Nanoparticulate drug delivery platforms for advancing bone infection therapies

INTRODUCTION

The ongoing surge of resistance of bacterial pathogens to antibiotic therapies and the consistently aging median member of the human race signal an impending increase in the incidence of chronic bone infection. Nanotechnological platforms for local and sustained delivery of therapeutics hold the greatest potential for providing minimally invasive and maximally regenerative therapies for this rare but persistent condition.

AREAS COVERED

Shortcomings of the clinically available treatment options, including poly(methyl methacrylate) beads and calcium sulfate cements, are discussed and their transcending using calcium-phosphate/polymeric nanoparticulate composites is foreseen. Bone is a composite wherein the weakness of each component alone is compensated for by the strength of its complement and an ideal bone substitute should be fundamentally the same.

EXPERT OPINION

Discrepancy between in vitro and in vivo bioactivity assessments is highlighted, alongside the inherent imperfectness of the former. Challenges entailing the cross-disciplinary nature of engineering a new generation of drug delivery vehicles are delineated and it is concluded that the future for the nanoparticulate therapeutic carriers belongs to multifunctional, synergistic and theranostic composites capable of simultaneously targeting, monitoring and treating internal organismic disturbances in a smart, feedback fashion and in direct response to the demands of the local environment.

Efficient targeting of fatty-acid modified oligonucleotides to live cell membranes through stepwise assembly

Lipid modifications provide efficient targeting of oligonucleotides to live cell membranes in a range of applications. Targeting efficiency is a function of the rate of lipid DNA insertion into the cell surface and its persistence over time. Here we show that increasing lipid hydrophobicity increases membrane persistence, but decreases the rate of membrane insertion due to the formation of nonproductive aggregates in solution. To ameliorate this effect, we split the net hydrophobicity of the membrane anchor between two complementary oligonucleotides. When prehybridized in solution, doubly anchored molecules also aggregate due to their elevated hydrophobicity. However, when added sequentially to cells, aggregation does not occur so membrane insertion is efficient. Hybridization between the two strands locks the complexes at the cell surface by increasing net hydrophobicity, increasing their total concentration and lifetime, and dramatically improving their utility in a variety of biomedical applications.

Sustained delivery of MGF peptide from microrods attracts stem cells and reduces apoptosis of myocytes

Local release of drugs may have many advantages for tissue repair but also presents major challenges. Bioengineering approaches allow microstructures to be fabricated that contain bioactive peptides for sustained local delivery. Heart tissue damage is associated with local increases in mechano growth factor (MGF), a member of the IGF-1 family. The E domain of MGF peptide is anti-apoptotic and a stem cell homing factor. The objectives of this study were to fabricate a microrod delivery device of poly (ethylene glycol) dimethacrylate (PEGDMA) hydrogel loaded with MGF peptide and to determine the elution profile and bioactivity of MGF. The injectable microrods are 30 kPa stiffness and 15 μm widths by 100 μm lengths, chosen to match heart stiffness and myocyte size. Successful encapsulation of native MGF peptide within microrods was achieved with delivery of MGF for 2 weeks, as measured by HPLC. Migration of human mesenchymal stem cells (hMSCs) increased with MGF microrod treatment (1.72 ± 0.23, p < 0.05). Inhibition of the apoptotic pathway in neonatal rat ventricular myocytes was induced by 8 h of hypoxia (1 % O2). Protection from apoptosis by MGF microrod treatment was shown by the TUNEL assay and increased Bcl-2 expression (2 ± 0.19, p < 0.05). Microrods without MGF regulated the cytoskeleton, adhesion, and proliferation of hMSCs, and MGF had no effect on these properties. Therefore, the combination microdevice provided both the mechanical cues and 2-week MGF bioactivity to reduce apoptosis and recruit stem cells, suggesting potential use of MGF microrods for cardiac regeneration therapy in vivo.

