Mathematical Modelling, Simulation and Optimisation of Microneedles for Transdermal Drug Delivery: Trends and Progress

Optimising distribution of hollow microneedle in arrays for transdermal drug delivery considering effects of tissue compression on drug permeability

Hollow microneedles (HMNs) are advanced transdermal drug delivery (TDD) devices that can create micro-perforations in the skin, enabling high molecular weight drugs to bypass the resistance of the skin's top layer, the stratum corneum. These MNs have been shown to enhance drug permeability K in skin with minimal or no discomfort. It is recognised that the variability and interplay of different MN parameters, including the MN distribution within an array, can significantly influence the HMN-based TDD rate. However, optimising these parameters can achieve consistent MN performance. In recognising this, our paper aims to develop a theoretical framework for optimising the HMN distribution in the arrays. The methodology involves formulating an optimisation function g based on different geometrical and physical properties of the MN and skin and associating this with the skin permeability (K) of drugs delivered via the HMNs. The methodology has been established to quantitatively assess the impact of MN design parameters on drug diffusion through the skin, including the effects of compressive strain caused by HMN insertion. The studyshows that an elevated value of g is associated with increased drug K in the skin. The results from the established framework have enabled us to determine the optimal design for various HMN configurations. The data generated from this optimisation technique is subsequently utilised to estimate the skin K of several high molecular weight drugs delivered through an HMN system.

Modelling receptor-mediated endocytosis in hollow microneedle-based verapamil delivery through viscoelastic skin

Drug delivered from the microneedle (MN) tip diffuses across the viscoelastic skin before entering the blood compartment and being absorbed. Reversible uptake kinetics between the blood and tissue compartments, reversible specific saturable binding with its receptors, and endocytosis are given due attention. Simulations predict that, unlike skin thinning, skin viscoelasticity and a higher Young's modulus value, as in an older person, inhibit verapamil diffusion within the skin, and metabolism stabilises the concentrations in the blood and tissue compartments. Simultaneously, the irreversible uptake kinetics improve drug concentrations in the tissue compartment, facilitating receptor-mediated endocytosis. The results also predict that internalised verapamil increases with time at slower internalisation rates; however, at higher rates, it attains a peak value before gradually diminishing. Furthermore, as the rate of lysosomal degradation escalates, the peak value of internalised concentration diminishes and shifts upward. A comprehensive sensitivity analysis has been performed because of uncertainty about several crucial parameters. Our findings align well with the existing literature.

Multiphysics modelling of the impact of skin deformation and strain on microneedle-based transdermal therapeutic delivery

Microneedle patches (MNs) hold enormous potential to facilitate the minimally-invasive delivery of drugs and vaccines transdermally. However, the micro-mechanics of skin deformation significantly influence the permeation of therapeutics through the skin. Previous studies often fail to appreciate the complexities in microneedle-skin mechanical interactions. This may impede the accuracy of MNs pre-clinical assessments. Here, we develop a multiphysics finite element model which simulates the biomechanics of microneedle skin penetration and the subsequent permeation of therapeutics. Employing the aqueous pore path hypothesis, we consider how strain (induced through the insertion of a MN), affects pore geometry in the skin and therefore the diffusion of therapeutics. Our models show that considering the insertion-induced skin deformation alone reduces the transdermal permeation of insulin by 25 %, while considering the effect of strain can reduce the overall permeation by a further 45 % over 24 h. Our model also indicates that once the mechanical strain is removed i.e. through removal or dissolution of the array, the permeation through the skin will recover. Furthermore, our results indicate that the delivery of high molecular weight compounds may be most susceptible to strain-induced changes in drug permeation. These findings could have significant implications for the preferred type of microneedle administration when targeting, for example, intradermal or transdermal delivery. STATEMENT OF SIGNIFICANCE: This manuscript presents an advanced computational model of microneedle insertion into human skin. Here, we adopt a multiphysics modelling strategy, where we predict the influence of microneedle insertion on skin deformation and strain and how that influences subsequent therapeutic permeation through the skin. Our model predicts that whether or not the microneedle remains in situ, the resultant change in tissue deformation and strain has a major impact on how quickly the therapeutic diffuses through the skin. This has important implications for transdermal device design, administration strategies and protocols and associated clinical studies, where either intradermal or transdermal therapeutic delivery is being targetted.

