively by crossing the stratum corneum, making them "a unique tool in diagnostics detection and therapeutic application inside the body." This review article discusses the recent advances in the applications especially related to the diagnostics and toxicity challenges of microneedles. In addition, this review article discusses the clinical state and commercial accessibility of microneedle technology-based devices in order to provide new information to scientists and researchers.

Three Dimensional Printing and Its Applications Focusing on Microneedles for Drug Delivery

Microneedles (MNs) are considered to be a novel smart injection system that causes significantly low skin invasion upon puncturing, due to the micron-sized dimensions that pierce into the skin painlessly. This allows transdermal delivery of numerous therapeutic molecules, such as insulin and vaccines. The fabrication of MNs is carried out through conventional old methods such as molding, as well as through newer and more sophisticated technologies, such as three-dimensional (3D) printing, which is considered to be a superior, more accurate, and more time- and production-efficient method than conventional methods. Three-dimensional printing is becoming an innovative method that is used in education through building intricate models, as well as being employed in the synthesis of fabrics, medical devices, medical implants, and orthoses/prostheses. Moreover, it has revolutionary applications in the pharmaceutical, cosmeceutical, and medical fields. Having the capacity to design patient-tailored devices according to their dimensions, along with specified dosage forms, has allowed 3D printing to stand out in the medical field. The different techniques of 3D printing allow for the production of many types of needles with different materials, such as hollow MNs and solid MNs. This review covers the benefits and drawbacks of 3D printing, methods used in 3D printing, types of 3D-printed MNs, characterization of 3D-printed MNs, general applications of 3D printing, and transdermal delivery using 3D-printed MNs.

Dissolvable microneedles for transdermal drug delivery showing skin pentation and modified drug release

Topical therapies for chronic skin diseases suffer from a low patient compliance due to the inconvenient treatment regimens of available products. Dissolvable microneedles (MN) with modified release offer an interesting possibility to increase the compliance by acting as a depot in the skin and thereby decreasing the dosing frequency. Furthermore, the bioavailability can be increased significantly by bypassing the barrier of the skin by the direct penetration of the MN into the skin. In this study the depot effect and skin penetration of an innovative dissolvable MN patch was assessed by insertion in ex vivo human skin and in vivo using minipigs. The MN patches are based on biodegradable polymers and the active pharmaceutical ingredients calcipotriol (Calci) and betamethasone-17-21-dipropionate (BDP) used to treat psoriasis. Using computed tomography (CT) and Coherent anti-Stokes Raman scattering (CARS) microscopy it was possible to visualize the skin penetration and follow the morphology of the MN as function of time in the skin. The depot effect was assessed by studying the modified in vitro release in an aqueous buffer and by comparing the drug release of a single application of a patch both ex vivo and in vivo to daily application of a marketed oleogel containing the same active pharmaceutical ingredients. The CT and CARS images showed efficient penetration of the MN patches into the upper dermis and a slow swelling process of the drug containing tip over a period of 8 days. Furthermore, CARS demonstrated that it can be used as a noninvasive technique with potential applicability in clinical settings. The in vitro release studies show a release of 54% over a time period of 30 days. The pharmacological relevance of MNs was confirmed in human skin explants and in vivo after single application and showed a similar response on calcipotriol and BDP mediated signaling events compared to daily application of the active oleogel. Altogether it was demonstrated that the MN can penetrate the skin and have the potential to provide a depot effect.

Metallic Microneedles for Transdermal Drug Delivery: Applications, Fabrication Techniques and the Effect of Geometrical Characteristics

Current procedures for transdermal drug delivery (TDD) have associated limitations including poor administration of nucleic acid, small or large drug molecules, pain and stress for needle phobic people. A painless micro-sized device capable of delivering drugs easily and efficiently, eliminating the disadvantages of traditional systems, has yet to be developed. While polymeric-based microneedle (MN) arrays have been used successfully and clinically as TDD systems, these devices lack mechanical integrity, piercing capacity and the ability to achieve tailored drug release into the systemic circulation. Recent advances in micro/nano fabrication techniques using Additive Manufacturing (AM), also known as 3D printing, have enabled the fabrication of metallic MN arrays, which offer the potential to overcome the limitations of existing systems. This review summarizes the different types of MNs used in TDD and their mode of drug delivery. The application of MNs in the treatment of a range of diseases including diabetes and cancer is discussed. The potential role of solid metallic MNs in TDD, the various techniques used for their fabrication, and the influence of their geometrical characteristics (e.g., shape, size, base diameter, thickness, and tip sharpness) on effective TDD are explored. Finally, the potential and the future directions relating to the optimization of metallic MN arrays for TDD are highlighted.

