Recent progress of 3D-printed microneedles for transdermal drug delivery

3D printed microneedles: revamping transdermal drug delivery systems

One of the advancements of the transdermal drug delivery system (TDDS) is the development of microneedles (MNs). These micron-sized needles are used for delivering various types of drugs to address the disadvantage of other transdermal techniques as well as oral drug delivery systems. MNs have high patient acceptance due to self-administration with minimally invasive and pain compared to the parenteral drug delivery. Over the years, various methods have been adopted to evolve the MNs and make them more cost-effective, accurate, and suitable for multiple applications. One such method is the 3D printing of MNs. The development of MN platforms using 3D printing has been made possible by improved features like precision, printing resolution, and the feasibility of using low-cost raw materials. In this review, we have tried to explain various types of MNs, fabrication methods, materials used in the formulation of MNs, and the recent applications that utilize 3D-printed MNs.

Additive manufacturing of microneedles for sensing and drug delivery

INTRODUCTION

Microneedles (MNs) are miniaturized, painless, and minimally invasive platforms that have attracted significant attention over recent decades across multiple fields, such as drug delivery, disease monitoring, disease diagnosis, and cosmetics. Several manufacturing methods have been employed to create MNs; however, these approaches come with drawbacks related to complicated, costly, and time-consuming fabrication processes. In this context, employing additive manufacturing (AM) technology for MN fabrication allows for the quick production of intricate MN prototypes with exceptional precision, providing the flexibility to customize MNs according to the desired shape and dimensions. Furthermore, AM demonstrates significant promise in the fabrication of sophisticated transdermal drug delivery systems and medical devices through the integration of MNs with various technologies.

AREAS COVERED

This review offers an extensive overview of various AM technologies with great potential for the fabrication of MNs. Different types of MNs and the materials utilized in their fabrication are also discussed. Recent applications of 3D-printed MNs in the fields of transdermal drug delivery and biosensing are highlighted.

EXPERT OPINION

This review also mentions the critical obstacles, including drug loading, biocompatibility, and regulatory requirements, which must be resolved to enable the mass-scale adoption of AM methods for MN production, and future trends.

Customizable Fabrication of Photothermal Microneedles with Plasmonic Nanoparticles Using Low-Cost Stereolithography Three-Dimensional Printing

Photothermal microneedle (MN) arrays have the potential to improve the treatment of various skin conditions such as bacterial skin infections. However, the fabrication of photothermal MN arrays relies on time-consuming and potentially expensive microfabrication and molding techniques, which limits their size and translation to clinical application. Furthermore, the traditional mold-and-casting method is often limited in terms of the size customizability of the photothermal array. To overcome these challenges, we fabricated photothermal MN arrays directly via 3D-printing using plasmonic Ag/SiO2 (2 wt % SiO2) nanoaggregates dispersed in ultraviolet photocurable resin on a commercial low-cost liquid crystal display stereolithography printer. We successfully printed MN arrays in a single print with a translucent, nanoparticle-free support layer and photothermal MNs incorporating plasmonic nanoaggregates in a selective fashion. The photothermal MN arrays showed sufficient mechanical strength and heating efficiency to increase the intradermal temperature to clinically relevant temperatures. Finally, we explored the potential of photothermal MN arrays to improve antibacterial therapy by killing two bacterial species commonly found in skin infections. To the best of our knowledge, this is the first time describing the printing of photothermal MNs in a single step. The process introduced here allows for the translatable fabrication of photothermal MN arrays with customizable dimensions that can be applied to the treatment of various skin conditions such as bacterial infections.

Flexible microneedles incorporating gold nanorods and tacrolimus for effective synergistic photothermal-chemotherapy of rheumatoid arthritis

Transdermal drug delivery systems for rheumatoid arthritis (RA) have garnered substantial attention due to their great potential to overcome limitations observed in conventional oral or injection strategies, including limited selectivity and adverse effects on extra-articular tissues. Microneedles (MNs) appear to be highly desirable carriers for transdermal drug delivery of RA. However, microneedles typically are unable to keep up with the flexibility of joints, which decreases the effectiveness of administration. In this study, we developed a flexible microneedles (FMNs) delivery system. And gelatin was employed for the fabrication of flexible backings for microneedles owing to its excellent ductility and biocompatibility. We achieved synergisticphotothermal-chemotherapy of RA by incorporating the chemical drug Tacrolimus (TAC) and the photothermal agent gold nanorods (AuNRs) into dissolving microneedles. Results showed a high mechanical strength of the proposed FMNs. In the animal model of adjuvant-induced arthritis (AA), it is indicated that the prepared FMNs inhibited the expression of related inflammatory cytokines such as IL-1ß and TNF-α while enhancing bone repair and other related factors. Thus, the combination therapy of FMNs-mediated hyperthermia and chemotherapy can serve as a novel and synergistic treatment option for RA.