Mathematical modelling of hollow microneedle-mediated transdermal drug delivery

Hollow microneedles represent a promising approach for overcoming the protective barrier of the stratum corneum, facilitating direct drug infusion into viable skin tissue and thereby enhancing the efficacy of transdermal delivery. However, delivery outcomes across different skin layers and into the systemic circulation can vary substantially due to the diverse properties of drug delivery systems, clinical settings, and environmental factors. The optimal strategies for enhancing the efficiency of hollow microneedle-mediated transdermal drug delivery remain to be elucidated. This study employs mathematical modelling and a reconstructed skin model with realistic anatomical structures to investigate drug transport and accumulation across different skin layers and into the bloodstream under different delivery conditions. The modelling results reveal the crucial role of interstitial fluid flow in determining drug transport in this transdermal delivery. Delivery outcomes of each skin layer and blood exhibit distinct responses to changes in delivery conditions. Specifically, increasing the vascular permeability or nanocarrier diffusivity raises drug concentration in the blood or reticular dermis, respectively, while leading to reductions in other skin layers. The use of microneedles with narrower infusion channels can only enhance drug availability in the viable epidermis. Optimisation requires a tailored approach to several parameters depending on the target skin layer, including drug release rate, infusion rate, infusion duration, and microneedle length. Environmental factors that promote trans-epidermal water loss can increase drug concentration in the viable epidermis but have a limited impact on deeper skin tissues. The findings support the selection or customisation of hollow microneedles and nanocarriers to address specific therapeutic needs, such as targeting specific skin layers or systemic circulation, while minimising the risk of side effects from high drug concentrations in normal tissues. This study provides guidance for optimising transdermal drug delivery systems.

Quality-by-design principles applied to the development and optimisation of lidocaine-loaded dissolving microneedle arrays - a proof-of-concept

The use of dissolving microneedle arrays (dMNA) for intradermal and transdermal drug delivery has been a growing trend in the field for the past decades. However, a lack of specific regulatory standards still hinders their clinical development and translation to market. It is also well-known that dMNA composition significantly impacts their performance, with each new formulation potentially presenting a challenge for developers, manufacturers and regulatory agencies. A systematic approach such as quality-by-design (QbD), which embeds quality from the very beginning of the product development process, allows the design and optimisation of a drug-loaded dMNA formulation with promising features. In this work, we defined the Quality Target Product Profile (QTPP) for lidocaine-loaded dMNA and optimised their composition through a sequential design of experiments (DoE) approach. The first step (DoE_1) confirmed the influence of all formulation components (PVP, PVA and sucrose) in the properties of the arrays and pre-optimised their settings for DoE_2. The array characterisation focused on previously defined critical quality attributes (drug content, dissolution time, mechanical strength, skin insertion and physical attributes). At its maximum desirability (85.15%), the optimised design space obtained in DoE_2 is predicted to produce Li-dMNA with high mechanical strength (< 10% needle height reduction), skin insertion (> 90% needle height) and Li-HCl loading (~ 5 mg), good physical attributes and dissolving in a maximum of 60 min. The flexible design space obtained allows for the production of dMNA that consistently meet the QTPP, reducing batch failure and end-product testing, which are common in the more rigid GMP approach. Overall, applying QbD principles to formulation development shows promise to increase product quality and facilitate translation of dMNA into the clinic.

Modelling insertion behaviour of PVP (Polyvinylpyrrolidone) and PVA (Polyvinyl Alcohol) microneedles

A comprehensive investigation into the effects of nonlinear material behaviour of polymeric (MN) and skin on the dynamics of the MN insertion in skin was undertaken in this study using experiments and numerical simulations. The nonlinearity of the material behaviour was incorporated by employing the Ramberg-Osgood and neo-Hookean equations for stress-strain relationships for the MN materials and skin, respectively. For this purpose, a characteristic type of dissolving MN array was selected. This type of MN is made by a combination of poly(vinyl alcohol) and poly(vinyl pyrrolidone). The numerical simulations were validated using experimental investigations where the MNs were fabricated using laser-engineered silicone micromould templates technology. Young's modulus, Poisson's ratio, and compression breaking force for the MN polymers were determined using a texture analyser. The alignment between experimental findings and simulation data underscores the accuracy of the parameters determined through mechanical testing and mathematical calculations for both MN materials (PVP/PVA) and skin behaviour during the MN insertion. This study has demonstrated a strong alignment between the experimental findings and computational simulations, confirming the accuracy of the established parameters for MNs and skin interactions for modelling MN insertion behaviour in skin, providing a solid foundation for future research in this area.