Evaluation of design and insertion analysis of a conical shaped polymeric based microneedle for transdermal drug delivery applications

Transportation of drug through parental routes are conventionally followed through hypodermic injection methods, where hypodermic injections are administered into the human skin for drug release. However, there are some issues observed when these hypodermic needles are being used, there are instances where the needle is being inserted leaves some needle fractures in the skin. To cater to the issue scientific researchers are voraciously working on designing and developing polymeric type of microneedle structures for various medical diagnostic applications for glucose monitoring, drug delivery, and other applications. This article presents the structural design of a conicalshaped polymeric microneedle and the insertion force while being pierced into the skin. Simulations at different insertion angles on microneedle are analyzed by arriving with total needle displacements in the process of insertion. The von mises stress is also analyzed with applied force at different insertion angles resulted in incremental change in stress exerted by the microneedle. The resultant stress is below the yield stress which makes the microneedle pierce into the skin without breakage.

The Finite Element Analysis Research on Microneedle Design Strategy and Transdermal Drug Delivery System

Microneedles (MNs) as a novel transdermal drug delivery system have shown great potential for therapeutic and disease diagnosis applications by continually providing minimally invasive, portable, cost-effective, high bioavailability, and easy-to-use tools compared to traditional parenteral administrations. However, microneedle transdermal drug delivery is still in its infancy. Many research studies need further in-depth exploration, such as safety, structural characteristics, and drug loading performance evaluation. Finite element analysis (FEA) uses mathematical approximations to simulate real physical systems (geometry and load conditions). It can simplify complex engineering problems to guide the precise preparation and potential industrialization of microneedles, which has attracted extensive attention. This article introduces FEA research for microneedle transdermal drug delivery systems, focusing on microneedle design strategy, skin mechanics models, skin permeability, and the FEA research on drug delivery by MNs.

Hydrogel-forming microarray patches with cyclodextrin drug reservoirs for long-acting delivery of poorly soluble cabotegravir sodium for HIV Pre-Exposure Prophylaxis

Hydrogel-forming microarray patches (HF-MAPs) offer minimally invasive, pain-free and prolonged drug delivery. These devices are designed to be self-administered and self-disabling, avoiding contaminated sharps waste generation. Cabotegravir sodium (CAB-Na) is a poorly soluble anti- human immunodeficiency virus (HIV) drug for the treatment and pre-exposure prophylaxis of HIV infection that lends itself to depot formation following intradermal delivery but presents significant challenges when delivered via HF-MAPs, whose nature is aqueous. Herein, we have investigated, for the first time, the use of hydroxypropyl-β-cyclodextrin (HP-β-CD) to enhance the solubility of CAB-Na, and its effect on intradermal delivery via HF-MAPs. Accordingly, tablet reservoirs containing CAB-Na and HP-β-CD were formulated. These novel reservoirs were combined with two different HF-MAP formulations (MAP1 (Gantrez S97® + poly (ethylene glycol) 10,000 + Na₂CO₃) and MAP2 (poly (vinyl pyrrolidone) 58 kDa + poly (vinyl alcohol) 85-120 kDa + citric acid)) to form fully integrated MAP devices which were tested in both ex vivo and in vivo settings. Ex vivo skin deposition results for MAP1 and MAP2 showed that 141 ± 40 μg and 342 ± 34 μg of CAB-Na was deposited into 0.5 cm² of excised neonatal porcine skin after 24 h, respectively. Based on these findings, the in vivo pharmacokinetics of MAP2 were investigated over 28 days using a Sprague-Dawley rat model. After 24 h patch application, MAP2 demonstrated an extended drug release profile and an observed C_max of 53.4 ± 10.16 μg/mL, superior to that of an FDA-approved CAB-nanosuspension administered via intramuscular application (C_max of 43.6 ± 5.3 μg/mL). Consequently, this tablet integrated MAP device is considered to be a viable option for the intradermal delivery of hydrophobic anti-HIV drugs.