Planar microdevices for enhanced in vivo retention and oral bioavailability of poorly permeable drugs

The development of novel oral drug delivery platforms for administering therapeutics in a safe and effective manner through the harsh gastrointestinal environment is of great importance. Here, the use of engineered thin planar poly(methyl methacrylate) (PMMA) microdevices is tested to enhance oral bioavailability of acyclovir, a poorly permeable drug. Acyclovir is loaded into the unidirectional drug releasing microdevice reservoirs using a drug entrapping photocross-linkable hydrogel matrix. An increase in acyclovir permeation across in vitro caco-2 monolayer is seen in the presence of microdevices as compared with acyclovir-entrapped hydrogels or free acyclovir solution. Cell proliferation studies show that microdevices are relatively nontoxic in nature for use in in vivo studies. Enhanced in vivo retention of microdevices is observed as their thin side walls experience minimal peristaltic shear stress as compared with spherical microparticles. Unidirectional acyclovir release and enhanced retention of microdevices achieve a 4.5-fold increase in bioavailability in vivo as compared with an oral gavage of acyclovir solution with the same drug mass. The enhanced oral bioavailability results suggest that thin, planar, bioadhesive, and unidirectional drug releasing microdevices will significantly improve the systemic and localized delivery of a broad range of oral therapeutics in the near future.

Nitinol-based nanotubular coatings for the modulation of human vascular cell function

In this study, we describe the synthesis of an upright nanotubular coating with discrete, exposed nanotubes on top of superelastic Nitinol via anodization and characterization of the surface elemental composition and nickel release rates. We demonstrate, for the first time, that this coating could improve re-endothelialization by increasing the cell spreading and migration of primary human aortic endothelial cells on Nitinol. We also show the potential for reducing neointimal hyperplasia by decreasing the proliferation and expression of collagen I and MMP-2 in primary human aortic smooth muscle cells (HASMC). Furthermore, we did not observe the nanotubular surface to induce inflammation through ICAM-1 expression in HASMC as compared to the flat control. This coating could be used to improve Nitinol stents by reducing restenosis rates and, given the extensive use of Nitinol in other implantable devices, act as a generalized coating strategy for other medical devices.

Novel functionalization of discrete polymeric biomaterial microstructures for applications in imaging and three-dimensional manipulation

Adapting ways to functionalize polymer materials is becoming increasingly important to their implementation in translational biomedical sciences. By tuning the mechanical, chemical, and biological qualities of these materials, their applications can be broadened, opening the door for more advanced integration into modern medical techniques. Here, we report on a method to integrate chemical functionalizations into discrete, microscale polymer structures, which are used for tissue engineering applications, for in vivo localization, and three-dimensional manipulation. Iron oxide nanoparticles were incorporated into the polymer matrix using common photolithographic techniques to create stably functional microstructures with magnetic potential. Using magnetic resonance imaging (MRI), we can promote visualization of microstructures contained in small collections, as well as facilitate the manipulation and alignment of microtopographical cues in a realistic tissue environment. Using similar polymer functionalization techniques, fluorine-containing compounds were also embedded in the polymer matrix of photolithographically fabricated microstructures. The incorporation of fluorine-containing compounds enabled highly sensitive and specific detection of microstructures in physiologic settings using fluorine MRI techniques ((19)F MRI). These functionalization strategies will facilitate more reliable noninvasive tracking and characterization of microstructured polymer implants as well as have implications for remote microstructural scaffolding alignment for three-dimensional tissue engineering applications.