Leveraging Numerical Simulation Technology to Advance Drug Preparation: A Comprehensive Review of Application Scenarios and Cases

BACKGROUND/OBJECTIVES

Numerical simulation plays an important role in pharmaceutical preparation recently. Mechanistic models, as a type of numerical model, are widely used in the study of pharmaceutical preparations. Mechanistic models are based on a priori knowledge, i.e., laws of physics, chemistry, and biology. However, due to interdisciplinary reasons, pharmacy researchers have greater difficulties in using computer models.

METHODS

In this paper, we highlight the application scenarios and examples of mechanistic modelling in pharmacy research and provide a reference for drug researchers to get started.

RESULTS

By establishing a suitable model and inputting preparation parameters, researchers can analyze the drug preparation process. Therefore, mechanistic models are effective tools to optimize the preparation parameters and predict potential quality problems of the product. With product quality parameters as the ultimate goal, the experiment design is optimized by mechanistic models. This process emphasizes the concept of quality by design.

CONCLUSIONS

The use of numerical simulation saves experimental cost and time, and speeds up the experimental process. In pharmacy experiments, part of the physical information and the change processes are difficult to obtain, such as the mechanical phenomena during tablet compression and the airflow details in the nasal cavity. Therefore, it is necessary to predict the information and guide the formulation with the help of mechanistic models.

Modeling of Dissolving Microneedle-Based Transdermal Drug Delivery: Effects of Dynamics of Polymers in Solution

Dissolving microneedle (DMN)-assisted transdermal drug delivery (TDD) has received attention from the scientific community in recent years due to its ability to control the rate of drug delivery through its design, the choice of polymers, and its composition. The dissolution of the polymer depends strongly on the polymer-solvent interaction and polymer physics. Here, we developed a mathematical model based on the physicochemical parameters of DMNs and polymer physics to determine the drug release profiles. An annular gap width is defined when the MN is inserted in the skin, accumulating interstitial fluid (ISF) from the surrounding skin and acting as a boundary layer between the skin and the MN. Poly(vinylpyrrolidone) (PVP) is used as a model dissolving polymer, and ceftriaxone is used as a representative drug. The model agrees well with the literature data for ex vivo permeation studies, along with the percent height reduction of the MN. Based on the suggested mathematical model, when loading 0.39 mg of ceftriaxone, the prediction indicates that approximately 93% of the drug will be cleared from the bloodstream within 24 h. The proposed modeling strategy can be utilized to optimize drug transport behavior using DMNs.

Microneedle transdermal drug delivery as a candidate for the treatment of gouty arthritis: Material structure, design strategies and prospects

Gouty arthritis (GA) is caused by monosodium urate (MSU) crystals deposition. GA is difficult to cure because of its complex disease mechanism and the tendency to reoccur. GA patients require long-term uric acid-lowering and anti-inflammatory treatments. In the past ten years, as a painless, convenient and well-tolerated new drug transdermal delivery method, microneedles (MNs) administration has been continuously developed, which can realize various drug release modes to deal with various complex diseases. Compared with the traditional administration methods (oral and injection), MNs are more conducive to the long-term independent treatment of GA patients because of their safe, efficient and controllable drug delivery ability. In this review, the pathological mechanism of GA and common therapeutic drugs for GA are summarized. After that, MNs drug delivery mechanisms were summarized: dissolution release mechanism, swelling release mechanism and channel-assisted release mechanism. According to drug delivery patterns of MNs, the mechanisms and applications of rapid-release MNs, long-acting MNs, intelligent-release MNs and multiple-release MNs were reviewed. Additionally, existing problems and future trends of MNs in the treatment of GA were also discussed. STATEMENT OF SIGNIFICANCE: Gout is an arthritis caused by metabolic disease "hyperuricemia". Epidemiological studies show that the number of gouty patients is increasing rapidly worldwide. Due to the complex disease mechanism and recurrent nature of gout, gouty patients require long-term therapy. However, traditional drug delivery modes (oral and injectable) have poor adherence, low drug utilization, and lack of local localized targeting. They may lead to adverse effects such as rashes and gastrointestinal reactions. As a painless, convenient and well-tolerated new drug transdermal delivery method, microneedles have been continuously developed, which can realize various drug release modes to deal with gouty arthritis. In this review, the material structure, design strategy and future outlook of microneedles for treating gouty arthritis will be reviewed.