Ciprofloxacin-loaded dissolving polymeric microneedles as a potential therapeutic for the treatment of S. aureus skin infections

Microneedles have been widely studied for many topical and transdermal therapeutics due to their ability to painlessly puncture the skin, thereby bypassing the stratum corneum, the main skin barrier. In this study, ciprofloxacin (CIP) was loaded into dissolving polymeric microneedles prepared by a two-layer centrifugation method as a potential treatment of skin infections such as cellulitis. The polymers used were polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP). Two formulations were investigated, namely CIP_MN1, composed of 10 mg ciprofloxacin incorporated into a polymer matrix of PVA and PVP with a weight ratio of (9:1), and CIP_MN2, composed of 10 mg ciprofloxacin incorporated into PVA polymer. CIP_MN1 and CIP_MN2 showed a mean microneedle height of 188 and 179 µm, respectively. Since Parafilm has been proven as a model to examine the perforation of microneedles in skin, it was used to evaluate the ability of microneedles to perforate the skin. CIP_MN1 showed almost complete perforation of Parafilm, 190 pores, compared to CIP_MN2 which created only 85 pores in Parafilm, and therefore CIP_MN1 was used for subsequent studies. Examining CIP_MN1 on agarose gel as an in vitro model of human skin showed that the formula was able to fully perforate the agarose gel. Moreover, this formula showed significantly greater antimicrobial activity (p < 0.0001) compared to a free gel of ciprofloxacin against Staphylococcus aureus in an agarose gel-based model. This was evidenced by a zone of inhibition of 29 mm for the microneedle formulation of ciprofloxacin (CIP_MN1) compared to 2 mm for the free gel of ciprofloxacin. Furthermore, the CIP_MN1 showed complete dissolution in human skin after 60 min from application. Finally, the skin deposition of CIP_MN1 was investigated in ex vivo excised human skin. CIP_MN1 showed significantly more deposition of ciprofloxacin in deeper skin layers compared to the free gel of ciprofloxacin, and the released ciprofloxacin from the microneedles tends to migrate to deeper layers with time. Collectively, these results suggest that CIP_MN1 can be a potential delivery system for the treatment of S. aureus skin infections.

The Role of 3D Printing Technology in Microengineering of Microneedles

Microneedles (MNs) are minimally invasive devices, which have gained extensive interest over the past decades in various fields including drug delivery, disease diagnosis, monitoring, and cosmetics. MN geometry and shape are key parameters that dictate performance and therapeutic efficacy, however, traditional fabrication methods, such as molding, may not be able to offer rapid design modifications. In this regard, the fabrication of MNs using 3D printing technology enables the rapid creation of complex MN prototypes with high accuracy and offers customizable MN devices with a desired shape and dimension. Moreover, 3D printing shows great potential in producing advanced transdermal drug delivery systems and medical devices by integrating MNs with a variety of technologies. This review aims to demonstrate the advantages of exploiting 3D printing technology as a new tool to microengineer MNs. Various 3D printing methods are introduced, and representative MNs manufactured by such approaches are highlighted in detail. The development of advanced MN devices is also included. Finally, clinical translation and future perspectives for the development of MNs using 3D printing are discussed.

Bioimaging of Dissolvable Microneedle Arrays: Challenges and Opportunities

The emergence of microneedle arrays (MNAs) as a novel, simple, and minimally invasive administration approach largely addresses the challenges of traditional drug delivery. In particular, the dissolvable MNAs act as a promising, multifarious, and well-controlled platform for micro-nanotransport in medical research and cosmetic formulation applications. The effective delivery mostly depends on the behavior of the MNAs penetrated into the body, and accurate assessment is urgently needed. Advanced imaging technologies offer high sensitivity and resolution visualization of cross-scale, multidimensional, and multiparameter information, which can be used as an important aid for the evaluation and development of new MNAs. The combination of MNA technology and imaging can generate considerable new knowledge in a cost-effective manner with regards to the pharmacokinetics and bioavailability of active substances for the treatment of various diseases. In addition, noninvasive imaging techniques allow rapid, receptive assessment of transdermal penetration and drug deposition in various tissues, which could greatly facilitate the translation of experimental MNAs into clinical application. Relying on the recent promising development of bioimaging, this review is aimed at summarizing the current status, challenges, and future perspective on in vivo assessment of MNA drug delivery by various imaging technologies.

Elongated microparticles tuned for targeting hyaluronic acid delivery to specific skin strata

OBJECTIVE

Microneedle or fractional laser applications are the most common topical delivery enhancement platforms. However, these methods of drug delivery are not skin strata specific. Drug delivery approaches which could target specific stratum of the skin remains a challenge. Elongated microparticles (EMPs) have been used in enhancing drug delivery into the skin. The aim of this study was to evaluate, for the first time, elongated silica microparticles with two different length profiles to enhance delivery of hyaluronic acid into different strata of human skin.