Does translational symmetry matter on the micro scale? Fibroblastic and osteoblastic interactions with the topographically distinct poly(ε-caprolactone)/hydroxyapatite thin films

Material composition and topography of the cell-contacting material interface are important considerations in the design of biomaterials at the nano and micro scales. This study is one of the first to have assessed the osteoblastic response to micropatterned polymer-ceramic composite surfaces. In particular, the effect of topographic variations of composite poly(ε-caprolactone)/hydroxyapatite (PCL/HAp) films on viability, proliferation, migration and osteogenesis of fibroblastic and osteoblastic MC3T3-E1 cells was evaluated. To that end, three different micropatterned PCL/HAp films were compared: flat and textured, the latter of which included films comprising periodically arranged and randomly distributed oval topographic features 10 μm in diameter, 20 μm in separation and 10 μm in height, comparable to the dimensions of MC3T3-E1 cells. PCL/HAp films were fabricated by the combination of a bottom-up, soft chemical synthesis of the ceramic, nanoparticulate phase and a top-down, photolithographic technique for imprinting fine, microscale features on them. X-ray diffraction analysis indicated an isotropic orientation of both the polymeric chains and HAp crystallites in the composite samples. Biocompatibility tests indicated no significant decrease in their viability when grown on PCL/HAp films. Fibroblast proliferation and migration onto PCL/HAp films proceeded slower than on the control borosilicate glass, with the flat composite film fostering more cell migration activity than the films containing topographic features. The gene expression of seven analyzed osteogenic markers, including procollagen type I, osteocalcin, osteopontin, alkaline phosphatase, and the transcription factors Runx2 and TGFβ-1, was, however, consistently upregulated in cells grown on PCL/HAp films comprising periodically ordered topographic features, suggesting that the higher levels of symmetry of the topographic ordering impose a moderate mechanochemical stress on the adherent cells and thus promote a more favorable osteogenic response. The obtained results suggest that topography can be a more important determinant of the cell/surface interaction than the surface chemistry and/or stiffness as well as that the regularity of the distribution of topographic features can be a more important variable than the topographic features per se.

Planar microdevices enhance transport of large molecular weight molecules across retinal pigment epithelial cells

Large molecular weight drug delivery to the posterior eye is challenging due to cellular barriers that hinder drug transport. Understanding how to enhance transport across the retinal barrier is important for the design of new drug delivery systems. A novel mechanism to enhance drug transport is the use of geometric properties, which has not been extensively explored in the retina. Planar SU-8/Poly(ethyleneglycol)dimethacrylate microdevices were constructed using photolithography to deliver FITC dextran across an in vitro retinal model. The model consists of retinal pigment epithelial (RPE) cells grown to confluence on transwell inserts, which provides an environment to investigate the influence of geometry on paracellular and transcellular delivery of encapsulated large molecules. Planar microdevices enhanced transport of large molecular weight dextrans across different models of RPE in a size dependent fashion. Increased drug permeation across the RPE was observed with the addition of microdevices as compared to a traditional bolus of FITC dextran. This phenomena was initiated by a non-toxic interaction between the microdevices and the retinal tight junction proteins. Suggesting that increased drug transport occurs via a paracellular pathway. These experiments provide evidence to support the future use of planar unidirectional microdevices for delivery of biologics in ocular applications.

Discrete microstructural cues for the attenuation of fibrosis following myocardial infarction

Chronic fibrosis caused by acute myocardial infarction (MI) leads to increased morbidity and mortality due to cardiac dysfunction. We have developed a therapeutic materials strategy that aims to mitigate myocardial fibrosis by utilizing injectable polymeric microstructures to mechanically alter the microenvironment. Polymeric microstructures were fabricated using photolithographic techniques and studied in a three-dimensional culture model of the fibrotic environment and by direct injection into the infarct zone of adult rats. Here, we show dose-dependent down-regulation of expression of genes associated with the mechanical fibrotic response in the presence of microstructures. Injection of this microstructured material into the infarct zone decreased levels of collagen and TGF-β, increased elastin deposition and vascularization in the infarcted region, and improved functional outcomes after six weeks. Our results demonstrate the efficacy of these discrete anti-fibrotic microstructures and suggest a potential therapeutic materials approach for combatting pathologic fibrosis.