Design of poly(vinyl pyrrolidone) and poly(ethylene glycol) microneedle arrays for delivering glycosaminoglycan, chondroitin sulfate, and hyaluronic acid

Osteoarthritis (OA) is a prevalent joint disorder characterized by cartilage and bone degradation. Medical therapies like glucosaminoglycan (GAG), chondroitin sulfate (CS), and hyaluronic acid (HA) aim to preserve joint function and reduce inflammation but may cause side effects when administered orally or via injection. Microneedle arrays (MNAs) offer a localized drug delivery method that reduces side effects. Thus, this study aims to demonstrate the feasibility of delivering GAG, CS, and HA using microneedles in vitro. An optimal needle geometry is crucial for the successful application of MNA. To address this, here we employ a multi-objective optimization framework using the non-dominated sorting genetic algorithm II (NSGA-II) to determine the ideal MNA design, focusing on preventing needle failure. Then, a three-step fabrication approach is followed to fabricate the MNAs. First, the master (male) molds are fabricated from poly(methyl methacrylate) using mechanical micromachining based on optimized needle geometry. Second, a micro-molding with Polydimethylsiloxane (PDMS) is used for the fabrication of production (female) molds. In the last step, the MNAs were fabricated by microcasting the hydrogels using the production molds. Light microscopy (LIMI) confirms the accuracy of the MNAs manufactured, and in vitro skin insertion tests demonstrate failure-free needle insertion. Subsequently, we confirmed the biocompatibility of MNAs by evaluating their impact on the L929 fibroblast cell line, human chondrocytes, and osteoblasts.

Fabrication of Hybrid Coated Microneedles with Donepezil Utilizing Digital Light Processing and Semisolid Extrusion Printing for the Management of Alzheimer's Disease

Microneedle (MN) patches are gaining increasing attention as a cost-effective technology for delivering drugs directly into the skin. In the present study, two different 3D printing processes were utilized to produce coated MNs, namely, digital light processing (DLP) and semisolid extrusion (SSE). Donepezil (DN), a cholinesterase inhibitor administered for the treatment of Alzheimer's disease, was incorporated into the coating material. Physiochemical characterization of the coated MNs confirmed the successful incorporation of donepezil as well as the stability and suitability of the materials for transdermal delivery. Optical microscopy and SEM studies validated the uniform weight distribution and precise dimensions of the MN arrays, while mechanical testing ensured the MNs' robustness, ensuring efficient skin penetration. In vitro studies were conducted to evaluate the produced transdermal patches, indicating their potential use in clinical treatment. Permeation studies revealed a significant increase in DN permeation compared to plain coating material, affirming the effectiveness of the MNs in enhancing transdermal drug delivery. Confocal laser scanning microscopy (CLSM) elucidated the distribution of the API, within skin layers, demonstrating sustained drug release and transcellular transport pathways. Finally, cell studies were also conducted on NIH3T3 fibroblasts to evaluate the biocompatibility and safety of the printed objects for transdermal applications.

Performance gain and electro-mechanical design optimization of microneedles for wearable sensor systems

Minimally invasive microneedle (MN) is an emerging technology platform for wearable and implantable diagnostics and therapeutics systems. These short MNs offer pain-free insertion and simple operation. Among the MN technologies proposed to enhance interstitial fluid (ISF) extraction, porous and swellable (P-S) hydrogels absorb analyte molecules across the entire lateral surface. Currently, the design, development, and optimization of the MNs rely on empirical, iterative approaches. Based on theory of fluid flow and analyte diffusion through geometrically complex biomimetic systems, here we derive a generalized physics-guided model for P-S MN sensors. The framework (a) quantifies MN extracting efficiency [Formula: see text] in terms of its geometric and physical properties, and (b) suggests strategies to optimize sensor response while satisfying the mechanical constraints related to various skin-types (e.g., mouse, pig, humans, etc.). Our results show that, despite the differences in geometry and composition, P-S MNs obey a universal scaling response, [Formula: see text] with [Formula: see text] being MN length, diffusivity, and radius, respectively, and [Formula: see text], [Formula: see text] and [Formula: see text] are the ratio between approximate vs. exact analytical solutions, the effective biofluid transfer coefficient between dermis and skin, and the exponent for the power-law approximation, respectively. These parameters quantify the biomolecule transfer through the dermis-to-MN interface at different scaling limits. P-S MNs outperform hollow MNs by a 2-6x enhancement factor; however, the buckling-limit of insertion defines the maximized functionality of the sensor. Our model, validated against experimental results and numerical simulations, offers a predictive design framework to significantly reduce the optimization time for P-S MN-based sensor platforms.