Optimization of stereolithography 3D printing of microneedle micro-molds for ocular drug delivery

Microneedles (MN) have emerged as an innovative technology for drug delivery, offering a minimally invasive approach to administer therapeutic agents. Recent applications have included ocular drug delivery, requiring the manufacture of sub-millimeter needle arrays in a reproducible and reliable manner. The development of 3D printing technologies has facilitated the fabrication of MN via mold production, although there is a paucity of information available regarding how the printing parameters may influence crucial issues such as sharpness and penetration efficacy. In this study, we have developed and optimized a 3D-printed MN micro-mold using stereolithography (SLA) 3D printing to prepare a dissolving ocular MN patch. The effects of a range of parameters including aspect ratio, layer thickness, length, mold shape and printing orientation have been examined with regard to both architecture and printing accuracy of the MN micro-mold, while the effects of printing angle on needle fidelity was also examined for a range of basic shapes (conical, pyramidal and triangular pyramidal). Mechanical strength and in vitro penetration of the polymeric (PVP/PVA) MN patch produced from reverse molds fabricated using MN with a range of shapes and height, and aspect ratios were assessed, followed by ex vivo studies of penetration into excised scleral and corneal tissues. The optimization process identified the parameters required to produce MN with the sharpest tips and highest dimensional fidelity, while the ex vivo studies indicated that these optimized systems would penetrate the ocular tissue with minimal applied pressure, thereby allowing ease of patient self-administration.

3D printing processes in precise drug delivery for personalized medicine

With the advent of personalized medicine, the drug delivery system will be changed significantly. The development of personalized medicine needs the support of many technologies, among which three-dimensional printing (3DP) technology is a novel formulation-preparing process that creates 3D objects by depositing printing materials layer-by-layer based on the computer-aided design method. Compared with traditional pharmaceutical processes, 3DP produces complex drug combinations, personalized dosage, and flexible shape and structure of dosage forms (DFs) on demand. In the future, personalized 3DP drugs may supplement and even replace their traditional counterpart. We systematically introduce the applications of 3DP technologies in the pharmaceutical industry and summarize the virtues and shortcomings of each technique. The release behaviors and control mechanisms of the pharmaceutical DFs with desired structures are also analyzed. Finally, the benefits, challenges, and prospects of 3DP technology to the pharmaceutical industry are discussed.

Design and fabrication of customizable microneedles enabled by 3D printing for biomedical applications

Microneedles (MNs) is an emerging technology that employs needles ranging from 10 to 1000 μm in height, as a minimally invasive technique for various procedures such as therapeutics, disease monitoring and diagnostics. The commonly used method of fabrication, micromolding, has the advantage of scalability, however, micromolding is unable to achieve rapid customizability in dimensions, geometries and architectures, which are the pivotal factors determining the functionality and efficacy of the MNs. 3D printing offers a promising alternative by enabling MN fabrication with high dimensional accuracy required for precise applications, leading to improved performance. Furthermore, enabled by its customizability and one-step process, there is propitious potential for growth for 3D-printed MNs especially in the field of personalized and on-demand medical devices. This review provides an overview of considerations for the key parameters in designing MNs, an introduction on the various 3D-printing techniques for fabricating this new generation of MNs, as well as highlighting the advancements in biomedical applications facilitated by 3D-printed MNs. Lastly, we offer some insights into the future prospects of 3D-printed MNs, specifically its progress towards translation and entry into market.

Three-Dimensional Printing as a Progressive Innovative Tool for Customized and Precise Drug Delivery

While using three-dimensional printing, materials are deposited layer by layer in accordance with the digital model created by computer-aided design software. Numerous research teams have shown interest in this technology throughout the last few decades to produce various dosage forms in the pharmaceutical industry. The number of publications has increased since the first printed medicine was approved in 2015 by Food and Drug Administration. Considering this, the idea of creating complex, custom-made structures that are loaded with pharmaceuticals for tissue engineering and dose optimization is particularly intriguing. New approaches and techniques for creating unique medication delivery systems are made possible by the development of additive manufacturing keeping in mind the comparative advantages it has over conventional methods of manufacturing medicaments. This review focuses on three-dimensional printed formulations grouped in orally disintegrated tablets, buccal films, implants, suppositories, and microneedles. The various types of techniques that are involved in it are summarized. Additionally, challenges and applications related to three-dimensional printing of pharmaceuticals are also being discussed.

Printable-Microencapsulated Ascorbic Acid for Personalized Topical Delivery

This study presents the successful development of printable-microencapsulated ascorbic acid (AA) for personalized topical delivery using laser printing technology. Rice flour with a 10% AA content was selected as an encapsulation material. Hydrophobic nanosilica was used to create negative electrostatic charges on the microencapsulated surfaces via a high-speed mixture. This process facilitated the microencapsulated AA fabrication using a commercial laser printer and produced a well-patterned design with some minor print defects, such as banding and scattering. The amount of encapsulated AA per area was 0.28 mg/cm2, and the RGB color code was 0,0,0. An emulsion carrier system comprising pentylene glycol (P5G) or diethylene glycol monoethyl ether (DEGEE), Tween 20, oleic acid, and deionized (DI) water at a ratio of 20:30:30:20 was developed to enhance AA transmission into the skin. The Franz diffusion cell technique was used to investigate topical absorption on Strat-M membranes using P5G and DEGEE as enhancers. The steady-state fluxes were 8.40 (±0.64) and 10.04 (±0.58) μg/h/cm2 for P5G and DEGEE, respectively. Cytotoxicity tests conducted on fibroblast cells revealed low cytotoxicity for the encapsulation products and carriers.