Compliant 3D microenvironment improves β-cell cluster insulin expression through mechanosensing and β-catenin signaling

Type 1 diabetes is chronic disease with numerous complications and currently no cure. Tissue engineering strategies have shown promise in providing a therapeutic solution, but maintenance of islet function and survival within these therapies represents a formidable challenge. The islet microenvironment may hold the key for proper islet maintenance. To elucidate the microenvironmental conditions necessary for improved islet function and survival, three-dimensional (3D) polyacrylamide cell scaffolds were fabricated with stiffnesses of 0.1 and 10 kPa to regulate the spatial and mechanical control of biosignals. Specifically, we show a significant increase in insulin mRNA expression of 3D primary mouse islet-derived and Min6-derived β-cell clusters grown on compliant 0.1 kPa scaffolds. Moreover, these compliant 0.1 kPa scaffolds also increase glucose sensitivity in Min6-derived β-cell clusters as demonstrated by the increased glucose stimulation index. Our data suggest that stiffness-specific insulin processing is regulated through the myosin light chain kinase (MLCK) and Rho-associated protein kinase (ROCK) mechanosensing pathways. Additionally, β-catenin is required for regulation of stiffness-dependent insulin expression. Through activation or inhibition of β-catenin signaling, reversible control of insulin expression is achieved on the compliant 0.1 kPa and overly stiff 10 kPa substrates. Understanding the role of the microenvironment on islet function can enhance the therapeutic approaches necessary to treat diabetes for improving insulin sensitivity and response.

Simultaneous bactericidal and osteogenic effect of nanoparticulate calcium phosphate powders loaded with clindamycin on osteoblasts infected with Staphylococcus aureus

Staphylococcus aureus internalized by bone cells and shielded from the immune system provides a reservoir of bacteria in recurring osteomyelitis. Its targeting by the antibiotic therapy may thus be more relevant for treating chronic bone infection than eliminating only the pathogens colonizing the bone matrix. Assessed was the combined osteogenic and antibacterial effect of clindamycin-loaded calcium phosphate nanoparticles of different monophasic compositions on co-cultures comprising osteoblasts infected with S. aureus. Antibiotic-carrying particles were internalized by osteoblasts and minimized the concentration of intracellular bacteria. In vitro treatments of the infected cells, however, could not prevent cell necrosis due to the formation of toxic byproducts of the degradation of the bacterium. Antibiotic-loaded particles had a positive morphological effect on osteoblasts per se, without reducing their viability, alongside stimulating the upregulation of expression of different bone growth markers in infected osteoblasts to a higher degree than achieved during the treatment with antibiotic only.

Membranes to achieve immunoprotection of transplanted islets

Transplantation of islet or beta cells is seen as the cure for type 1 diabetes since it allows physiological regulation of blood glucose levels without requiring any compliance from the patients. In order to circumvent the use of immunosuppressive drugs (and their side effects), semipermeable membranes have been developed to encapsulate and immunoprotect transplanted cells. This review presents the historical developments of immunoisolation and provides an update on the current research in this field. A particular emphasis is laid on the fabrication, characterization and performance of membranes developed for immunoisolation applications.

In vitro analysis of nanoparticulate hydroxyapatite/chitosan composites as potential drug delivery platforms for the sustained release of antibiotics in the treatment of osteomyelitis