Long-acting microneedle formulations

The minimally-invasive and painless nature of microneedle (MN) application has enabled the technology to obviate many issues with injectable drug delivery. MNs not only administer therapeutics directly into the dermal and ocular space, but they can also control the release profile of the active compound over a desired period. To enable prolonged delivery of payloads, various MN types have been proposed and evaluated, including dissolving MNs, polymeric MNs loaded or coated with nanoparticles, fast-separable MNs hollow MNs, and hydrogel MNs. These intricate yet intelligent delivery platforms provide an attractive approach to decrease side effects and administration frequency, thus offer the potential to increase patient compliance. In this review, MN formulations that are loaded with various therapeutics for long-acting delivery to address the clinical needs of a myriad of diseases are discussed. We also highlight the design aspects, such as polymer selection and MN geometry, in addition to computational and mathematical modeling of MNs that are necessary to help streamline and develop MNs with high translational value and clinical impact. Finally, up-scale manufacturing and regulatory hurdles along with potential avenues that require further research to bring MN technology to the market are carefully considered. It is hoped that this review will provide insight to formulators and clinicians that the judicious selection of materials in tandem with refined design may offer an elegant approach to achieve sustained delivery of payloads through the simple and painless application of a MN patch.

Enhanced Skin Permeation and Controlled Release of β-Sitosterol Using Cubosomes Encrusted with Dissolving Microneedles for the Management of Alopecia

The use of synthetic medication for treating alopecia is restricted because of systemic exposure and related negative effects. Beta-sitosterol (β-ST), a natural chemical, has lately been studied for its potential to promote hair development. The cubosomes with dissolving microneedles (CUBs-MN_D) created in this study may be a useful starting point for the creation of a sophisticated dermal delivery system for β-ST. Cubosomes (CUBs) were prepared by the emulsification method, using glyceryl monooleate (GMO) as a lipid polymer. CUBs were loaded with dissolving microneedles (MN_D) fabricated with HA and a PVP-K90 matrix. An ex vivo skin permeation study and an in vivo hair growth efficacy test of β-ST were performed with both CUB and CUB-MN_D. The average particle size of the CUBs was determined to be 173.67 ± 0.52 nm, with a low polydispersity index (0.3) and a high zeta potential value that prevents the aggregate formation of dispersed particles. When compared to CUBs alone, CUBs-MN_D displayed higher permeating levels of β-ST at all-time points. In the animals from the CUB-MN_D group, significant hair development was observed. According to the results of the current investigation, CUBs that integrate dissolving microneedles of β-ST are superior in terms of transdermal skin penetration and activity for the treatment of alopecia.

Coupled Diffusion-Binding-Deformation Modelling for Phase-Transition Microneedles-Based Drug Delivery

Phase-transition microneedles (PTMNs)-based transdermal drug delivery (TDD) is gaining popularity due to its non-invasiveness and ability to deliver a wide range of drugs. PTMNs absorb interstitial skin fluid (ISF) and transport drugs from microneedle (MNs) domain to the skin without polymer dissolution. To establish PTMNs for practical use, one needs to understand and optimise the key parameters governing drug transport mechanisms to achieve controlled drug delivery. In addressing this point, we have developed a coupled diffusion-binding-deformation model to understand the effect of physicochemical parameters (e.g., swelling capacity, drug binding) of MN and skin mechanical properties on overall drug transport behaviour. The contact mechanics at the MN and skin interface is introduced to account for the resistive force exerted by the deformed skin to MN swelling. The model is validated with the reported data of in vitro insulin delivery using polyvinyl alcohol (PVA) MN. The drug binding parameters are estimated from the fitting of the cumulative release of insulin within 6 hours of MN insertion. To predict the in vivo data of insulin delivery using the PVA MN, one-compartment model of drug pharmacokinetics is incorporated. It is shown in the paper that the model is able to predict the final insulin concentration in blood and in good agreement with the reported experimental data. The proposed model is concluded to be a tool for the predictive design and development of PTMNs-based TDD systems.