METHODS

Two types of EMPs - long (milled EMPs) or short (etched EMPs) length ranges were characterized. A prototypical liquid formulation (Fluorescent hyaluronic acid) with and without EMP enhancement were evaluated for hyaluronic acid delivery in ex-vivo human skin. High Performance Liquid Chromatography (HPLC), Typhoon fluorescence scanning system, Laser Scanning Confocal Microscopy (LSCM) and Reflectance Confocal Microscopy (RCM) were used to validate F-HA stability, visualize fluorescein in the skin, image the depth of F-HA delivery in the skin and define EMP penetration in skin strata, respectively. Statistical analysis was conducted using GraphPad Prism 6 software (GraphPad Software Inc, USA).

RESULTS

Fluorescein-hyaluronic acid was stable and EMP enhanced skin penetration. Reflectance confocal microscopy revealed that "etched EMP" penetrated the skin to the stratum spinosum level. The vast majority (97.8%; p < 0.001) of the etched EMP did not penetrate completely through the viable epidermis and no obvious penetration into the dermis. In contrast, milled EMP showed 41-fold increase in penetration compared to the etched EMP but penetrated beyond the dermoepidermal junction.

CONCLUSION

EMPs can enhance delivery of hyaluronic acid. Using EMPs with defined length distributions, which can be tuned for a specific stratum of the skin, can achieve targeted hyaluronic acid delivery.

Fracture behaviour of human skin in deep needle insertion can be captured using validated cohesive zone finite-element method

Medical needles have shown an appreciable contribution to the development of novel medical devices and surgical technologies. A better understanding of needle-skin interactions can advance the design of medical needles, modern surgical robots, and haptic devices. This study employed finite element (FE) modelling to explore the effect of different mechanical and geometrical parameters on the needle's force-displacement relationship, the required force for the skin puncture, and generated mechanical stress around the cutting zone. To this end, we established a cohesive FE model, and identified its parameters by a three-stage parameter identification algorithm to closely replicate the experimental data of needle insertion into the human skin available in the literature. We showed that a bilinear cohesive model with initial stiffness of 5000 MPa/mm, failure traction of 2 MPa, and separation length of 1.6 mm can lead to a model that can closely replicate experimental results. The FE results indicated that while the coefficient of friction between the needle and skin substantially changes the needle reaction force, the insertion velocity does not have a noticeable effect on the reaction force. Regarding the geometrical parameters, needle cutting angle is the prominent factor in terms of stress fields generated in the skin tissue. However, the needle diameter is more influential on the needle reaction force. We also presented an energy study on the frictional dissipation, damage dissipation, and strain energy throughout the insertion process.

Insights into the mechanics of solid conical microneedle array insertion into skin using the finite element method

In order to develop optimum microneedle designs, researchers must first develop robust, repeatable and adaptable test methods which are representative of in vivo conditions. However, there is a lack of experimental tools which can accurately comparatively interrogate functional microneedle penetration of tissue. In this study, we seek to develop a state of the art finite element model of microneedle insertion into and penetration of human skin. The developed model employs a 3D hyperelastic, anisotropic pre-stressed multi-layered material which more accurately reflects in vivo skin conditions, while the microneedle is modeled as an array, which can capture the influence of adjacent microneedles on the overall response. Using the developed finite element model, we highlight the importance of accurate computational modeling which can decipher the mechanics of microneedle insertion, including the influence of its position within an array and how it correlates well with experimental observations. In particular, we have concluded that, for our model microneedle array, increasing skin pretension from 0 to 10% strain reduces the penetration force by 13%, ultimate local deformation about the microneedle by 22% and the ultimate penetration efficiency by 15%. We have also concluded that the presence of a base plate limits the penetration efficiency by up to 24%, while the penetration efficiency across a 5 × 1 microneedle array may vary by 27%. This model elucidates, for the first time, the combined effects of skin tension and needle geometry on accurately predicting microneedle penetration efficiency. STATEMENT OF SIGNIFICANCE: Microneedles arrays (MNAs) are medical devices with microscale protrusions, typically designed to penetrate the outermost layer of the skin, that upon optimisation, could lead to disruptive minimally-invasive disease management. However, the mechanics of MNA insertion are complex, due in part to a 'bed of nails' effect, and difficult to elucidate experimentally. Therefore, comparisons between designs, functional assessment of production batches and ultimately the likelihood of clinical translation are challenging to predict. Here, we have develop the most sophisticated in silico model of MNA insertion into pre-tensioned human skin to predict the extent of MNA penetration and therefore the likelihood of successful therapeutic delivery. Researchers can customise this model to predict the penetration efficiency of any MNA design.

