DRUG DELIVERY: FUNDAMENTALS AND APPLICATIONS

The National Nanotechnology Initiative defines nanotechnology as any technology conducted or containing features at 1–100 nm. Nanodevices are systems designed on the micro- or nanoscale, with nanoscale features. When attempting to design features in the nanometer range, not only is fabrication a challenge, but verification and visualization of the results is a hurdle in itself. As this chapter will discuss, both micro- and nanoscale features offer a range of possibilities for improving medical technologies and drug delivery, but it is only with advances in fabrication technologies and analytical techniques that we are able to design, control, and characterize these features.

Nanoscale features confer unique and tunable properties to materials that can be leveraged by scientists and engineers to design materials and devices with novel properties and functions. For example, relative to their mass, materials with nanoscale features have much higher surface areas and a greater potential for surface interactions and for significant intermolecular forces than standard materials. When considering bulk material properties, physical and chemical properties are independent of size; aluminum will have the same material properties whether it is 1 in.² or 1 ft². However, nanosized particles exhibit properties such as electrical conductivity, chemical reactivity, and melting point, among others, which can depend on the particle size. Scientists can leverage these size dependencies to fine-tune material properties by controlling particle size.

In drug delivery and targeting, a multitude of barriers must be overcome to successfully deliver a therapeutic to its site of action. While the physicochemical properties and intended target of a drug often dictate these challenges, drug delivery systems (DDS) mitigate these obstacles to improve efficacy and bioavailability. Through their unique properties, micro- and nanofeatures bolster drug delivery systems by increasing membrane permeation, improving mucoadhesion, controlling drug release, and minimizing immune response. Because molecular and cellular biology occurs primarily at the nanoscale, utilizing nanoscale features allows scientists to interact precisely and directly with key components in biological systems. For example, nanopillar arrays can interact with individual cells to change cell morphology and increase drug permeability through a cell layer.

In this chapter, we will describe top-down micro- and nanofabrication techniques, outline some challenges in drug delivery that are being addressed through micro- and nanofabrication, and give examples of some specific applications of nanotechnology to drug delivery.

19.2 TOP-DOWN MICRO- AND NANOFABRICATION TECHNIQUES

Here we describe the fabrication and analytical technologies that have made manipulation and measurement at the nanoscale possible. Micro- and nanofabrication leverage materials and techniques originating in the microelectronics field to offer new approaches for overcoming traditional challenges in drug delivery. As a “top-down” process, micro- and nanofabrication provides a high level of control over size, shape, and surface features. Modifications of basic techniques such as lithography, molding, and extrusion allow for micro- and nanoscale manipulation of a range of materials, including polymers, as well as metals and metal oxides. The development of new fabrication techniques and materials with novel properties continues to expand the application of micro- and nanofeatures in drug delivery systems, but here we will cover only the basic fabrication techniques that are most relevant to creating nanotechnologies for drug delivery.

19.2.1 SOLVENT CASTING

Solvent casting is a technique to form uniform layers of a material, most often a polymer, on a surface. The general idea behind solvent casting is to dissolve the material of interest in an organic solvent, to cast the solution into the desired shape, and to allow the solvent to evaporate, leaving behind the material of interest in its solid form. This technique typically works best with materials that in their solid state have structural integrity and using volatile solvents. A key component is finding a compatible solvent for the material of interest. Solvent casting is utilized in micro- and nanolithography in a number of ways. It is often used in conjunction with spin-coating systems to create uniform thin films. Spin-coating systems use centrifugal force to coat a substrate with a solution, by spinning the substrate. Solvent casting can also be used to fill molds to form materials into specific shapes. Templates and molds, especially on the micro- and nanoscale, can be combined with spin-coating systems, to create thin films with topographies defined by the template or mold over which the solution is cast.

19.2.2 ETCHING

Etching is a process by which a material is removed or degraded. When controlled, etching is a powerful technique in fabrication processes to selectively remove materials, leaving protected areas of the same material, or compatible materials, behind. There are a number of different types of etching that are important in micro- and nanofabrication, including wet etching, photoetching, and reactiveion etching. The type of etching is defined by how the material is being removed. In wet etching, a material is in contact with a solution that either dissolves the material or chemically attacks it, causing it to degrade. In photoetching, material is removed by exposure to UV light, and in reactive-ion etching, chemically reactive plasma is generated by an electromagnetic field that when directed at the substrate surface, removes the material that it contacts. Etching is most useful when combined with lithographic techniques that protect regions and patterns of a material, leaving only the exposed areas for etching. Etching can also be useful in fabrication processes to remove sacrificial templates or scaffolds. The etching process can be controlled by the duration and strength of exposure to the etchant.