Microneedle arrays integrated with microfluidic systems: Emerging applications and fluid flow modeling

Microneedle arrays are patches of needles at micro- and nano-scale, which are competent and versatile technologies that have been merged with microfluidic systems to construct more capable devices for biomedical applications, such as drug delivery, wound healing, biosensing, and sampling body fluids. In this paper, several designs and applications are reviewed. In addition, modeling approaches used in microneedle designs for fluid flow and mass transfer are discussed, and the challenges are highlighted.

Glucose-Responsive Silk Fibroin Microneedles for Transdermal Delivery of Insulin

Microneedles (MNs) have attracted great interest as a drug delivery alternative to subcutaneous injections for treating diabetes mellitus. We report MNs prepared from polylysine-modified cationized silk fibroin (SF) for responsive transdermal insulin delivery. Scanning electron microscopy analysis of MNs' appearance and morphology revealed that the MNs were well arranged and formed an array with 0.5 mm pitch, and the length of single MNs is approximately 430 μm. The average breaking force of an MN is above 1.25 N, which guarantees that it can pierce the skin quickly and reach the dermis. Cationized SF MNs are pH-responsive. MNs dissolution rate increases as pH decreases and the rate of insulin release are accelerated. The swelling rate reached 223% at pH = 4, while only 172% at pH = 9. After adding glucose oxidase, cationized SF MNs are glucose-responsive. As the glucose concentration increases, the pH inside the MNs decreases, the MNs' pore size increases, and the insulin release rate accelerates. In vivo experiments demonstrated that in normal Sprague Dawley (SD) rats, the amount of insulin released within the SF MNs was significantly smaller than that in diabetic rats. Before feeding, the blood glucose (BG) of diabetic rats in the injection group decreased rapidly to 6.9 mmol/L, and the diabetic rats in the patch group gradually reduced to 11.7 mmol/L. After feeding, the BG of diabetic rats in the injection group increased rapidly to 33.1 mmol/L and decreased slowly, while the diabetic rats in the patch group increased first to 21.7 mmol/L and then decreased to 15.3 mmol/L at 6 h. This demonstrated that the insulin inside the microneedle was released as the blood glucose concentration increased. Cationized SF MNs are expected to replace subcutaneous injections of insulin as a new modality for diabetes treatment.

Microneedle-Mediated Transdermal Delivery of Biopharmaceuticals

Transdermal delivery provides numerous benefits over conventional routes of administration. However, this strategy is generally limited to a few molecules with specific physicochemical properties (low molecular weight, high potency, and moderate lipophilicity) due to the barrier function of the stratum corneum layer. Researchers have developed several physical enhancement techniques to expand the applications of the transdermal field; among these, microneedle technology has recently emerged as a promising platform to deliver therapeutic agents of any size into and across the skin. Typically, hydrophilic biomolecules cannot penetrate the skin by passive diffusion. Microneedle insertion disrupts skin integrity and compromises its protective function, thus creating pathways (microchannels) for enhanced permeation of macromolecules. Microneedles not only improve stability but also enhance skin delivery of various biomolecules. Academic institutions and industrial companies have invested substantial resources in the development of microneedle systems for biopharmaceutical delivery. This review article summarizes the most recent research to provide a comprehensive discussion about microneedle-mediated delivery of macromolecules, covering various topics from the introduction of the skin, transdermal delivery, microneedles, and biopharmaceuticals (current status, conventional administration, and stability issues), to different microneedle types, clinical trials, safety and acceptability of microneedles, manufacturing and regulatory issues, and the future of microneedle technology.

Development of an Automated Design Tool for FEM-Based Characterization of Solid and Hollow Microneedles

Microneedle design for biomedical applications, such as transdermal drug delivery, vaccination and transdermal biosensing, has lately become a rapidly growing research field. In this sense, finite element analysis has been extendedly used by microneedle designers to determine the most suitable structural parameters for their prototypes, and also to predict their mechanical response and efficiency during the insertion process. Although many proposals include computer-aided tools to build geometrical models for mechanical analysis, there is a lack of software utilities intended to automate the design process encompassing geometrical modeling, simulation setup and postprocessing of results. This work proposes a novel MATLAB-based design tool for microneedle arrays that permits personalized selection of the basic characteristics of a mechanical model. The tool automatically exports the selected options to an ANSYS batch file, including instructions to run a static and a linear buckling analysis. Later, the subsequent simulation results can be retrieved for on-screen display and potential postprocessing. In addition, this work reviews recent proposals (2018-2022) about finite element model characterization of microneedles to establish the minimum set of features that any tool intended for automating a design process should provide.

