Multiphoton microscopy of transdermal quantum dot delivery using two photon polymerization-fabricated polymer microneedles

Overcoming challenges in dermal and transdermal delivery of herbal therapeutics with polymeric microneedles

Natural products are generally preferred medications owing to their low toxicity and irritancy potential. However, a good number of herbal therapeutics (HT) exhibit solubility, permeability and stability issues that eventually affect oral bioavailability. Transdermal administration has been successful in resolving some of these issues which has lead in commercialization of a few herbal transdermal products. Polymeric Microneedles (MNs) has emerged as a promising platform in transdermal delivery of HT that face problems in permeating the skin. Several biocompatible and biodegradable polymers used in the fabrication of MNs have been discussed. MNs have been exploited for cutaneous delivery of HT in management of skin ailments like skin cancer, acne, chronic wounds and hypertrophic scar. Considering the clinical need, MNs are explored for systemic delivery of potent HT for management of diverse disorders like asthma, disorders of central nervous system and nicotine replacement as it obviates first pass metabolism and elicits a quicker onset of therapeutic response. MNs of HT have found good number of aesthetic applications in topical delivery of HT to the skin. Interestingly, MNs have emerged as an attractive option as a minimally invasive diagnostic aid in sampling biomarkers from plants, skin and ocular interstitial fluid. The review updates the progress made by MN technology of HT for multiple therapeutic interventions along with the future challenges. An attempt is made to illustrate the challenging formulation strategies employed in the fabrication of polymeric MNs of HT. Efforts are on to extend the potential applications of polymeric MNs to HT for diverse therapeutic applications.

Photochemically Activated 3D Printing Inks: Current Status, Challenges, and Opportunities

3D printing with light is enabled by the photochemistry underpinning it. Without fine control over the ability to photochemically gate covalent bond formation by the light at a certain wavelength and intensity, advanced photoresists with functions spanning from on-demand degradability, adaptability, rapid printing speeds, and tailored functionality are impossible to design. Herein, recent advances in photoresist design for light-driven 3D printing applications are critically assessed, and an outlook of the outstanding challenges and opportunities is provided. This is achieved by classing the discussed photoresists in chemistries that function photoinitiator-free and those that require a photoinitiator to proceed. Such a taxonomy is based on the efficiency with which photons are able to generate covalent bonds, with each concept featuring distinct advantages and drawbacks.

Biocompatible polymers with tunable mechanical properties and conductive functionality on two-photon 3D printing

Two-photon polymerization (TPP)-based 3D printing technology utilizes the two-photon absorption process of near-infrared radiation, enabling the fabrication of micro- and nano-scale three-dimensional structures with extremely high resolution. It has been widely applied in scientific fields closely related to living organisms, such as tissue engineering, drug delivery, and biosensors. Nevertheless, the existing photoresist materials have poor mechanical tunability and are hardly able to be doped with functional materials, resulting in constraints on the preparation of functional devices with micro-nano structures. In this paper, TPP printable polymer formulas with good mechanical tunability, high resolution, strong functional scalability, and excellent biocompatibility are proposed, by using the synergistic effects of a hydroxyl group-containing photocurable resin prepolymer, UV acrylate monomer, long-chain hydrophilic crosslinking monomer and photo-initiator. This can ensure the printability and help to improve the flexibility of the printed polymer, thereby solving the problem the photosensitive materials suitable for two-photon 3D printing in previous research had in balancing the formability and flexibility. The results of nanoindenter analysis showed that the Young's modulus of the printed structure can be adjusted between 0.3 GPa and 1.43 GPa, realizing mechanical tunability. Also, complex structures, such as micro-scaffold structures and high aspect ratio hollow microneedles were printed to explore the structural stability as well as the feasibility of biodevice application. Meanwhile, the proposed polymer formula can be functionalized to be conductive by doping with functional nanomaterial MXene. Finally, the biocompatibility of the proposed polymer formula was studied by culturing with human normal lung epithelial cells. The results indicated a good potential for biodevice applications.