19.2.3 LITHOGRAPHY

Lithography was first developed as a printing technique in the late 1700s, that involved creating a template stamp (by creating a grease-based image on lithographic limestone and subsequently etching the stone that was not protected by the grease-based image), which could then be used to transfer ink onto paper. In micro- and nanofabrication, lithography typically refers to photolithography, one of the most commonly used techniques.

Photolithography uses a similar approach to print lithography, but with different materials and more precision (Figure 19.1). Photolithography is made possible by the development of photopatternable materials called photoresists. Photoresists are polymeric materials that cross-link, polymerize, or cleave, when exposed to UV light. Photoresists that polymerize or cross-link upon UV exposure are called negative photoresists because the areas exposed to UV light are the areas that are retained in the final template, leaving the inverse of the UV-blocking photomask. Positive photoresists are degraded with UV exposure, leaving only the areas of photoresist that were blocked from exposure by the photomask pattern.

To create patterns using photoresists, exposure to UV radiation is limited by a micropatterned photomask with the desired features. Photomasks can be designed for either positive or negative photoresists to achieve the desired patterns. For example, when using a negative photoresist, the desired pattern will be clear with all other areas blocked out, and for a positive photoresist, the desired pattern will be blocked out, with all other areas transparent.

FIGURE 19.1 The photolithography process. (a) A polymer substrate layer (poly(methyl methacrylate) [PMMA]) and a positive photoresist layer are solvent cast through spin coating onto a support substrate. (b) Photoresist is exposed to ultraviolet irradiation with a photomask in place to block PMMA degradation. (c) Reactive-ion etching is used to etch away exposed PMMA substrate. (d, e) A second photomask and etching step are used to get more advanced features. (f) Finally, the photoresist is removed leaving just the polymer substrate with the desired pattern.

The photoresist pattern can be the final template, or the photoresist can be used to transfer the pattern to an underlying substrate in a subsequent etching step. In the latter case, the photoresist is first coated over the substrate of interest before UV exposure. The negative pattern of the photoresist (i.e., the areas where the photoresist is absent) allows the underlying substrate to be exposed for etching, resulting in a final pattern on the substrate that reflects the positive photoresist pattern from the areas blocked to etching by the photoresist. Lithography is often applied in a layer-by-layer approach, leveraging solvent casting and spin coating to deposit each layer and create complex patterns of multiple materials. For example, after depositing and lithographically patterning one material onto a substrate, a second material may be deposited through solvent casting on top of the first layer and lithographically patterned with a different pattern. In such a way, one is able to produce hierarchical features with multiple materials.

Photolithography is the basis for many microfabrication procedures and leverages additional techniques such as solvent casting and etching in the process. Depending on the materials being used and the precision of the masks, photolithography can be applied in various approaches to create a breadth of designs, structures, and devices, limited only by the creativity of the designer. The resolution of photolithography is limited by the wavelength of the light used to treat the photoresist. While standard photolithography can achieve at best 1 μm resolution, special light sources such as deep ultraviolet light can improve resolution to as small as 50 nm.

While photolithography allows the designer precise control over shapes and sizes, it can only be applied to a flat substrate and is not flexible enough to create geometric shapes that are not flat on the ends, such as spheres or arches. A further limitation of photolithography is the need for a cleanroom environment, free of particles and dust.

While photolithography is the most commonly used lithographic technique in micro- and nanofabrication, there are a number of other lithographic approaches, and modifications of existing approaches, that are also being used. Techniques such as multibeam interference lithography, probe lithography, electron-beam lithography, and nanoimprint lithography complement photolithography, to increase patterning capabilities by expanding the range of materials that can be patterned, as well as the sizes and shapes of patterns that can be achieved.

19.2.4 MOLDING

Molding is a technique that uses a stamp or a mold to transfer a pattern onto a surface. There are a number of different molding techniques utilized in micro- and nanofabrication. Hard-pattern molding uses a micro- or nanopatterned template as a stamp or mold for repeated patterning of polymer surfaces. Micro- and nanofeature hard mold templates are created by using other techniques such as reactive-ion etching or electron-beam lithography to transfer a pattern into a hard substrate. A monomer, polymer, or prepolymer is then applied to the mold and cross-linked or cured. There are several techniques such as nanoimprint lithography and step–flash imprint lithography that use hard-pattern molds to transfer patterns in the lithographic process.

Soft-pattern molding, also known as soft lithography, uses a micropatterned elastomeric material as the stamp or mold. Replica molding transfers a patterned master to polydimethylsiloxane (PDMS) and then solidifies a photocurable or thermally curable prepolymer against the PDMS mold to produce a replica of the original master. Solvent-assisted micromolding uses a solvent to swell or dissolve a polymer against a PDMS mold. Upon evaporation of the solvent, the polymer substrate solidifies around the PDMS mold and is thus inversely patterned with the mold features.