The clinical and translational prospects of microneedle devices, with a focus on insulin therapy for diabetes mellitus as a case study

Microneedles have the clinical advantage of being able to deliver complex drugs across the skin in a convenient and comfortable manner yet haven't successfully transitioned to medical practice. Diabetes mellitus is a complicated disease, which is commonly treated with multiple daily insulin injections, contributing to poor treatment adherence. Firstly, this review determines the clinical prospect of microneedles, alongside considerations that ought to be addressed before microneedle technology can be translated from bench to bedside. Thereafter, we use diabetes as a case study to consider how microneedle-based-technology may be successfully harnessed. Here, publications referring to insulin microneedles were evaluated to understand whether insertion efficiency, angle of insertion, successful dose delivery, dose adjustability, material biocompatibility and therapeutic stability are being addressed in early stage research. Moreover, over 3,000 patents from 1970-2019 were reviewed with the search term '"microneedle" AND "insulin"' to understand the current status of the field. In conclusion, the reporting of early stage microneedle research demonstrated a lack of consistency relating to the translational factors addressed. Additionally, a more rational design, based on a patient-centred approach is required before microneedle-based delivery systems can be used to revolutionise the lives of people living with diabetes following regulatory approval.

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.

The Rise of Polymeric Microneedles: Recent Developments, Advances, Challenges, and Applications with Regard to Transdermal Drug Delivery

The current scenario of the quest for microneedles (MNs) with biodegradability and biocompatibility properties is a potential research area of interest. Microneedles are considered to be robust, can penetrate the skin's deep-seated layers, and are easy to manufacture, and their applications from the clinical perspective are still ongoing with standard escalation. This review paper focuses on some of the pivotal variants of polymeric microneedles which are specifically dissolvable and swell-based MNs. It further explores the drug dissolution kinetics and insertion behavior mechanisms with an emphasis on the need for mathematical modeling of MNs. This review further evaluates the multifarious fabrication methods, with an update on the advances in the fabrication of polymeric MNs, the choice of materials used for the fabrication, the challenges in polymeric MN fabrication, and the prospects of polymeric MNs with applications pertinent to healthcare, by exclusively focusing on the procurable literature over the last decade.

Super-swelling Hydrogel-forming Microneedle based Transdermal Drug Delivery: Mathematical Modelling, Simulation and Experimental Validation

Super-swelling hydrogel-forming microneedles (HFMNs) based transdermal drug delivery (TDD) is gaining significant interest due to their non-invasiveness and ability to deliver a wide range of drugs. The HFMNs swell by imbibing interstitial skin fluid (ISF), and they facilitate drug transport from the reservoir attached at the base into the skin without polymer dissolution. To develop HFMNs for practical applications, a complete understanding of the drug transport mechanism is required, allowing for controlled TDD and geometrical optimisation. A three-phase system consisting of a reservoir, microneedle, and skin is considered. A mathematical model is developed to incorporate the drug binding within the matrix of the compartment, which was not considered earlier. Super-swelling nature of the HFMNs is incorporated through the swelling ratio obtained experimentally for a polymer. The results are validated with in vitro diffusion studies of ibuprofen sodium (IBU) across excised porcine skin, showing that around 20% of the loaded IBU in lyophilised wafer was delivered in 24 hours. It was observed that increasing IBU solubility in reservoir can achieve high drug transport across the skin. The developed model is shown to be in good agreement with the experimental data. It is concluded that the proposed model can be considered a tool with predictive design and development of super-swelling HFMNs based TDD systems.

Potential of Microneedle Systems for COVID-19 Vaccination: Current Trends and Challenges

To prevent the coronavirus disease 2019 (COVID-19) pandemic and aid restoration to prepandemic normality, global mass vaccination is urgently needed. Inducing herd immunity through mass vaccination has proven to be a highly effective strategy for preventing the spread of many infectious diseases, which protects the most vulnerable population groups that are unable to develop immunity, such as people with immunodeficiencies or weakened immune systems due to underlying medical or debilitating conditions. In achieving global outreach, the maintenance of the vaccine potency, transportation, and needle waste generation become major issues. Moreover, needle phobia and vaccine hesitancy act as hurdles to successful mass vaccination. The use of dissolvable microneedles for COVID-19 vaccination could act as a major paradigm shift in attaining the desired goal to vaccinate billions in the shortest time possible. In addressing these points, we discuss the potential of the use of dissolvable microneedles for COVID-19 vaccination based on the current literature.