Innovative Fabrication of Hollow Microneedle Arrays Enabling Blood Sampling with a Self-Powered Microfluidic Patch

Microneedles are gaining a lot of attention in the context of sampling cutaneous biofluids such as capillary blood. Their minimal invasiveness and user-friendliness make them a prominent substitute for venous puncture or finger-pricking. Although the latter is suitable for self-sampling, the impracticality of manual handling and the difficulty of obtaining enough qualitative sample is driving the search for better solutions. In this context, hollow microneedle arrays (HMNAs) are particularly interesting for completely integrating sample-to-answer solutions as they create a duct between the skin and the sampling device. However, the fabrication of sharp-tipped HMNAs with a high aspect ratio (AR) is challenging, especially since a length of ≥1500 μm is desired to reach the blood capillaries. In this paper, we first described a novel two-step fabrication protocol for HMNAs in stainless steel by percussion laser drilling and subsequent micro-milling. The HMNAs were then integrated into a self-powered microfluidic sampling patch, containing a capillary pump which was optimized to generate negative pressure differences up to 40.9 ± 1.8 kPa. The sampling patch was validated in vitro, showing the feasibility of sampling 40 μL of liquid. It is anticipated that our proof-of-concept is a starting point for more sophisticated all-in-one biofluid sampling and point-of-care testing systems.

Revolutionizing Therapeutic Delivery with Microneedle Technology for Tumor Treatment

The tumor is an uncontrolled growth of tissue that can be localized (benign) or possesses the capability of metastasis (malignant). The conventional methods of tumor diagnosis, such as acupuncture, endoscopy, and histopathology, and treatment methods, such as injections, chemotherapy, surgery, and radiotherapy, are invasive, expensive, and pose severe safety and management issues for the patients. Microneedle technology is a recently developed approach for active transdermal drug delivery. It is minimally invasive, self-administrable, bypasses the first-pass effect, and effectively delivers chemotherapeutics and drugs at low doses, thus, overcoming the drawbacks of conventional delivery systems. This review provides an idea of the types, materials utilized in the fabrication, and techniques used for the preparation of microneedles (MNs), as well as their application in tumor diagnosis and treatment. Additionally, emphasis is given to the case studies related to MNs-assisted tumor therapy, such as photothermal therapy, gene therapy, photodynamic therapy, chemotherapy, immunotherapy, and various combination therapies. MNs also serve as a tool for diagnosis by the bio-sampling of blood and interstitial skin fluid, as well as biosensing various cancer biomarkers. The combined therapy and diagnostics provide theranostic MNs for enhanced and personalized tumor therapy. The limitations and prospects of MNs development are also discussed.

Microneedles-based drug delivery strategies: A breakthrough approach for the management of pain

Pain is a personalized event or body alarm system that can limit a patient's activities and lead to negative repercussions. The commercially available conventional treatment strategies like oral, parenteral, and topical drug delivery systems for pain management are associated with side effects and poor patient compliance. The transdermal route is eminent for its painless distribution. Among transdermal drug delivery system, microneedles (MNs) are gaining attention for their application with delivery at the deeper dermal layer because it bypasses the major barrier of the skin, easily accesses the skin dermal microcirculation, prevents damage to dermal blood vessels, and can be simply inserted into the skin without utilizing any additional applicator devices. Hence, considered a promising drug delivery strategy with high patient compliance. This review highlights the recent advancements of MNs in pain management. The present work mainly emphasizes all the case studies reported from the past 10 years that utilize MNs containing therapeutics in the treatment of chronic pain-associated diseases like rheumatoid arthritis, neuropathic pain, osteoarthritis, psoriatic arthritis, and atopic dermatitis. These studies have proven the efficacious application of MNs in the management of chronic pain and inflammation. The review also covered the clinical trials, patents, and future goals of pain management by using MNs.