A Bibliometric Analysis of 3D Printing in Personalized Medicine Research from 2012 to 2022

In recent years, the 3D printing of personalized drug formulations has attracted the attention of medical practitioners and academics. However, there is a lack of data-based analyses on the hotspots and trends of research in this field. Therefore, in this study, we performed a bibliometric analysis to summarize the 3D printing research in the field of personalized drug formulation from 2012 to 2022. This study was based on the Web of Science Core Collection Database, and a total of 442 eligible publications were screened. Using VOSviewer and online websites for bibliometric analysis and scientific mapping, it was observed that annual publications have shown a significant growth trend over the last decade. The United Kingdom and the United States, which account for 45.5% of the total number of publications, are the main drivers of this field. The International Journal of Pharmaceutics and University College London are the most prolific and cited journals and institutions. The researchers with the most contributions are Basit, Abdul W. and Goyanes Alvaro. The keyword analysis concluded that the current research hotspots are "drug release" and "drug dosage forms". In conclusion, 3D printing has broad application prospects in the field of personalized drugs, which will bring the pharmaceutical industry into a new era of innovation.

Microneedles: materials, fabrication, and biomedical applications

The microneedles have attracted great interests for a wide range of transdermal biomedical applications, such as biosensing and drug delivery, due to the advantages of being painless, semi-invasive, and sustainable. The ongoing challenges are the materials and fabrication methods of the microneedles in order to obtain a specific shape, configuration and function of the microneedles to achieve a target biomedical application. Here, this review would introduce the types of materials of the microneedles firstly. The hardness, Young's modulus, geometric structure, processability, biocompatibility and degradability of the microneedles are explored as well. Then, the fabrication methods for the solid and hollow microneedles in recent years are reviewed in detail, and the advantages and disadvantages of each process are analyzed and compared. Finally, the biomedical applications of the microneedles are reviewed, including biosensing, drug delivery, body fluid extraction, and nerve stimulation. It is expected that this work provides the fundamental knowledge for developing new microneedle devices, as well as the applications in a variety of biomedical fields.

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.

Fabrication and optimization of 3D printed gelatin methacryloyl microneedle arrays based on vat photopolymerization

Microneedles (MNs) are micrometer-sized arrays that can penetrate the skin in a minimally invasive manner; these devices offer tremendous potential for the transdermal delivery of therapeutic molecules. Although there are many conventional techniques for manufacturing MNs, most of them are complicated and can only fabricate MNs with specific geometries, which restricts the ability to adjust the performance of the MNs. Herein, we present the fabrication of gelatin methacryloyl (GelMA) MN arrays using the vat photopolymerization 3D printing technique. This technique allows for the fabrication of high-resolution and smooth surface MNs with desired geometries. The existence of methacryloyl groups bonded to the GelMA was verified by ¹H NMR and FTIR analysis. To examine the effects of varying needle heights (1000, 750, and 500 µm) and exposure times (30, 50, and 70 s) on GelMA MNs, the height, tip radius, and angle of the needles were measured; their morphological and mechanical properties were also characterized. It was observed that as the exposure time increased, the height of the MNs increased; moreover, sharper tips were obtained and tip angles decreased. In addition, GelMA MNs exhibited good mechanical performance with no breakage up to 0.3 mm displacement. These results indicate that 3D printed GelMA MNs have great potential for transdermal delivery of various therapeutics.

Biphasic burst and sustained transdermal delivery in vivo using an AI-optimized 3D-printed MN patch

The objective of the present study was to fabricate microneedles for delivering lipophilic active ingredients (APIs) using digital light processing (DLP) printing technology and quality by design (QbD) supplemented by artificial intelligence (AI) algorithms. In the present study, dissolvable microneedle (MN) patches using ibuprofen (IBU) as a model drug were successfully fabricated with DLP printing technology at ∼ 750 μm height, ∼250 μm base diameter, and tip with radius of curvature (RoC) of ∼ 15 μm. MN patches were comprised of IBU, photoinitiator, Lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate (LAP), polyethylene glycol dimethacrylate (PEGDAMA)550 and distilled water and were developed using the QbD optimization approach. Optimization of print fidelity and needle morphology were achieved using AI implementing a semi-supervised machine learning approach. Mechanical strength tests demonstrated that IBU MNs formed pores both on Parafilm M® and human cadaver skin. IBU-MNs consisting of 0.23 %w/v and 0.49 %w/v LAP with 10 %w/v water showed ∼ 2 mg/cm2 sustained drug permeation at 72 h in skin permeation experiments with flux of ∼ 40 μg/cm2/h. Pharmacokinetic studies in rats displayed biphasic rapid first-order absorption with sustained zero-order input of Ko = 150ug/hr, AUC0-48h = 62812.02 ± 11128.39 ng/ml*h, Tmax = 2.66 ± 1.12 h, and Cmax = 3717.43 ± 782.25 ng/ml (using 0.23 %w/v LAP IBU MN patch). An in vitro in vivo relation (IVIVR) was conducted identifying a polynomial relationship between patch release and fraction absorbed in vivo. This study demonstrates fabrication of dissolvable DLP-printed microneedle patches for lipophilic API delivery with biphasic rapid first-order and sustained zero-order release.