Microneedles in Drug Delivery: Progress and Challenges

In recent years, an innovative transdermal delivery technology has attracted great interest for its ability to distribute therapeutics and cosmeceuticals for several applications, including vaccines, drugs, and biomolecules for skin-related problems. The advantages of microneedle patch technology have been extensively evaluated in the latest literature; hence, the academic publications in this area are rising exponentially. Like all new technologies, the microneedle patch application has great potential but is not without limitations. In this review, we will discuss the possible limitations by highlighting the areas where a great deal of improvements are required. Emphasising these concerns early on should help scientists and technologists to address the matters in a timely fashion and to use their resources wisely.

Dissolving Microneedles Developed in Association with Nanosystems: A Scoping Review on the Quality Parameters of These Emerging Systems for Drug or Protein Transdermal Delivery

The largest organ of the body provides the main challenge for the transdermal delivery of lipophilic or high molecular weight drugs. To cross the main barrier of the skin, the stratum corneum, many techniques have been developed and improved. In the last 20 years, the association of microneedles with nanostructured systems has gained prominence for its versatility and for enabling targeted drug delivery. Currently, the combination of these mechanisms is pointed to as an emerging technology; however, some gaps need to be answered to transcend the development of these devices from the laboratory scale to the pharmaceutical market. It is known that the lack of regulatory guidelines for quality control is a hindrance to market conquest. In this context, this study undertakes a scoping review of original papers concerning methods applied to evaluate both the quality and drug/protein delivery of dissolving and hydrogel-forming microneedles developed in association with nanostructured systems.

Nanomedicines and microneedles: a guide to their analysis and application

The fast-advancing progress in the research of nanomedicine and microneedle applications in the past two decades has suggested that the combination of the two concepts could help to overcome some of the challenges we are facing in healthcare. They include poor patient compliance with medication and the lack of appropriate administration forms that enable the optimal dose to reach the target site. Nanoparticles as drug vesicles can protect their cargo and deliver it to the target site, while evading the body's defence mechanisms. Unfortunately, despite intense research on nanomedicine in the past 20 years, we still haven't answered some crucial questions, e.g. about their colloidal stability in solution and their optimal formulation, which makes the translation of this exciting technology from the lab bench to a viable product difficult. Dissolvable microneedles could be an effective way to maintain and stabilise nano-sized formulations, whilst enhancing the ability of nanoparticles to penetrate the stratum corneum barrier. Both concepts have been individually investigated fairly well and many analytical techniques for tracking the fate of nanomaterials with their precious cargo, both in vitro and in vivo, have been established. Yet, to the best of our knowledge, a comprehensive overview of the analytical tools encompassing the concepts of microneedles and nanoparticles with specific and successful examples is missing. In this review, we have attempted to briefly analyse the challenges associated with nanomedicine itself, but crucially we provide an easy-to-navigate scheme of methods, suitable for characterisation and imaging the physico-chemical properties of the material matrix.

Panorama of dissolving microneedles for transdermal drug delivery

Recently, microfabrication technology has been developed to increase the permeability of drugs for transdermal delivery. Microneedles are ultra-small needles usually in the micron size range (different dimensions in micron), generate pores, and allow for delivery of local medication in the systemic circulation via skin. The microneedles have been available in dissolving, solid, coated, hollow, and hydrogel-based microneedles. Dissolving microneedles have been fabricated using micro-molding, photo-polymerization, drawing lithography and droplet blowing techniques. Dissolving microneedles could be a valuable option for the delivery of low molecular weight drugs, peptides, enzymes, vaccines and bio-therapeutics. It consists of water-soluble materials including maltose, polyvinyl pyrrolidone, chondroitin sulfate, dextran, hyaluronic acid, and albumin. The microneedles have almost dissolved after patch removal, leaving only blunt stubs behind, which are easily removable. In this review, we summarize the major building blocks, classification, fabrication techniques, characterization, diffusion models and application of microneedles in diverse area. We also reviewed the regulatory aspects, computational studies, patents, clinical data, and market trends of microneedles.