3D-printed microneedle arrays for drug delivery

Microneedle arrays provide an efficient tool for transdermal drug delivery in a minimally invasive and painless manner, showing great potential applications in medicine. However, it remains challenging to fabricate the desired microneedle arrays, because of their micron-scale size and fine structure. Novel manufacturing technologies are very wanted for the development of microneedle arrays, which would solidly advance the clinical translation of microneedle arrays. 3D printing technology is a powerful manufacturing technology with superiority in fabricating personalized and complex structures. Currently, 3D printing technology has been employed to fabricate microneedle arrays, which could push more microneedle arrays into clinic and inspire the development of future microneedle arrays. This work reviews the art of 3D printing microneedle arrays, the benefits of fabricating microneedle arrays with 3D printing, and the considerations for clinical translation of 3D-printed microneedle arrays. This work provides an overview of the current 3D-printed microneedle arrays in drug delivery.

Bioimaging of Dissolvable Microneedle Arrays: Challenges and Opportunities

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

Fabrication of hollow microneedles using liquid crystal display (LCD) vat polymerization 3D printing technology for transdermal macromolecular delivery

The present study aimed to fabricate a hollow microneedle device consisting of an array and a reservoir by means of 3D printing technology for transdermal peptide delivery. Hollow microneedles (HMNs) were fabricated using a biocompatible resin material, while PLA filament was used for the reservoirs. The fabricated microdevice was characterized by means of optical microscopy, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), contact angle measurements and leakage inspection studies to ensure the passageway of liquid formulations. Mechanical failure and penetration tests were carried out and supported by Finite Element Analysis (FEA). The cytocompatibility of the microneedle arrays was assessed to human keratinocytes (HaCaT). Finally, the transport of the model peptide octreotide acetate across artificial membranes was assessed in Franz cells using the aforementioned HMN design.

High-resolution two-photon polymerization: the most versatile technique for the fabrication of microneedle arrays

Microneedle patches have received much interest in the last two decades as drug/vaccine delivery or fluid sampling systems for diagnostic and monitoring purposes. Microneedles are manufactured using a variety of additive and subtractive micromanufacturing techniques. In the last decade, much attention has been paid to using additive manufacturing techniques in both research and industry, such as 3D printing, fused deposition modeling, inkjet printing, and two-photon polymerization (2PP), with 2PP being the most flexible method for the fabrication of microneedle arrays. 2PP is one of the most versatile and precise additive manufacturing processes, which enables the fabrication of arbitrary three-dimensional (3D) prototypes directly from computer-aided-design (CAD) models with a resolution down to 100 nm. Due to its unprecedented flexibility and high spatial resolution, the use of this technology has been widespread for the fabrication of bio-microdevices and bio-nanodevices such as microneedles and microfluidic devices. This is a pioneering transformative technology that facilitates the fabrication of complex miniaturized structures that cannot be fabricated with established multistep manufacturing methods such as injection molding, photolithography, and etching. Thus, microstructures are designed according to structural and fluid dynamics considerations rather than the manufacturing constraints imposed by methods such as machining or etching processes. This article presents the fundamentals of 2PP and the recent development of microneedle array fabrication through 2PP as a precise and unique method for the manufacture of microstructures, which may overcome the shortcomings of conventional manufacturing processes.

Two-Photon Polymerisation 3D Printing of Microneedle Array Templates with Versatile Designs: Application in the Development of Polymeric Drug Delivery Systems

PURPOSE

To apply a simple and flexible manufacturing technique, two-photon polymerisation (2PP), to the fabrication of microneedle (MN) array templates with high precision and low cost in a short time.