3D-printed morphology-customized microneedles: understanding the correlation between their morphologies and the received qualities

Despite remarkable progress in the last decade in transdermal microneedle drug delivery systems, great difficulties in precisely manufacturing microneedles with sophisticated microstructures still strongly retard their practical applications. Herein we propose morphology-customized microneedles (spiral, conical, cylindroid, ring-like, arrow-like and tree-like) fabricated by stereolithography (SLA) based 3D-printing technique, and in-depth investigate the correlation between the customized morphologies and the received qualities of the corresponding microneedles such as the mechanical properties and skin penetration behavior, drug loading capacity and the drug release profiles. Results indicated that 3D-printed morphology-customized microneedles not only enhanced the mechanical strength but also improved both drug loading capacity and drug release behavior, which resulted from their highly controllable and 3D-printable morphologies (surface area and volume). And the in vivo study demonstrated that the 3D-printed morphology-customized microneedles successfully promoted the transdermal delivery of the loaded drug (verapamil hydrochloride) with an enhanced therapeutic efficacy for the treatment of hypertrophic scar.

3D Printed Hollow Microneedles for Treating Skin Wrinkles Using Different Anti-Wrinkle Agents: A Possible Futuristic Approach

Skin wrinkles are an inevitable phenomenon that is brought about by aging due to the degradation of scleroprotein fibers and significant collagen reduction, which is the fundamental basis of anti-wrinkle technology in use today. Conventional treatments such as lasering and Botulinum toxin have some drawbacks including allergic skin reactions, cumbersome treatment procedures, and inefficient penetration of the anti-wrinkle products into the skin due to the high resistance of stratum corneum. Bearing this in mind, the cosmetic industry has exploited the patient-compliant technology of microneedles (MNs) to treat skin wrinkles, developing several products based on solid and dissolvable MNs incorporated with antiwrinkle formulations. However, drug administration via these MNs is limited by the high molecular weight of the drugs. Hollow MNs (HMNs) can deliver a wider array of active agents, but that is a relatively unexplored area in the context of antiwrinkle technology. To address this gap, we discuss the possibility of bioinspired 3D printed HMNs in treating skin wrinkles in this paper. We compare the previous and current anti-wrinkling treatment options, as well as the techniques and challenges involved with its manufacture and commercialization.

Three-Dimensional-Printed Oral Films Based on LCD: Influence Factors of the Film Printability and Received Qualities

As an oral mucosal drug delivery system, oral films have been of wide concern in recent years because of their advantages such as rapid absorption, being easy to swallow and avoiding the first-pass effect common for mucoadhesive oral films. However, the currently utilized manufacturing approaches including solvent casting have many limitations, such as solvent residue and difficulties in drying, and are not suitable for personalized customization. To solve these problems, the present study utilizes liquid crystal display (LCD), a photopolymerization-based 3D printing technique, to fabricate mucoadhesive films for oral mucosal drug delivery. The designed printing formulation includes PEGDA as the printing resin, TPO as the photoinitiator, tartrazine as the photoabsorber, PEG 300 as the additive and HPMC as the bioadhesive material. The influence of printing formulation and printing parameters on the printing formability of the oral films were elucidated in depth, and the results suggested that PEG 300 in the formulation not only provided the necessary flexibility of the printed oral films, but also improved drug release rate due to its role as pore former in the produced films. The presence of HPMC could greatly improve the adhesiveness of the 3D-printed oral films, but excessive HPMC increased the viscosity of the printing resin solution, which could strongly hinder the photo-crosslinking reaction and reduce printability. Based on the optimized printing formulation and printing parameters, the bilayer oral films containing a backing layer and an adhesive layer were successfully printed with stable dimensions, adequate mechanical properties, strong adhesion ability, desirable drug release and efficient in vivo therapeutic efficacy. All these results indicated that an LCD-based 3D printing technique is a promising alternative to precisely fabricate oral films for personalized medicine.