Translation of Polymeric Microneedles for Treatment of Human Diseases: Recent Trends, Progress, and Challenges

The ongoing search for biodegradable and biocompatible microneedles (MNs) that are strong enough to penetrate skin barriers, easy to prepare, and can be translated for clinical use continues. As such, this review paper is focused upon discussing the key points (e.g., choice polymeric MNs) for the translation of MNs from laboratory to clinical practice. The review reveals that polymers are most appropriately used for dissolvable and swellable MNs due to their wide range of tunable properties and that natural polymers are an ideal material choice as they structurally mimic native cellular environments. It has also been concluded that natural and synthetic polymer combinations are useful as polymers usually lack mechanical strength, stability, or other desired properties for the fabrication and insertion of MNs. This review evaluates fabrication methods and materials choice, disease and health conditions, clinical challenges, and the future of MNs in public healthcare services, focusing on literature from the last decade.

3D Printing-A "Touch-Button" Approach to Manufacture Microneedles for Transdermal Drug Delivery

Microneedles (MNs) represent the concept of attractive, minimally invasive puncture devices of micron-sized dimensions that penetrate the skin painlessly and thus facilitate the transdermal administration of a wide range of active substances. MNs have been manufactured by a variety of production technologies, from a range of materials, but most of these manufacturing methods are time-consuming and expensive for screening new designs and making any modifications. Additive manufacturing (AM) has become one of the most revolutionary tools in the pharmaceutical field, with its unique ability to manufacture personalized dosage forms and patient-specific medical devices such as MNs. This review aims to summarize various 3D printing technologies that can produce MNs from digital models in a single step, including a survey on their benefits and drawbacks. In addition, this paper highlights current research in the field of 3D printed MN-assisted transdermal drug delivery systems and analyzes parameters affecting the mechanical properties of 3D printed MNs. The current regulatory framework associated with 3D printed MNs as well as different methods for the analysis and evaluation of 3D printed MN properties are outlined.

Finger-Actuated Microneedle Array for Sampling Body Fluids

The application of microneedles (MNs) for minimally invasive biological fluid sampling is rapidly emerging, offering a user-friendly approach with decreased insertion pain and less harm to the tissues compared to conventional needles. Here, a finger-powered microneedle array (MNA) integrated with a microfluidic chip was conceptualized to extract body fluid samples. Actuated by finger pressure, the microfluidic device enables an efficient approach for the user to collect their own body fluids in a simple and fast manner without the requirement for a healthcare worker. The processes for extracting human blood and interstitial fluid (ISF) from the body and the flow across the device, estimating the amount of the extracted fluid, were simulated. The design in this work can be utilized for the minimally invasive personalized medical equipment offering a simple usage procedure.

Engineering Microneedle Patches for Improved Penetration: Analysis, Skin Models and Factors Affecting Needle Insertion

Transdermal microneedle (MN) patches are a promising tool used to transport a wide variety of active compounds into the skin. To serve as a substitute for common hypodermic needles, MNs must pierce the human stratum corneum (~ 10 to 20 µm), without rupturing or bending during penetration. This ensures that the cargo is released at the predetermined place and time. Therefore, the ability of MN patches to sufficiently pierce the skin is a crucial requirement. In the current review, the pain signal and its management during application of MNs and typical hypodermic needles are presented and compared. This is followed by a discussion on mechanical analysis and skin models used for insertion tests before application to clinical practice. Factors that affect insertion (e.g., geometry, material composition and cross-linking of MNs), along with recent advancements in developed strategies (e.g., insertion responsive patches and 3D printed biomimetic MNs using two-photon lithography) to improve the skin penetration are highlighted to provide a backdrop for future research.

Flexible and porous microneedles of PDMS for continuous glucose monitoring

Microneedle (MN) is a key technology of the biomedical engineering field due to its capability of accessing the biological information in a minimally invasive manner. One of the huge demands for next-generation healthcare monitoring is continuous monitoring, especially of blood glucose concentration. For this, MN should be kept inserted into the human skin for a certain period of time, enduring stresses induced by daily human motion and at the same time measuring biomarkers in ISF. However, conventional MNs for biosensing are not suitable for a long term insertion due to the rigid structure and biological risks of MN breakage. In this study, a novel MN structure is proposed and investigated by combining flexible "sponge-like" porous PDMS matrix and coating by biodissolving hyaluronic acid (HA). The fabricated porous MNs coated with HA show ideal mechanical characteristics, by which the MNs are rigid enough to penetrate the skin and become flexible after insertion into the skin. It is also shown that the MN array successfully extracts ISF in vitro and in vivo not by capillary action but by repeated compressions. The results show the applicability of the flexible MNs to continuous blood glucose monitoring.