METHODS

Seven different MN array templates were produced by 2PP 3D printing, varying needle height (900-1300 μm), shape (conical, pyramidal, cross-shaped and with pedestal), base width (300-500 μm) and interspacing (100-500 μm). Silicone MN array moulds were fabricated from these templates and used to produce dissolving and hydrogel-forming MN arrays. These polymeric MN arrays were evaluated for their insertion in skin models and their ability to deliver model drugs (cabotegravir sodium and ibuprofen sodium) to viable layers of the skin (ex vivo and in vitro) for subsequent controlled release and/or absorption.

RESULTS

The various templates obtained with 2PP 3D printing allowed the reproducible fabrication of multiple MN array moulds. The polymeric MN arrays produced were efficiently inserted into two different skin models, with sharp conical and pyramidal needles showing the highest insertion depth values (64-90% of needle height). These results correlated generally with ex vivo and in vitro drug delivery results, where the same designs showed higher drug delivery rates after 24 h of application.

CONCLUSION

This work highlights the benefits of using 2PP 3D printing to prototype variable MN array designs in a simple and reproducible manner, for their application in drug delivery.

Recent advances on microneedle arrays-mediated technology in cancer diagnosis and therapy

Regarding the increasing prevalence of cancer throughout the globe, the development of novel alternatives for conventional therapies is inevitable to circumvent limitations such as low efficacy, complications, and high cost. Recently, microneedle arrays (MNs) have been introduced as a novel, minimally invasive, and low-cost approach. MNs can delivery both small molecule and macromolecular drugs or even nanoparticles (NPs) to the tumor tissue in a safe and controlled manner. Relying on the recent promising outcomes of MNs in transdermal delivery of anticancer agents, this review is aimed to summarize constituent materials, fabrication methods, advantages, and limitations of different types of MNs used in cancer therapy applications. This review paper also presents the potential use of MNs in transdermal delivery of NPs for effective chemotherapy, gene therapy, immunotherapy, photodynamic, and photothermal therapy. Additionally, MNs are currently explored as routine point-of-care health monitoring devices for transdermal detection of cancer biomarkers or physiologically relevant analytes which will be addressed in this paper. Despite the promising potential of MNs for cancer therapy and diagnosis, several limitations have impeded their therapeutic efficacy and real-time applicability that are addressed in this paper.

Microneedle-based transdermal electrochemical biosensors based on Prussian blue-gold nanohybrid modified screen-printed electrodes

We report on the fabrication of a microneedle-based electrochemical biosensor for use in transdermal biosensing, which includes a screen-printed electrode containing a Prussian blue-gold nanohybrid as the working electrode. The Prussian blue gold nanohybrid is made from polyethylenime (PEI)- mediated simultaneous synthesis of Prussian blue (PBNP) and gold nanoparticles (AuNP), which forms a PBNP-AuNP nanohybrid with a remarkable change in the Prussian blue character. PEI-protected polycrystalline PBNPs can be synthesized in acidic media from the single precursor potassium ferricyanide [K₃ Fe(CN)₆ ] at 60°C. Since PEI also enables the controlled formation of gold nanoparticles (AuNPs) in the presence of formaldehyde, nanohybrids containing PBNPs and AuNPs may be prepared. Two different methods of PEI mediated synthesis of AuNP in the presence of PBNP were considered. In Method 1, AuNP and PBNP were made independently and mixed together in an appropriate ratio. In Method 2, PBNPs were made first, followed by PEI- and formaldehyde-mediated reduction of gold cations in the presence of PBNP. PBNP-AuNPs display a remarkable change in Prussian blue behavior such that the absorption maxima of PBNP-AuNPs made through Method 1 tend to increase at 670 nm as a function of gold concentration as compared with the control; the reverse was observed when PBNP-AuNPs were made through Method 2. As made PBNPs and PBNP-AuNPs made through Method 1 display excellent catalytic activity toward both reduction and oxidation of hydrogen peroxide based on peroxidase mimetic activity. In addition, the as-synthesized PBNPs displayed superparamagnetic behavior that can be manipulated in the presence of AuNPs. The results from peroxidase mimetic activity, chemiluminescence, cyclic voltammetry, and amperometry showed suitable analytical performance of the as-made PBNP-AuNP nanohybrid for biomedical applications.