Nanoparticulate composites of hydroxyapatite (HAp) and chitosan were synthesized by ultrasound-assisted sequential precipitation and characterized for their microstructure at the atomic scale, surface charge, drug release properties, and combined antibacterial and osteogenic response. Crystallinity of HAp nanoparticles was reduced because of the interference of the surface layers of chitosan with the dissolution/reprecipitation-mediated recrystallization mechanism that conditions the transition from the as-precipitated amorphous calcium phosphate phase to the most thermodynamically stable one--HAp. Embedment of 5-10 nm sized, narrowly dispersed HAp nanoparticles within the polymeric matrix mitigated the burst release of the small molecule model drug, fluorescein, bound to HAp by physisorption, and promoted sustained-release kinetics throughout the 3 weeks of release time. The addition of chitosan to the particulate drug carrier formulation, however, reduced the antibacterial efficacy against S aureus. Excellent cell spreading and proliferation of osteoblastic MC3T3-E1 cells evidenced on microscopic conglomerates of HAp nanoparticles in vitro also markedly diminished on HAp/chitosan composites. Mitochondrial dehydrogenase activity exhibited normal values only for HAp/chitosan particle concentrations of up to 2 mg/cm(2) and significantly dropped, by about 50%, at higher particle concentrations (4 and 8 mg/cm(2)). The gene expression of osteocalcin, a mineralization inductor, and the transcription factor Runx2 was downregulated in cells incubated in the presence of 3 mg/cm(2) HAp/chitosan composite particles, whereas the expression of osteopontin, a potent mineralization inhibitor, was upregulated, further demonstrating the partially unfavorable osteoblastic cell response to the given particles. The peak in the expression of osteogenic markers paralleling the osteoblastic differentiation was also delayed most for the cell population incubated with HAp/chitosan particles. Overall, the positive effect of chitosan coating on the drug elution profile of HAp nanoparticles as carriers for the controlled delivery of antibiotics in the treatment of osteomyelitis was compensated for by the lower bacteriostatic efficiency and the comparatively unviable cell response to the composite material, especially at higher dosages.

The effect of nanotopography on modulating protein adsorption and the fibrotic response

Understanding and modulating the cellular response to implanted biomaterials is crucial for the field of tissue engineering and regenerative medicine. Since cells typically reside in an extracellular matrix containing nanoscale architecture, identifying synthetic nanostructures that induce desirable cellular behaviors could greatly impact the field. Using nanoimprint lithography, nanostructured patterns were generated on thin film polymeric materials. The ability of these surfaces to influence protein adsorption, fibroblast proliferation and morphology, and fibrotic markers was investigated. Nanostructured features with aspect ratios greater than five allowed for less protein adsorption, resulting in decreased fibroblast proliferation and rounded cellular morphology. These nanofeatures also induced significantly lower gene expression of collagen 1α2, collagen 3α1, and growth factors such as connective tissue growth factor, integrin linked kinase, transforming growth factor β1 (TGF-β1), and epidermal growth factor, key factors associated with a fibrotic response. The results demonstrate that select nanostructured surfaces could be used to modulate the fibrotic behavior in cells and have the potential to be used as antifibrotic architecture for medical implants or tissue engineering scaffolds.

Osteogenic and antimicrobial nanoparticulate calcium phosphate and poly-(D,L-lactide-co-glycolide) powders for the treatment of osteomyelitis

Development of a material for simultaneous sustained and localized delivery of antibiotics and induction of spontaneous regeneration of hard tissues affected by osteomyelitis stands for an important clinical need. In this work, a comparative analysis of the bacterial and osteoblastic cell response to two different nanoparticulate carriers of clindamycin, an antibiotic commonly prescribed in the treatment of bone infection, one composed of calcium phosphate and the other comprising poly-(D,L-lactide-co-glycolide)-coated calcium phosphate, was carried out. Three different non-cytotoxic phases of calcium phosphate, exhibiting dissolution and drug release profiles in the range of one week to two months to one year, respectively, were included in the analysis: monetite, amorphous calcium phosphate and hydroxyapatite. Spherical morphologies and narrow size distribution of both types of nanopowders were confirmed in transmission and scanning electron microscopic analyses. The antibiotic-containing powders exhibited sustained drug release contingent upon the degradation rate of the carrier. Assessment of the antibacterial performance of the antibiotic-encapsulated powders against Staphylococcus aureus, the most common pathogen isolated from infected bone, yielded satisfactory results both in broths and on blood agar plates for all the analyzed powders. In contrast, no cytotoxic behavior was detected upon the incubation of the antibiotic powders with the osteoblastic MC3T3-E1 cell line for up to three weeks. The cells were shown to engage in a close contact with the antibiotic-containing particles, irrespective of their internal or surface phase composition, polymeric or mineral. At the same time, both types of particles upregulated the expression of osteogenic markers osteocalcin, osteopontin, Runx2 and protocollagen type I, suggesting their ability to promote osteogenesis and enhance remineralization of the infected site in addition to eliminating the bacterial source of infection.