Current trends in polymer microneedle for transdermal drug delivery

Transdermal drug delivery using microneedles is increasingly gaining interest due to the issues associated with oral drug delivery routes. Gastrointestinal route exposes the drug to acid and enzymes present in the stomach, leading to denaturation of the compound and resulting in poor bioavailability. Microneedle transdermal drug delivery addresses the problems linked to oral delivery and to relieves the discomfort of patients associated with injections to increase patient compliance. Microneedles can be broadly classified into five types: solid microneedles, coated microneedles, dissolving microneedles, hollow microneedles, and hydrogel-forming microneedles. The materials used for the preparation of microneedles dictate the different applications and features present in the microneedle. Polymeric microneedle arrays present an improved method for transdermal administration of drugs as they penetrate the skin stratum corneum barrier with minimal invasiveness. The review summarizes the importance of polymeric microneedle and discussed some of the most important therapeutic drugs in research, mainly protein drugs, vaccines and small molecule drugs in regenerative medicine.

Finite Element Analysis for Predicting Skin Pharmacokinetics of Nano Transdermal Drug Delivery System Based on the Multilayer Geometry Model

Background

Skin pharmacokinetics is an indispensable indication for studying the drug fate after administration of transdermal drug delivery systems (TDDS). However, the heterogeneity and complex skin structured with stratum corneum, viable epidermis, dermis, and subcutaneous tissue inevitably leads the drug diffusion coefficient (Kp) to vary depending on the skin depth, which seriously limits the development of TDDS pharmacokinetics in full thickness skin.

Methods

A multilayer geometry skin model was established and the Kp of drug in SC, viable epidermis, and dermis was obtained using the technologies of molecular dynamics simulation, in vitro permeation experiments, and in vivo microdialysis, respectively. Besides, finite element analysis (FEA) based on drug Kps in different skin layers was applied to simulate the paeonol nanoemulsion (PAE-NEs) percutaneous dynamic penetration process in two and three dimensions. In addition, PAE-NEs skin pharmacokinetics profile obtained by the simulation was verified by in vivo experiment.

Results

Coarse-grained modeling of molecular dynamic simulation was successfully established and the Kp of PAE in SC was 2.00×10⁻⁶ cm²/h. The Kp of PAE-NE in viable epidermis and in dermis detected using penetration test and microdialysis probe technology, was 1.58×10⁻⁵ cm²/h and 3.20×10⁻⁵ cm²/h, respectively. In addition, the results of verification indicated that PAE-NEs skin pharmacokinetics profile obtained by the simulation was consistent with that by in vivo experiment.

Discussion

This study demonstrated that the FEA combined with the established multilayer geometry skin model could accurately predict the skin pharmacokinetics of TDDS.

Intradermal and transdermal drug delivery using microneedles - Fabrication, performance evaluation and application to lymphatic delivery

The progress in microneedle research is evidenced by the transition from simple 'poke and patch' solid microneedles fabricated from silicon and stainless steel to the development of bioresponsive systems such as hydrogel-forming and dissolving microneedles. In this review, we provide an outline on various microneedle fabrication techniques which are currently employed. As a range of factors, including materials, geometry and design of the microneedles, affect the performance, it is important to understand the relationships between them and the resulting delivery of therapeutics. Accordingly, there is a need for appropriate methodologies and techniques for characterization and evaluation of microneedle performance, which will also be discussed. As the research expands, it has been observed that therapeutics delivered via microneedles has gained expedited access to the lymphatics, which makes them a favorable delivery method for targeting the lymphatic system. Such opportunity is valuable in the area of vaccination and treatment of lymphatic disorders, which is the final focus of the review.

Transdermal Microneedles-A Materials Perspective

Transdermal drug delivery is an emerging field in the pharmaceutical remit compared with conventional methods (oral and parenteral). Microneedle (MN)-based devices have gained significant interest as a strategy to overcome the skin's formidable barrier: the stratum corneum. This approach provides a less invasive, more efficient, patient friendly method of drug delivery with the ability to incorporate various therapeutic agents including macromolecules (proteins and peptides), anti-cancer agents and other hydrophilic and hydrophobic compounds. This short review attempts to assess the various materials involved in the fabrication of MNs as well as incorporation of other excipients to improve drug delivery for novel medical devices. The focus will be on polymers, metals and other inorganic materials utilised for MN drug delivery, as well as their application, limitations and future work to be carried out.