Hard-pattern and soft-pattern moldings are the broadest categories of molding techniques, but in recent years a number of more specialized molding techniques have been developed to overcome challenges in specific applications. For example, in order to prevent thermal degradation of drugs incorporated directly into polymer devices, a low-temperature vacuum-molding technique was developed to cure the molded polymer without the need for elevated temperatures.

The approaches introduced here are not an exhaustive representation of fabrication techniques used in micro- and nanofabrication, but rather an overview of the basic techniques that are critical to fabrication processes. Many specialized and novel techniques are emerging based upon these basic approaches. Most advanced fabrication procedures do not simply utilize a single technique but rather combine approaches in multistep processes to create complex systems. In Section 19.4, we will discuss specific examples of micro- and nanotechnologies in drug delivery that utilize these approaches in their fabrication processes.

19.3 NANOSCALE CHARACTERIZATION

A systematic approach to developing nanodevices requires not only the ability to create and control structures at the nanoscale but also the power to measure and characterize these features. Whether the aim is to demonstrate improved drug delivery or to gain a mechanistic understanding of the impact of nanofeatures, we must be able to verify the nanofeatures that we have created, their dimensions, and their properties. While standard microscopy is not powerful enough for nanoscale resolution, techniques such as scanning electron microscopy (SEM) or atomic force microscopy (AFM) provide means for directly observing and measuring our nanofeatures. Because nanodevices are often composed of micro or even macro components in combination with nanofeatures, other analytical techniques such as profilometry for microscale measurements are also critical in nanodevice technology.

19.3.1 SCANNING ELECTRON MICROSCOPY

Because of the wavelike nature of light, a light microscope can magnify an image up to, at best, 1500×. While this may be adequate to make out micron-sized features, it falls short of the resolution needed to visualize nanofeatures. The SEM, however, is able to magnify up to 1 million times, which is sufficient to resolve features of the order of a single nanometer. It scans the sample using a beam of electrons, which excite atoms in the sample, producing secondary electrons that are emitted by the excited atoms. The quantity of secondary electrons produced depends on the angle between the surface of the sample and the scanning electron beam. By detecting and analyzing the secondary electrons produced, relative to the position of the scanning beam, an image is produced.

In order to ensure consistent and adequate electron excitation, nonconductive surfaces are coated with a layer of conductive metal such as iridium or gold prior to imaging. This layer is thin enough so as to not obstruct any surface features of the underlying sample but also provides consistent and adequate electron density for excitation. However, once coated and imaged by SEM, samples cannot be recovered for use. SEM is a powerful tool for visualizing micro- and nanofeatures as well as for quantifying dimensions through comparison to scale bars that accompany the magnified images (Figure 19.2).

19.3.2 ATOMIC FORCE MICROSCOPY

Not commercially available until the end of the 1980s, AFM is capable of subnanometer resolution. In AFM, an image is collected from a probe that passes across the sample surface. The AFM probe has a nanometer-sized cantilever that does not actually touch the surface, but when brought in close proximity to the sample surface, forces between the sample and the cantilever deflect the cantilever. The force of deflection is measured and translated into an image of the surface. The AFM is able to measure different forces depending on the situation; some of these forces include mechanical contact, electrostatic, magnetic, and van der Waals forces.

19.3.3 PROFILOMETRY

Profilometry is another technique that uses a probe to scan the surface of a sample (Figure 19.3). However, profilometry does not measure the forces between the probe and the surface, but rather the displacement of the probe due to changes in surface features. Profilometry is not sensitive enough to detect submicron features, but it is still an important tool in the characterization of nanodevices, which often involve both micro- and nanofeatures. Profilometry is also useful in determining the thickness of thin films, surface roughness, and microscale topographical features. Compared to SEM or AFM, profilometry is much less expensive and time intensive.

FIGURE 19.2 Scanning electron microscope images. (a) Cells attached to micron-sized structures on a poly(propylene) film. (b) Nanosized pores in a poly(caprolactone) film.

FIGURE 19.3 Profilometer used to measure changes in height across a surface. The scanning electron microscopy image (a) shows a microdevice with three wells on the surface. The dashed line indicates the path of the profilometry measurement, which measures changes in height across the surface. The profilometry data (b) are represented with height on the y-axis and distance on the x-axis. (From Chirra, H. and Desai, T.A.: Multi-reservoir bioadhesive microdevices for independent rate-controlled delivery of multiple drugs. Small. 2012. 8(24). 3839–3846. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

SEM, AFM, and profilometry are examples of advanced instrumentation that allow direct visualization and measurement of key features at the micro- and nanoscale. The following sections provide specific examples of how nanotechnologies are being used in drug delivery.