Effect of calcium phosphate particle shape and size on their antibacterial and osteogenic activity in the delivery of antibiotics in vitro

Powders composed of four morphologically different calcium phosphate particles were prepared by precipitation from aqueous solutions: flaky, brick-like, elongated orthogonal, and spherical. The particles were then loaded with either clindamycin phosphate as the antibiotic of choice or fluorescein, a model molecule used to assess the drug release properties. A comparison was carried out of the effect of such antibiotic-releasing materials on: sustained drug release profiles; Staphylococcus aureus growth inhibition; and osteogenic propensities in vitro. Raman spectroscopic analysis indicated the presence of various calcium phosphate phases, including monetite (flaky and elongated orthogonal particles), octacalcium phosphate (brick-shaped particles), and hydroxyapatite (spherical particles). Testing the antibiotic-loaded calcium phosphate powders for bacterial growth inhibition demonstrated satisfying antibacterial properties both in broths and on agar plates. All four calcium-phosphate-fluorescein powders exhibited sustained drug release over 21 days. The calcium phosphate sample with the highest specific surface area and the smallest, spherical particle size was the most effective in both drug loading and release, consequently having the highest antibacterial efficiency. Moreover, the highest cell viability, the largest gene expression upregulation of three different osteogenic markers--osteocalcin, osteopontin, and Runx2--as well as the least disrupted cell cytoskeleton and cell morphologies were also noticed for the calcium phosphate powder composed of the smallest, spherical nanosized particles. Still, all four powders exerted a viable effect on osteoblastic MC3T3-E1 cells in vitro, as evidenced by both morphological assessments on fluorescently stained cells and measurements of their mitochondrial activity. The obtained results suggest that the nanoscale particle size and the corresponding coarseness of the surface of particle conglomerates as the cell attachment points may present a favorable starting point for the development of calcium-phosphate-based osteogenic drug delivery devices.

Ocular biocompatibility and structural integrity of micro- and nanostructured poly(caprolactone) films

The identification of biomaterials that are well tolerated in the eye is important for the development of new ocular drug delivery devices and implants, and the application of micro- and nanoengineered devices to biomedical treatments is predicated on the long-term preservation within the target organ or tissue of the very small functional design elements. This study assesses the ocular tolerance and durability of micro- and nanostructured biopolymer thin films injected or implanted into the rabbit eye. Structured poly(caprolactone) (PCL) thin films were placed in adult rabbit eyes for survival studies, with serial ophthalmic examinations over 6 months. Morphologic abnormalities and device/tissue reactions were evaluated by histologic studies, and scanning electron microscopy (SEM) of films was used to determine the structural integrity. Structured PCL thin films (20- to 40-μm thick) were constructed to design specifications with 50-μm linear microgrooves or arrays of nanopores with ~30-nm diameters. After up to 9 months of ocular residency, SEM on devices retrieved from the eye showed preservation of micro- and nanostructural features. In ocular safety evaluations carried out over 6 months, serial examinations in 18 implanted eyes showed no evidence of chronic inflammation, cataractogenesis, or retinal toxicity. Postoperative ocular inflammation was seen in 67% of eyes for 1 week, and persistent corneal edema occurred in 1 eye. Histology revealed no ocular inflammation or morphologic abnormalities of ocular tissues. Thin-film/tissue responses such as cellular reaction, fibrosis, or surface biodeposits were not seen. Micro- and nanostructured PCL thin films exhibited acceptable ocular tolerance and maintained the structural integrity of design features while residing in the eye. Thin-film micro- and nanostructured PCL appears to be a feasible biomaterial for intraocular therapeutic applications.