Digital Manufacturing for Microfluidics

The microfluidics field is at a critical crossroads. The vast majority of microfluidic devices are presently manufactured using micromolding processes that work very well for a reduced set of biocompatible materials, but the time, cost, and design constraints of micromolding hinder the commercialization of many devices. As a result, the dissemination of microfluidic technology-and its impact on society-is in jeopardy. Digital manufacturing (DM) refers to a family of computer-centered processes that integrate digital three-dimensional (3D) designs, automated (additive or subtractive) fabrication, and device testing in order to increase fabrication efficiency. Importantly, DM enables the inexpensive realization of 3D designs that are impossible or very difficult to mold. The adoption of DM by microfluidic engineers has been slow, likely due to concerns over the resolution of the printers and the biocompatibility of the resins. In this article, we review and discuss the various printer types, resolution, biocompatibility issues, DM microfluidic designs, and the bright future ahead for this promising, fertile field.

Progress in Microneedle-Mediated Protein Delivery

The growing demand for patient-compliance therapies in recent years has led to the development of transdermal drug delivery, which possesses several advantages compared with conventional methods. Delivering protein through the skin by transdermal patches is extremely difficult due to the presence of the stratum corneum which restricts the application to lipophilic drugs with relatively low molecular weight. To overcome these limitations, microneedle (MN) patches, consisting of micro/miniature-sized needles, are a promising tool to perforate the stratum corneum and to release drugs and proteins into the dermis following a non-invasive route. This review investigates the fabrication methods, protein delivery, and translational considerations for the industrial scaling-up of polymeric MNs for dermal protein delivery.

Application of Micro-Scale 3D Printing in Pharmaceutics

3D printing, as one of the most rapidly-evolving fabrication technologies, has released a cascade of innovation in the last two decades. In the pharmaceutical field, the integration of 3D printing technology has offered unique advantages, especially at the micro-scale. When printed at a micro-scale, materials and devices can provide nuanced solutions to controlled release, minimally invasive delivery, high-precision targeting, biomimetic models for drug discovery and development, and future opportunities for personalized medicine. This review aims to cover the recent advances in this area. First, the 3D printing techniques are introduced with respect to the technical parameters and features that are uniquely related to each stage of pharmaceutical development. Then specific micro-sized pharmaceutical applications of 3D printing are summarized and grouped according to the provided benefits. Both advantages and challenges are discussed for each application. We believe that these technologies provide compelling future solutions for modern medicine, while challenges remain for scale-up and regulatory approval.

Facile design and development of photoluminescent graphene quantum dots grafted dextran/glycol-polymeric hydrogel for thermoresponsive triggered delivery of buprenorphine on pain management in tissue implantation

A novel nano-formulations of biocompatible, biodegradable and thermo-responsive graphene quantum dots (GQDs) loaded dextran/poly(N-isopropylacrylamide) (Dex/PNIPAM) copolymeric matrix was synthesized and analyzed the materials characterization, sustained drug delivery system, tissue feasibility in the tissue implantation site. This research report was aimed to grafting and functionalizing thermo-responsive (Dex/PNIPAM) copolymeric composite with presence of graphene quantum dots to achieve thermal responsive drug delivery (TrDD) with no harm effect in the implantation site. The synthesized GQD by using ionic liquid were evaluated by spectroscopic (DLS, PL, XRD and Raman spectroscopy) and Transmission electron microscopic analysis (TEM). The ultra-small GQDs loaded Dex/PNIPAM and was appeared to be asymmetric and open uniform porous structure, which can be significantly favorable for cell uptake and greatly influenced to be an effective drug carrier into the cellular compartment with good fluid flow. The PNIPAM polymeric composite were exhibited sustained and enhanced drug release percentages with increasing temperature at above low critical solution temperature (LCST) is 39 °C comparable to the cumulative drug release profile of below LCST (32 °C), which demonstrated that thermo-responsive polymer was played a significant role in the delivery system. The treated group of GQDs-Dex/PNIPAM was observed that no inflammation and shows noteworthy stromal cell infiltration, demonstrating that the synthesized drug carriers did not harm to the nerves and tissues and only was responsible for the pain management.