Modeling of Microneedle Arrays in Transdermal Drug Delivery Applications

The use of computational tools for the development of technologies in fields such as medicine and engineering has facilitated the process of designing new components and devices for these areas. In this work, two proposals focused on a hollow microneedle array (MNA) for the administration of an analgesic drug are shown and evaluated by means of a computational fluid dynamics (CFD) simulation distributed in three stages. In the first stage, the behavior of lidocaine through the MNA was evaluated as a workflow. Then, the possible entry of the drug into the organism, which was established as a porous aqueous medium, was modeled. Finally, a joint simulation was performed to understand the general behavior in the interaction between the outflow of an MNA and the body to which lidocaine is administered. The input parameters to the simulation were set at a velocity of 0.05 m∙s^-1, at a pressure of 2000 Pa, the dominant behavior was defined as laminar flow, and a resistive pressure at the inlet of 400 Pa. Our results indicate that the vertical flow exhibits a better fluid distribution across the MNAs and favorable infiltration behavior, representing better delivery of the analgesic to the skin capillaries.

The Use of Micro-Ribbons and Micro-Fibres in the Formulation of 3D Printed Fast Dissolving Oral Films

Three-dimensional printing (3DP) allows production of novel fast dissolving oral films (FDFs). However, mechanical properties of the films may not be desirable when certain excipients are used. This work investigated whether adding chitosan micro-ribbons or cellulose microfibres will achieve desired FDFs by fused deposition modelling 3DP. Filaments containing polyvinyl alcohol (PVA) and paracetamol as model drug were manufactured at 170 °C. At 130 °C, filaments containing polyvinylpyrrolidone (PVP) and paracetamol were also created. FDFs were printed with plain or mesh patterns at temperatures of 200 °C (PVA) or 180 °C (PVP). Both chitosan micro-ribbons and cellulose micro-fibres improved filament mechanical properties at 1% w/w concentration in terms of flexibility and stiffness. The filaments were not suitable for printing at higher concentrations of chitosan micro-ribbons and cellulose micro-fibres. Furthermore, mesh FDFs containing only 1% chitosan micro-ribbons disintegrated in distilled water within 40.33 ± 4.64 s, while mesh FDFs containing only 7% croscarmellose disintegrated in 55.33 ± 2.86 s, and croscarmellose containing films showed signs of excipient scorching for PVA polymer. Cellulose micro-fibres delayed disintegration of PVA mesh films to 108.66 ± 3.68 s at 1% w/w. In conclusion, only chitosan micro-ribbons created a network of hydrophilic channels within the films, which allowed faster disintegration time at considerably lower concentrations.

Improved Topical Drug Delivery: Role of Permeation Enhancers and Advanced Approaches

The delivery of drugs via transdermal routes is an attractive approach due to ease of administration, bypassing of the first-pass metabolism, and the large skin surface area. However, a major drawback is an inability to surmount the skin's stratum corneum (SC) layer. Therefore, techniques reversibly modifying the stratum corneum have been a classical approach. Surmounting the significant barrier properties of the skin in a well-organised, momentary, and harmless approach is still challenging. Chemical permeation enhancers (CPEs) with higher activity are associated with certain side effects restricting their advancement in transdermal drug delivery. Furthermore, complexity in the interaction of CPEs with the skin has led to difficulty in elucidating the mechanism of action. Nevertheless, CPEs-aided transdermal drug delivery will accomplish its full potential due to advancements in analytical techniques, synthetic chemistry, and combinatorial studies. This review focused on techniques such as drug-vehicle interaction, vesicles and their analogues, and novel CPEs such as lipid synthesis inhibitors (LSIs), cell-penetrating peptides (CPPs), and ionic liquids (ILs). In addition, different types of microneedles, including 3D-printed microneedles, have been focused on in this review.

Manufacturing of a Transdermal Patch in 3D Printing

Diabetes mellitus is an endocrine disorder that affects glucose metabolism, making the body unable to effectively use the insulin it produces. Transdermal drug delivery (TDD) has attracted strong interest from researchers, as it allows minimally invasive and painless insulin administration, showing advantages over conventional delivery methods. Systems composed of microneedles (MNs) assembled in a transdermal patch provide a unique route of administration, which is innovative with promising results. This paper presents the design of a transdermal patch composed of 25 microneedles manufactured with 3D printing by stereolithography with a class 1 biocompatible resin and a printing angle of 0°. Finite element analysis with ANSYS software is used to obtain the mechanical behavior of the microneedle (MN). The values obtained through the analysis were: a Von Misses stress of 18.057 MPa, a maximum deformation of 2.179×10-3, and a safety factor of 4. Following this, through a flow simulation, we find that a pressure of 1.084 Pa and a fluid velocity of 4.800 ms were necessary to ensure a volumetric flow magnitude of 4.447×10-5cm3s. Furthermore, the parameters found in this work are of great importance for the future implementation of a transdermal drug delivery device.

Deep eutectic solvents: Recent advances in fabrication approaches and pharmaceutical applications

Deep eutectic solvents (DESs) have received increasing attention in the past decade owing to their distinguished properties including biocompatibility, tunability, thermal and chemical stability. Particularly, DESs have joined forces in pharmaceutical industry, not only to efficiently separate actives from natural products, but also to dramatically increase solubility and permeability of drugs, both are critical for the drug absorption and efficacy. As a result, lately DESs have been extensively and practically adopted as versatile drug delivery systems for different routes such as nasal, transdermal and oral administration with enhanced bioavailability. This review summarizes the emerging progress of DESs by introducing applied fabrication approaches with advantages and limitations thereof, and by highlighting the pharmaceutical applications of DESs.