Radially branched deployment for more efficient cell transplantation at the scale of the human brain

BACKGROUND

In preclinical studies, cell transplantation into the brain has shown great promise for the treatment of a wide range of neurological diseases. However, the use of a straight cannula and syringe for cell delivery to the human brain does not approximate cell distribution achieved in animal studies. This technical deficiency may limit the successful clinical translation of cell transplantation.

OBJECTIVE

To develop a stereotactic device that effectively distributes viable cells to the human brain. Our primary aims were to (1) minimize the number of transcortical penetrations required for transplantation, (2) reduce variability in cell dosing and (3) increase cell survival.

METHODS

We developed a modular cannula system capable of radially branched deployment (RBD) of a cell delivery catheter at variable angles from the longitudinal device axis. We also developed an integrated catheter-plunger system, eliminating the need for a separate syringe delivery mechanism. The RBD prototype was evaluated in vitro and in vivo with subcortical injections into the swine brain. Performance was compared to a 20G straight cannula with dual side ports, a device used in current clinical trials.

RESULTS

RBD enabled therapeutic delivery in a precise 'tree-like' pattern branched from a single initial trajectory, thereby facilitating delivery to a volumetrically large target region. RBD could transplant materials in a radial pattern up to 2.0 cm from the initial penetration tract. The novel integrated catheter-plunger system facilitated manual delivery of small and precise volumes of injection (1.36 ± 0.13 µl per cm of plunger travel). Both dilute and highly concentrated neural precursor cell populations tolerated transit through the device with high viability and unaffected developmental potential. While reflux of infusate along the penetration tract was problematic with the use of the 20G cannula, RBD was resistant to this source of cell dose variability in agarose. RBD enabled radial injections to the swine brain when used with a modern clinical stereotactic system.

CONCLUSIONS

By increasing the total delivery volume through a single transcortical penetration in agarose models, RBD strategy may provide a new approach for cell transplantation to the human brain. Incorporation of RBD or selected aspects of its design into future clinical trials may increase the likelihood of successful translation of cell-based therapy to the human patient.

Nanostructure-mediated transport of biologics across epithelial tissue: enhancing permeability via nanotopography

Herein, we demonstrate that nanotopographical cues can be utilized to enable biologics >66 kDa to be transported across epithelial monolayers. When placed in contact with epithelial monolayers, nanostructured thin films loosen the epithelial barrier and allow for significantly increased transport of FITC-albumin, FITC-IgG, and a model therapeutic, etanercept. Our work highlights the potential to use drug delivery systems which incorporate nanotopography to increase the transport of biologics across epithelial tissue.

Nano- and microfabrication for overcoming drug delivery challenges

This highlight article describes current nano- and microfabrication techniques for creating drug delivery devices. We first review the main physiological barriers to delivering therapeutic agents. Then, we describe how novel fabrication methods can be utilized to combine many features into a single physiologically relevant device to overcome drug delivery challenges.

Microdomain heterogeneity in 3D affects the mechanics of neonatal cardiac myocyte contraction

Cardiac muscle cells are known to adapt to their physical surroundings, optimizing intracellular organization and contractile function for a given culture environment. A previously developed in vitro model system has shown that the inclusion of discrete microscale domains (or microrods) in three dimensions (3D) can alter long-term growth responses of neonatal ventricular myocytes. The aim of this work was to understand how cellular contact with such a domain affects various mechanical changes involved in cardiac muscle cell remodeling. Myocytes were maintained in 3D gels over 5 days in the presence or absence of 100-μm-long microrods, and the effect of this local heterogeneity on cell behavior was analyzed via several imaging techniques. Microrod abutment resulted in approximately twofold increases in the maximum displacement of spontaneously beating myocytes, as based on confocal microscopy scans of the gel xy-plane or the myocyte long axis. In addition, microrods caused significant increases in the proportion of aligned myofibrils (≤20° deviation from long axis) in fixed myocytes. Microrod-related differences in axial contraction could be abrogated by long-term interruption of certain signals of the RhoA-/Rho-associated kinase (ROCK) or protein kinase C (PKC) pathway. Furthermore, microrod-induced increases in myocyte size and protein content were prevented by ROCK inhibition. In all, the data suggest that microdomain heterogeneity in 3D appears to promote the development of axially aligned contractile machinery in muscle cells, an observation that may have relevance to a number of cardiac tissue engineering interventions.