Determination of Transdermal Rate of Metallic Microneedle Array through an Impedance Measurements-Based Numerical Check Screening Algorithm

Microneedle systems have been widely used in health monitoring, painless drug delivery, and medical cosmetology. Although many studies on microneedle materials, structures, and applications have been conducted, the applications of microneedles often suffered from issues of inconsistent penetration rates due to the complication of skin-microneedle interface. In this study, we demonstrated a methodology of determination of transdermal rate of metallic microneedle array through impedance measurements-based numerical check screening algorithm. Metallic sheet microneedle array sensors with different sizes were fabricated to evaluate different transdermal rates. In vitro sensing of hydrogen peroxide confirmed the effect of transdermal rate on the sensing outcomes. An FEM simulation model of a microneedle array revealed the monotonous relation between the transdermal state and test current. Accordingly, two methods were primely derived to calculate the transdermal rate from the test current. First, an exact logic method provided the number of unpenetrated tips per sheet, but it required more rigorous testing results. Second, a fuzzy logic method provided an approximate transdermal rate on adjacent areas, being more applicable and robust to errors. Real-time transdermal rate estimation may be essential for improving the performance of microneedle systems, and this study provides various fundaments toward that goal.

Process optimization of Ca2+ cross-linked alginate-based swellable microneedles for enhanced transdermal permeability: More applicable to acidic drugs

We describe a swellable microneedle (SMN) consisting of Ca²⁺ cross-linked alginate, which expands the types of natural polymers available for SMN fabrication. After investigation of different fabrication methods, the alginate in situ hydrogel-based SMN with a flat substrate was successfully constructed, whose gelation was triggered by ethylenediaminetetraacetic acid calcium disodium salt and D-(+)-glucono-1,5-lactone. With the addition of polyvinyl alcohol and trehalose, SMN possessed good mechanical properties. The biocompatibility of SMN was demonstrated through the tests of in vitro cytotoxicity and in vivo skin irritation. With the assistance of SMN, the in vitro transdermal delivery efficiencies of drugs were significantly improved throughout 16 h. 3-O-ethyl ascorbic acid (EAA, pH = 4.81) exhibited a cumulative release of up to 83.83 ± 6.30%, which was consistent with zero-order kinetics, while tranexamic acid (TA, pH = 6.90) showed the most significant increase in delivery efficiency, which was consistent with the Higuchi model and Ritger-Peppas model. The SMN remained intact after the 16 h of EAA transdermal delivery, indicating its better suitability for acidic drugs. We believe that this technology has the potential to expand the range of drugs available for transdermal administration as well as the breadth of patient care applications.

Emergence of microneedles as a potential therapeutics in diabetes mellitus

Diabetes mellitus is a severe condition in which the pancreas produces inadequate insulin or the insulin generated is ineffective for utilisation by the body; as a result, insulin therapy is required for control blood sugar levels in patients having type 1 diabetes and is widely recommended in advanced type 2 diabetes patients with uncontrolled diabetes despite dual oral therapy, while subcutaneous insulin administration using hypodermic injection or pump-mediated infusion is the traditional route of insulin delivery and causes discomfort, needle phobia, reduced adherence, and risk of infection. Therefore, transdermal insulin delivery has been extensively explored as an appealing alternative to subcutaneous approaches for diabetes management which not only is non-invasive and easy, but also avoids first-pass metabolism and prevents gastrointestinal degradation. Microneedles have been commonly investigated in human subjects for transdermal insulin administration because they are minimally invasive and painless. The different types of microneedles developed for the transdermal delivery of anti-diabetic drugs are discussed in this review, including solid, dissolving, hydrogel, coated, and hollow microneedles. Numerous microneedle products have entered the market in recent years. But, before the microneedles can be effectively launched into the market, a significant amount of investigation is required to address the numerous challenges. In conclusion, the use of microneedles in the transdermal system is an area worth investigating because of its significant benefits over the oral route in the delivery of anti-diabetic medications and biosensing of blood sugar levels to assure improved clinical outcomes in diabetes management.

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.