Beneath the Skin: A Review of Current Trends and Future Prospects of Transdermal Drug Delivery Systems

The ideal drug delivery system has a bioavailability comparable to parenteral dosage forms but is as convenient and easy to use for the patient as oral solid dosage forms. In recent years, there has been increased interest in transdermal drug delivery (TDD) as a non-invasive delivery approach that is generally regarded as being easy to administer to more vulnerable age groups, such as paediatric and geriatric patients, while avoiding certain bioavailability concerns that arise from oral drug delivery due to poor absorbability and metabolism concerns. However, despite its many merits, TDD remains restricted to a select few drugs. The physiology of the skin poses a barrier against the feasible delivery of many drugs, limiting its applicability to only those drugs that possess physicochemical properties allowing them to be successfully delivered transdermally. Several techniques have been developed to enhance the transdermal permeability of drugs. Both chemical (e.g., thermal and mechanical) and passive (vesicle, nanoparticle, nanoemulsion, solid dispersion, and nanocrystal) techniques have been investigated to enhance the permeability of drug substances across the skin. Furthermore, hybrid approaches combining chemical penetration enhancement technologies with physical technologies are being intensively researched to improve the skin permeation of drug substances. This review aims to summarize recent trends in TDD approaches and discuss the merits and drawbacks of the various chemical, physical, and hybrid approaches currently being investigated for improving drug permeability across the skin.

Recent advances in polymer microneedles for drug transdermal delivery: Design strategies and applications

In recent years, the transdermal drug delivery based on microneedles (MNs) technology has received extensive attention, which offers a safer and painless alternative to hypodermic needle injection. They can pierce the stratum corneum and deliver drugs to the epidermis and dermis-structures of skin, showing prominent properties such as minimally invasive, bypassing first-pass metabolism, and self-administered. A range of materials have been used to fabricate MNs, such as silicon, metal, glass, and polymers. Among them, polymer MNs have gained increasing attention from pharmaceutical and cosmetic companies as one of the promising drug delivery methods. Microneedle products have recently become available on the market, and some of them are under evaluation for efficacy and safety. This paper focuses on current state of polymer MNs in the drug transdermal delivery. The materials and methods for the fabrication of polymer MNs and their drug administration are described. The recent progresses of polymer MNs for treatment of cancer, vaccine delivery, blood glucose regulation, androgenetic alopecia, obesity, tissue healing, myocardial infarction and gout are reviewed. The challenges of MNs technology are summarized and the future development trend of MNs is also prospected. This article is protected by copyright. All rights reserved.

Recent Development of Drug Delivery Systems through Microfluidics: From Synthesis to Evaluation

Conventional drug administration usually faces the problems of degradation and rapid excretion when crossing many biological barriers, leading to only a small amount of drugs arriving at pathological sites. Therapeutic drugs delivered by drug delivery systems to the target sites in a controlled manner greatly enhance drug efficacy, bioavailability, and pharmacokinetics with minimal side effects. Due to the distinct advantages of microfluidic techniques, microfluidic setups provide a powerful tool for controlled synthesis of drug delivery systems, precisely controlled drug release, and real-time observation of drug delivery to the desired location at the desired rate. In this review, we present an overview of recent advances in the preparation of nano drug delivery systems and carrier-free drug delivery microfluidic systems, as well as the construction of in vitro models on-a-chip for drug efficiency evaluation of drug delivery systems. We firstly introduce the synthesis of nano drug delivery systems, including liposomes, polymers, and inorganic compounds, followed by detailed descriptions of the carrier-free drug delivery system, including micro-reservoir and microneedle drug delivery systems. Finally, we discuss in vitro models developed on microfluidic devices for the evaluation of drug delivery systems, such as the blood–brain barrier model, vascular model, small intestine model, and so on. The opportunities and challenges of the applications of microfluidic platforms in drug delivery systems, as well as their clinical applications, are also discussed.

The effects of molecular weight of hyaluronic acid on transdermal delivery efficiencies of dissolving microneedles

Hyaluronic acid (HA) is widely adopted to fabricate dissolving microneedles for transdermal drug delivery applications, yet the structure-activity relationship between molecular weight of HA and transdermal delivery efficiency of microneedles (HA-MNs) has not been fully explored, particularly in the transdermal delivery of small molecule drugs. Herein, we report the fabrication of three types of HA-MNs of various molecular weights (10k, 74k and 290k Da), which incorporate rhodamine B as the model drug. We assess the influence of molecular weight of HA on the mechanical properties of HA-MNs and transdermal delivery of rhodamine B in vitro and in vivo. The mechanical strength of all types of HA-MNs exceeds the minimal force requirement for skin penetration, with the highest values of compression force found in 10k-HA-MN. Interestingly, 74k-HA-MN that owns a medium mechanical strength, exhibits the highest efficiency in transdermal delivery of rhodamine B in a porcine skin and a Franz cell transdermal model. Further in vivo fluorescence imaging of HA-MN-treated mice reveals a tunable transdermal delivery of rhodamine B, which is controllable according to the molecular weight of HA. Importantly, 74k-HA-MN treatment demonstrates the highest initial delivering amount and longest retention time of rhodamine B in mice. In addition, histological examinations of puncture sites of the skin tissues confirm the complete recovery of skin and excellent biocompatibility of HA-MNs.