Addressing Unmet Clinical Needs with 3D Printing Technologies

Recent advances in 3D printing have enabled the creation of novel 3D constructs and devices with an unprecedented level of complexity, properties, and functionalities. In contrast to manufacturing techniques developed for mass production, 3D printing encompasses a broad class of fabrication technologies that can enable 1) the creation of highly customized and optimized 3D physical architectures from digital designs; 2) the synergistic integration of properties and functionalities of distinct classes of materials to create novel hybrid devices; and 3) a biocompatible fabrication approach that facilitates the creation and cointegration of biological constructs and systems. This progress report describes how these capabilities can potentially address a myriad of unmet clinical needs. First, the creation of 3D-printed prosthetics to regain lost functionalities by providing structural support for skeletal and tubular organs is highlighted. Second, novel drug delivery strategies aided by 3D-printed devices are described. Third, the advancement of medical research heralded by 3D-printed tissue/organ-on-chips systems is discussed. Fourth, the developments of 3D-printed tissue and organ regeneration are explored. Finally, the potential for seamless integration of engineered organs with active devices by leveraging the versatility of multimaterial 3D printing is envisioned.

The upcoming 3D-printing revolution in microfluidics

In the last two decades, the vast majority of microfluidic systems have been built in poly(dimethylsiloxane) (PDMS) by soft lithography, a technique based on PDMS micromolding. A long list of key PDMS properties have contributed to the success of soft lithography: PDMS is biocompatible, elastomeric, transparent, gas-permeable, water-impermeable, fairly inexpensive, copyright-free, and rapidly prototyped with high precision using simple procedures. However, the fabrication process typically involves substantial human labor, which tends to make PDMS devices difficult to disseminate outside of research labs, and the layered molding limits the 3D complexity of the devices that can be produced. 3D-printing has recently attracted attention as a way to fabricate microfluidic systems due to its automated, assembly-free 3D fabrication, rapidly decreasing costs, and fast-improving resolution and throughput. Resins with properties approaching those of PDMS are being developed. Here we review past and recent efforts in 3D-printing of microfluidic systems. We compare the salient features of PDMS molding with those of 3D-printing and we give an overview of the critical barriers that have prevented the adoption of 3D-printing by microfluidic developers, namely resolution, throughput, and resin biocompatibility. We also evaluate the various forces that are persuading researchers to abandon PDMS molding in favor of 3D-printing in growing numbers.

The upcoming 3D-printing revolution in microfluidics

In the last two decades, the vast majority of microfluidic systems have been built in poly(dimethylsiloxane) (PDMS) by soft lithography, a technique based on PDMS micromolding. A long list of key PDMS properties have contributed to the success of soft lithography: PDMS is biocompatible, elastomeric, transparent, gas-permeable, water-impermeable, fairly inexpensive, copyright-free, and rapidly prototyped with high precision using simple procedures. However, the fabrication process typically involves substantial human labor, which tends to make PDMS devices difficult to disseminate outside of research labs, and the layered molding limits the 3D complexity of the devices that can be produced. 3D-printing has recently attracted attention as a way to fabricate microfluidic systems due to its automated, assembly-free 3D fabrication, rapidly decreasing costs, and fast-improving resolution and throughput. Resins with properties approaching those of PDMS are being developed. Here we review past and recent efforts in 3D-printing of microfluidic systems. We compare the salient features of PDMS molding with those of 3D-printing and we give an overview of the critical barriers that have prevented the adoption of 3D-printing by microfluidic developers, namely resolution, throughput, and resin biocompatibility. We also evaluate the various forces that are persuading researchers to abandon PDMS molding in favor of 3D-printing in growing numbers.