Recent advances of controlled drug delivery using microfluidic platforms

Conventional systematically-administered drugs distribute evenly throughout the body, get degraded and excreted rapidly while crossing many biological barriers, leaving minimum amounts of the drugs at pathological sites. Controlled drug delivery aims to deliver drugs to the target sites at desired rates and time, thus enhancing the drug efficacy, pharmacokinetics, and bioavailability while maintaining minimal side effects. Due to a number of unique advantages of the recent microfluidic lab-on-a-chip technology, microfluidic lab-on-a-chip has provided unprecedented opportunities for controlled drug delivery. Drugs can be efficiently delivered to the target sites at desired rates in a well-controlled manner by microfluidic platforms via integration, implantation, localization, automation, and precise control of various microdevice parameters. These features accordingly make reproducible, on-demand, and tunable drug delivery become feasible. On-demand self-tuning dynamic drug delivery systems have shown great potential for personalized drug delivery. This review presents an overview of recent advances in controlled drug delivery using microfluidic platforms. The review first briefly introduces microfabrication techniques of microfluidic platforms, followed by detailed descriptions of numerous microfluidic drug delivery systems that have significantly advanced the field of controlled drug delivery. Those microfluidic systems can be separated into four major categories, namely drug carrier-free micro-reservoir-based drug delivery systems, highly integrated carrier-free microfluidic lab-on-a-chip systems, drug carrier-integrated microfluidic systems, and microneedles. Microneedles can be further categorized into five different types, i.e. solid, porous, hollow, coated, and biodegradable microneedles, for controlled transdermal drug delivery. At the end, we discuss current limitations and future prospects of microfluidic platforms for controlled drug delivery.

Insulin delivery systems combined with microneedle technology

Diabetes, a metabolic disorder of glucose, is a serious chronic disease and an important public health problem. Insulin is one of the hormones for modulating blood glucose level and the products of which is indispensable for most diabetes patients. Introducing microneedles (MNs) to insulin delivery is promising to pave the way for modulating glucose level noninvasively of diabetes patients, as which born to be painless, easy to handle and no need of any power supply. In this work, we review the process of insulin delivery systems (IDSs) based on MN technology in terms of two categories: drug free MNs and drug loaded MNs. Drug free MNs include solid MNs ("poke and patch"), hollow MNs ("poke and flow") and reservoir-based swelling MNs ("poke and swell R-type"), and drug loaded MNs include coated MNs ("coat and poke"), dissolving MNs ("poke and release") and insulin incorporated swelling MNs ("poke and swell I-type"). Majority researches of MN-based IDSs have been conducted by using hollow MNs or dissolving MNs, and almost all clinical trials for MN-based IDSs have employed hollow MNs. "Poke and patch" approach dramatically increase skin permeability compared to traditional transdermal patch, but MNs fabricated from silicon or metal may leave sharp waste in the skin and cause a safety issue. "Poke and flow" approach, similar to transitional subcutaneous (SC) injection, is capable of producing faster insulin absorption and action than SC injection but may associate with blockage, leakage and low flow rate. Coated MNs are able of retaining the activity of drug, which loaded in a solid phase, for a long time, however have been relatively less studied for insulin application as the low drug dosing. "Poke and release" approach leaves no biohazardous sharp medical waste and is capable of rapid drug release. "Poke and swell R-type" can be seen as a combination of "poke and flow" and "poke and patch" approach, while "poke and swell I-type" is an approach between "coat and poke" and "poke and release" approach. Insulin MNs are promising for painless diabetes therapeutics, and additional efforts for addressing fundamental issues including the drug loading, the PK/PD profile, the storage and the safety of insulin MNs will accelerate the clinical transformation.

Microfabrication for Drug Delivery

This review is devoted to discussing the application of microfabrication technologies to target challenges encountered in life processes by the development of drug delivery systems. Recently, microfabrication has been largely applied to solve health and pharmaceutical science issues. In particular, fabrication methods along with compatible materials have been successfully designed to produce multifunctional, highly effective drug delivery systems. Microfabrication offers unique tools that can tackle problems in this field, such as ease of mass production with high quality control and low cost, complexity of architecture design and a broad range of materials. Presented is an overview of silicon- and polymer-based fabrication methods that are key in the production of microfabricated drug delivery systems. Moreover, the efforts focused on studying the biocompatibility of materials used in microfabrication are analyzed. Finally, this review discusses representative ways microfabrication has been employed to develop systems delivering drugs through the transdermal and oral route, and to improve drug eluting implants. Additionally, microfabricated vaccine delivery systems are presented due to the great impact they can have in obtaining a cold chain-free vaccine, with long-term stability. Microfabrication will continue to offer new, alternative solutions for the development of smart, advanced drug delivery systems.