Microneedles in diagnostic, treatment and theranostics: An advancement in minimally-invasive delivery system

Microneedle (MN) technology plays an important role in biomedical engineering for their less intrusive access to the skin due to minimally or painless penetration, enhancement of drug permeability, improvement of detectability of biomolecules in the epidermal and dermal layers with therapeutic efficacy and safety. Furthermore, MNs possess some major disadvantages like difficulty in scale-up technique, variation in drug delivery pattern with respect to external environment of skin, blockage of arrays due to dermal tissues, induction of inflammation or allergy at the site of administration and restriction of dosing range based on the size of active. Additionally, microneedle acts as a transdermal theranostic device for monitoring the physiological parameters in clinical studies. The investigation of drug transfer mechanisms through microneedles includes coat and poke, poke and flow, poke and patch and poke and release method. This review article discusses different categories of microneedles with fabrication methods such as photolithography, laser cutting, 3D printing, etc. in therapeutic applications for treating cancer, diabetes, arthritis, obesity, neurological disorders, and glaucoma. Biosensing devices based on microneedles may detect target analytes directly in the interstitial fluid by penetrating the stratum corneum of the skin and thus microneedles-based devices can be considered as a single tool in diagnostic sensing and therapeutic administration of drugs inside the body. Moreover, the clinical status and commercial availability of microneedle devices are discussed in this review article to offer new insights to researchers and scientists. Continuous monitoring particularly for the determination of blood glucose concentration is one of the most important requirements for the development of next-generation healthcare devices. The aim of this review article focuses mainly on the theranostic applications of microneedles in various medical conditions such as malaria, glaucoma, cancer, etc.

Microneedle-mediated transdermal nanodelivery systems: a review

The greatest limitation in the development of transdermal drug delivery systems is that only a few drugs can permeate the skin due to the barrier function of the stratum corneum. Active and passive methods are generally available for improving the ability of drug transdermal delivery. However, nanoparticles, as a passive approach, exhibit capacity-constrained permeation enhancement. Thus, microneedle-mediated nanoparticles possess enormous potential and broad prospects. Microneedles promote the penetration of macromolecules by creating microchannels on the skin surface. In this review, the prevailing subknowledge on microneedles (mechanism, classification, and applications of microneedles combined with nanoparticles) is discussed to provide a guideline for readers and a basic reference for further in-depth studies of this novel drug delivery system.

Toward closed-loop drug delivery: Integrating wearable technologies with transdermal drug delivery systems

The recent advancement and prevalence of wearable technologies and their ability to make digital measurements of vital signs and wellness parameters have triggered a new paradigm in the management of diseases. Drug delivery as a function of stimuli or response from wearable, closed-loop systems can offer real-time on-demand or preprogrammed drug delivery capability and offer total management of disease states. Here we review the key opportunities in this space for development of closed-loop systems, given the advent of digital wearable technologies. Particular considerations and focus are given to closed-loop systems combined with transdermal drug delivery technologies.

3D-Printed Products for Topical Skin Applications: From Personalized Dressings to Drug Delivery

3D printing has been widely used for the personalization of therapies and on-demand production of complex pharmaceutical forms. Recently, 3D printing has been explored as a tool for the development of topical dosage forms and wound dressings. Thus, this review aims to present advances related to the use of 3D printing for the development of pharmaceutical and biomedical products for topical skin applications, covering plain dressing and products for the delivery of active ingredients to the skin. Based on the data acquired, the important growth in the number of publications over the last years confirms its interest. The semisolid extrusion technique has been the most reported one, probably because it allows the use of a broad range of polymers, creating the most diverse therapeutic approaches. 3D printing has been an excellent field for customizing dressings, according to individual needs. Studies discussed here imply the use of metals, nanoparticles, drugs, natural compounds and proteins and peptides for the treatment of wound healing, acne, pain relief, and anti-wrinkle, among others. The confluence of 3D printing and topical applications has undeniable advantages, and we would like to encourage the research groups to explore this field to improve the patient's life quality, adherence and treatment efficacy.

Additive manufacturing in drug delivery: Innovative drug product design and opportunities for industrial application

Additive manufacturing (AM) or 3D printing is enabling new directions in product design. The adoption of AM in various industrial sectors has led to major transformations. Similarly, AM presents new opportunities in the field of drug delivery, opening new avenues for improved patient care. In this review, we discuss AM as an innovative tool for drug product design. We provide a brief overview of the different AM processes and their respective impact on the design of drug delivery systems. We highlight several enabling features of AM, including unconventional release, customization, and miniaturization, and discuss several applications of AM for the fabrication of drug products. This includes products that have been approved or are in development. As the field matures, there are also several new challenges to broad implementation in the pharmaceutical landscape. We discuss several of these from the regulatory and industrial perspectives and provide an outlook for how these issues may be addressed. The introduction of AM into the field of drug delivery is an enabling technology and many new drug products can be created through productive collaboration of engineers, materials scientists, pharmaceutical scientists, and industrial partners.