Multi-reservoir bioadhesive microdevices for independent rate-controlled delivery of multiple drugs

A variety of oral administrative systems such as enterically coated tablets, capsules, particles, and liposomes have been developed to improve oral bioavailability of drugs. However, they suffer from poor intestinal localization and therapeutic efficacy due to the various physiological conditions and high shear fluid flow. Fabrication of novel microdevices combined with the introduction of controlled release, improved adhesion, selective targeting, and tissue permeation may overcome these issues and potentially diminish the toxicity and high frequency of conventional oral administration. Herein, thin, asymmetric, poly(methyl methacrylate) (PMMA) microdevices are fabricated with multiple reservoirs using photolithography and reactive ion etching. They are loaded with different individual model drug in each reservoir. Enhanced bioadhesion of the microdevices is observed in the presence of a conjugated of targeting protein (tomato lectin) to the PMMA surface. As compared to drug encompassing hydrogels, an increase in drug permeation across the caco-2 monolayer is noticed in the presence of a microdevice loaded with the same drug-hydrogel system. Also, the release of multiple drugs from their respective reservoirs is found to be independent from each other. The use of different hydrogel systems in each reservoir shows differences in the controlled release of the respective drugs over the same release period. These results suggest that, in the future, microfabricated unidirectional multi-drug releasing devices will have an impact on the oral administration of a broad range of therapeutics.

Size-controlled insulin-secreting cell clusters

The search for an effective cure for type I diabetes from the transplantation of encapsulated pancreatic β-cell clusters has so far produced sub-optimal clinical outcomes. Previous efforts have not controlled the size of transplanted clusters, a parameter implicated in affecting long-term viability and the secretion of therapeutically sufficient insulin. Here we demonstrate a method based on covalent attachment of patterned laminin for fabricating uniformly size-controlled insulin-secreting cell clusters. We show that cluster size within the range 40-120μm in diameter affects a variety of therapeutically relevant cellular responses including insulin expression, content and secretion. Our studies elucidate two size-dependent phenomena: (1) as the cluster size increases from 40μm to 60μm, glucose stimulation results in a greater amount of insulin produced per cell; and (2) as the cluster size increases beyond 60μm, sustained glucose stimulation results in a greater amount of insulin secreted per cell. Our study describes a method for producing uniformly sized insulin-secreting cell clusters, and since larger cluster sizes risk nutrient availability limitations, our data suggest that 100-120μm clusters may provide optimal viability and efficacy for encapsulated β-cell transplants as a treatment for type I diabetes and that further in vivo evaluation is warranted.

Emerging microtechnologies for the development of oral drug delivery devices

The development of oral drug delivery platforms for administering therapeutics in a safe and effective manner across the gastrointestinal epithelium is of much importance. A variety of delivery systems such as enterically coated tablets, capsules, particles, and liposomes have been developed to improve oral bioavailability of drugs. However, orally administered drugs suffer from poor localization and therapeutic efficacy due to various physiological conditions such as low pH, and high shear intestinal fluid flow. Novel platforms combining controlled release, improved adhesion, tissue penetration, and selective intestinal targeting may overcome these issues and potentially diminish the toxicity and high frequency of administration associated with conventional oral delivery. Microfabrication along with appropriate surface chemistry, provide a means to fabricate these platforms en masse with flexibility in tailoring the shape, size, reservoir volume, and surface characteristics of microdevices. Moreover, the same technology can be used to include integrated circuit technology and sensors for designing sophisticated autonomous drug delivery devices that promise to significantly improve point of care diagnostic and therapeutic medical applications. This review sheds light on some of the fabrication techniques and addresses a few of the microfabricated devices that can be effectively used for controlled oral drug delivery applications.