3D-Printed Microfluidics

The advent of soft lithography allowed for an unprecedented expansion in the field of microfluidics. However, the vast majority of PDMS microfluidic devices are still made with extensive manual labor, are tethered to bulky control systems, and have cumbersome user interfaces, which all render commercialization difficult. On the other hand, 3D printing has begun to embrace the range of sizes and materials that appeal to the developers of microfluidic devices. Prior to fabrication, a design is digitally built as a detailed 3D CAD file. The design can be assembled in modules by remotely collaborating teams, and its mechanical and fluidic behavior can be simulated using finite-element modeling. As structures are created by adding materials without the need for etching or dissolution, processing is environmentally friendly and economically efficient. We predict that in the next few years, 3D printing will replace most PDMS and plastic molding techniques in academia.

Microneedles: A New Frontier in Nanomedicine Delivery

This review aims to concisely chart the development of two individual research fields, namely nanomedicines, with specific emphasis on nanoparticles (NP) and microparticles (MP), and microneedle (MN) technologies, which have, in the recent past, been exploited in combinatorial approaches for the efficient delivery of a variety of medicinal agents across the skin. This is an emerging and exciting area of pharmaceutical sciences research within the remit of transdermal drug delivery and as such will undoubtedly continue to grow with the emergence of new formulation and fabrication methodologies for particles and MN. Firstly, the fundamental aspects of skin architecture and structure are outlined, with particular reference to their influence on NP and MP penetration. Following on from this, a variety of different particles are described, as are the diverse range of MN modalities currently under development. The review concludes by highlighting some of the novel delivery systems which have been described in the literature exploiting these two approaches and directs the reader towards emerging uses for nanomedicines in combination with MN.

Microneedle-based transdermal sensor for on-chip potentiometric determination of K(+)

The determination of electrolytes is invaluable for point of care diagnostic applications. An ion selective transdermal microneedle sensor is demonstrated for potassium by integrating a hollow microneedle with a microfluidic chip to extract fluid through a channel towards a downstream solid-state ion-selective-electrode (ISE). 3D porous carbon and 3D porous graphene electrodes, made via interference lithography, are compared as solid-state transducers for ISE's and evaluated for electrochemical performance, stability, and selectivity. The porous carbon K(+) ISE's show better performance than the porous graphene K(+) ISE's, capable of measuring potassium across normal physiological concentrations in the presence of interfering ions with greater stability. This new microfluidic/microneedle platform shows promise for medical applications.

Advanced nanobiomaterial strategies for the development of organized tissue engineering constructs

Nanobiomaterials, a field at the interface of biomaterials and nanotechnologies, when applied to tissue engineering applications, are usually perceived to resemble the cell microenvironment components or as a material strategy to instruct cells and alter cell behaviors. Therefore, they provide a clear understanding of the relationship between nanotechnologies and resulting cellular responses. This review will cover recent advances in nanobiomaterial research for applications in tissue engineering. In particular, recent developments in nanofibrous scaffolds, nanobiomaterial composites, hydrogel systems, laser-fabricated nanostructures and cell-based bioprinting methods to produce scaffolds with nanofeatures for tissue engineering are discussed. As in native niches of cells, where nanofeatures are constantly interacting and influencing cellular behavior, new generations of scaffolds will need to have these features to enable more desirable engineered tissues. Moving forward, tissue engineering will also have to address the issues of complexity and organization in tissues and organs.