Hydrophilic Excipient-Independent Drug Release from SLA-Printed Pellets

Three-dimensional (3D) printing technology, specifically stereolithography (SLA) technology, has recently created exciting possibilities for the design and fabrication of sophisticated dosages for oral administration, paving a practical way to precisely manufacture customized pharmaceutical dosages with both personalized properties and sustained drug release behavior. However, the sustained drug release achieved in prior studies largely relies on the presence of hydrophilic excipients in the printing formulation, which unfortunately impedes the printability and formability of the corresponding printing formulations. The current study developed and prepared mini-sized oral pellets using the SLA technique and successfully accomplished a hydrophilic excipient-independent drug release behavior. With ibuprofen as the model drug, the customized photopolymerizable printing formulation included polyethylene glycol diacrylate (PEGDA) as a monomer and diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) as a photoinitiator. The produced mini-sized pellets were thoroughly investigated for various factors, including their printability, physical properties, microscopic features, drug content, and drug-release profiles. The drug release profiles from the printed pellets that were larger size (3 mm and 6 mm) followed the Ritger–Peppas model, demonstrating that the release was influenced by both the diffusion of the dissolved drug and by the erosion of the hydrophilic excipients (PEG400). The profiles from the smaller printed pellets (1 mm and 2 mm) followed first release kinetics, not only illustrating that the release was impacted only by drug diffusion, but also indicating that there is a size boundary between the dependent and independent hydrophilic excipients. These results could create practical benefits to the pharmaceutical industry in terms of the design and development personalized dosages using the SLA printing technique with controllable drug release by manipulating size alone.

Wearable Glucose Monitoring and Implantable Drug Delivery Systems for Diabetes Management

The global cost of diabetes care exceeds $1 trillion each year with more than $327 billion being spent in the United States alone. Despite some of the advances in diabetes care including continuous glucose monitoring systems and insulin pumps, the technology associated with managing diabetes has largely remained unchanged over the past several decades. With the rise of wearable electronics and novel functional materials, the field is well-poised for the next generation of closed-loop diabetes care. Wearable glucose sensors implanted within diverse platforms including skin or on-tooth tattoos, skin-mounted patches, eyeglasses, contact lenses, fabrics, mouthguards, and pacifiers have enabled noninvasive, unobtrusive, and real-time analysis of glucose excursions in ambulatory care settings. These wearable glucose sensors can be integrated with implantable drug delivery systems, including an insulin pump, glucose responsive insulin release implant, and islets transplantation, to form self-regulating closed-loop systems. This review article encompasses the emerging trends and latest innovations of wearable glucose monitoring and implantable insulin delivery technologies for diabetes management with a focus on their advanced materials and construction. Perspectives on the current unmet challenges of these strategies are also discussed to motivate future technological development toward improved patient care in diabetes management.

An overview on the advantages and limitations of 3D printing of microneedles

The use of 3D printing (3DP) technology, which has been continuously evolving since the 1980s, has recently become common in healthcare services. The introduction of 3DP into the pharmaceutical industry particularly aims at the development of patient-centered dosage forms based on structure design. It is still a new research direction with potential to create the targeted release of drug delivery systems in freeform geometries. Although the use of 3DP technology for solid oral dosage forms is more preferable, studies on transdermal applications of the technology are also increasing. Microneedle sequences are one of the transdermal drug delivery (TDD) methods which are used to bypass the minimally invasive stratum corneum with novel delivery methods for small molecule drugs and vaccines. Microneedle arrays have advantages over many traditional methods. It is attractive with features such as ease of application, controlled release of active substances and patient compliance. Recently, 3D printers have been used for the production of microneedle patches. After giving a brief overview of 3DP technology, this article includes the materials necessary for the preparation of microneedles and microneedle patches specifically for penetration enhancement, preparation methods, quality parameters, and their application to TDD. In addition, the applicability of 3D microneedles in the pharmaceutical industry has been evaluated.

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.

Recent advances of oral film as platform for drug delivery

Orally drug delivery film has received extensive interest duo to a distinct set of its advantageous properties compared to the traditional orally administered dosages, including faster rate of drug absorption, higher bioavailability and better patient compliance for children and elders with swallowing deficiencies. In particular, its potential capacity of delivering proteins and peptides has further attracted great attention. Lately, tremendous advances have been made in designing and developing both novel mucoadhesive films and orodispersible films to fulfill specific accomplishments of drug delivery. This review aims to summarize those newly developed oral films, discussing their formulation strategies, manufacturing methods as well as advantages and limitations thereof. Conclusions and future perspectives are also provided in brief.