The Effects of Geometry on Skin Penetration and Failure of Polymer Microneedles

Microneedles are small-scale devices that may be used for drug delivery and biosensing. In this study, the forces required for mechanical failure, the modes of mechanical failure, as well as the mechanisms for microneedle penetration into porcine skin were examined. Microneedles produced from the acrylate-based polymer e-Shell 200 using an indirect rapid prototyping approach involving two-photon polymerization and poly(dimethylsiloxane) micromolding were found to possess sufficient strength for penetration of porcine skin. The failure forces were an order of magnitude greater than the forces necessary for full insertion into the skin. Bending was the most common form of failure; an increasing aspect ratio and a decreasing tip diameter were associated with lower failure forces. Video captured during skin penetration revealed that microneedle penetration into the skin occurred by means of a series of insertions and not by means of a single insertion event. Images obtained during and after skin penetration confirmed microneedle penetration of skin as well as transdermal delivery of lucifer yellow dye. These findings shed insight into the mechanisms of microneedle penetration and failure, facilitating design improvements for polymer microneedles.

Multiplexed microneedle-based biosensor array for characterization of metabolic acidosis

The development of a microneedle-based biosensor array for multiplexed in situ detection of exercise-induced metabolic acidosis, tumor microenvironment, and other variations in tissue chemistry is described. Simultaneous and selective amperometric detection of pH, glucose, and lactate over a range of physiologically relevant concentrations in complex media is demonstrated. Furthermore, materials modified with a cell-resistant (Lipidure(®)) coating were shown to inhibit macrophage adhesion; no signs of coating delamination were noted over a 48-h period.

Fabrication of microscale medical devices by two-photon polymerization with multiple foci via a spatial light modulator

Two-photon polymerization is an appealing technique for producing microscale devices due to its flexibility in producing structures with a wide range of geometries as well as its compatibility with materials suitable for biomedical applications. The greatest limiting factor in widespread use of two-photon polymerization is the slow fabrication times associated with line-by-line, high-resolution structuring. In this study, a recently developed technology was used to produce microstructures by two-photon polymerization with multiple foci, which significantly reduces the production time. Computer generated hologram pattern technology was used to generate multiple laser beams in controlled positions from a single laser. These multiple beams were then used to simultaneously produce multiple microstructures by two-photon polymerization. Arrays of micro-Venus structures, tissue engineering scaffolds, and microneedle arrays were produced by multifocus two-photon polymerization. To our knowledge, this work is the first demonstration of multifocus two-photon polymerization technology for production of a functional medical device. Multibeam fabrication has the potential to greatly improve the efficiency of two-photon polymerization production of microscale devices such as tissue engineering scaffolds and microneedle arrays.

Modification of microneedles using inkjet printing

In this study, biodegradable acid anhydride copolymer microneedles containing quantum dots were fabricated by means of visible light dynamic mask micro-stereolithography-micromolding and inkjet printing. Nanoindentation was performed to obtain the hardness and the Young's modulus of the biodegradable acid anhydride copolymer. Imaging of quantum dots within porcine skin was accomplished by means of multiphoton microscopy. Our results suggest that the combination of visible light dynamic mask micro-stereolithography-micromolding and inkjet printing enables fabrication of solid biodegradable microneedles with a wide range of geometries as well as a wide range of pharmacologic agent compositions.

Integrated carbon fiber electrodes within hollow polymer microneedles for transdermal electrochemical sensing

In this study, carbon fiber electrodes were incorporated within a hollow microneedle array, which was fabricated using a digital micromirror device-based stereolithography instrument. Cell proliferation on the acrylate-based polymer used in microneedle fabrication was examined with human dermal fibroblasts and neonatal human epidermal keratinocytes. Studies involving full-thickness cadaveric porcine skin and trypan blue dye demonstrated that the hollow microneedles remained intact after puncturing the outermost layer of cadaveric porcine skin. The carbon fibers underwent chemical modification in order to enable detection of hydrogen peroxide and ascorbic acid; electrochemical measurements were demonstrated using integrated electrode-hollow microneedle devices.