19.4 APPLICATIONS OF NANOTECHNOLOGY IN DRUG DELIVERY

19.4.1 MICRO- AND NANOTOPOGRAPHY TO OVERCOME EPITHELIAL BARRIERS

In drug delivery, we face the general challenge of passing molecules through biological membranes in practically all routes of administration. As described in detail in Chapter 4, these membranes limit passage in and out of the tissues, organs, and individual cells, conferring protection against invasion of pathogens and exposure to toxins. At the outermost surface, the skin presents a stratified epithelial barrier to drug entry into the body, while the plasma membrane of individual cells prevents the free diffusion of substances in and out of the cell interior. Additionally, mucus membranes line various tracts (gastrointestinal, respiratory, reproductive, etc.) that are regularly exposed to pathogens and foreign substances. In all cases, these membranes protect the body against external agents, but these barriers also limit the delivery of drugs via these sites.

As described in Chapter 4, the tight junctions between adjacent cells and epithelial cells restrict the diffusion of large molecules between cells via the paracellular route. Transcellular transport is typically limited to drugs that are low molecular weight and lipophilic. With an increased focus on delivery of macromolecules and particles, new approaches that do not require hypodermic needle injections are being explored to improve delivery of large molecules through epithelial layers.

Micro- and nanotopography, in the form of wires, needles, pegs, and grates, can be leveraged in drug delivery systems to overcome challenges associated with passage through epithelial barriers. Recent developments in transdermal drug delivery utilize microneedles to penetrate the skin with “needle-free” systems that minimize pain and offer the potential for sustained local, or systemic, delivery. Nanofeatures have also been shown to enhance mucoadhesion, improving transport through, and reducing clearance from, mucus layers that overlay epithelial cells. These techniques are described further here.

19.4.1.1 Microneedles

Some of the most successful advances in the micro- and nanotechnology for drug delivery have been in the development of microneedle systems for transdermal drug delivery. This technology is also described in Chapter 9 (Section 9.3.3.4). Transdermal delivery has long been a focus in drug delivery as the skin is an easily accessible site, with high potential for controlled or sustained release for local or systemic delivery. For successful transdermal delivery, a drug must permeate through the multiple layers comprising skin, starting with the toughest barrier to pass, the stratum corneum. Microneedles are designed to penetrate through the stratum corneum and form channels into the dermis through which drug can diffuse (Chapter 9, Figures 9.3 and 9.5). With microscale length and width, these needles are long enough to penetrate the toughest layer of the skin but short and narrow enough that they do not contact nerve endings in the dermis and thus cause no pain (Tuan-Mahmood et al. 2013). The microneedles alone can be enough to achieve sufficient transdermal delivery, or the technology can be combined with additional chemical, enzymatic, or mechanical components to enhance drug permeation.

There are a variety of different microneedle approaches, including the following systems:

• Using a microneedle patch to introduce pores into the skin before applying a topical drug

• Incorporating the drug directly into the microneedle structure

• Using microneedles that dissolve or break off within the dermis

• Using hollow microneedles that allow drug solutions to be actively delivered through the microneedle array from an external source

Microneedles are most commonly fabricated using molding and etching techniques and often utilize molds made from lithographic procedures. With these fabrication tools, the geometry, length, and array density of the microneedles can be precisely designed and produced. There are many recent reports of how microneedle design affects performance, but studies tend to focus on microneedle size and shape effects.

For example, needle length and array density have been shown to affect drug flux in a solid silicon microneedle array applied to the skin prior to application of acyclovir to the treated area (Yan et al. 2010). The results showed that needles longer than 600 μm increase drug flux and that with these longer needles, a needle density less than 2000 needles/cm² further increases flux. Another study demonstrated that for dissolving microneedles made from polysaccharides, a lower aspect ratio (height to width), with pyramidal rather than conical tips, increases mechanical strength of the microneedles thereby improving insertion into the skin (Lee et al. 2008). These reports highlight the importance of microneedle design, but additional work is still needed to better understand the complexities of both shape and size on the effect of microneedles made from different materials for the delivery of different therapeutic agents.

While work still continues on optimizing, characterizing, and designing microneedle systems, several commercial products and clinical studies have already proven the potential of microneedles for commercial application. Among the commercially available microneedle products are the Mi-Roll^® Derma Rolling System and the MicronJet^®. The Mi-Roll^® is an FDA-approved skin treatment system composed of microneedles that, when applied to the skin, are intended to increase collagen production in addition to creating microchannels to enhance the effect of topical treatments and creams. The MicronJet^® is an attachment with microneedles that fits standard syringes for intradermal injection. In clinical studies, low-dose influenza vaccines delivered with the hollow microneedle MicronJet^® system were compared to standard intramuscular influenza vaccines (Van Damme et al. 2009). These results showed a comparable immune response obtained using the MicronJet^® delivery system, as obtained with standard vaccination.

Varieties of other microneedle systems are currently under development for vaccination via the transdermal route and are described in detail in Chapter 17. These technologies include the ZP^™ Patch; the hollow microstructured transdermal system (hMTS^™); and the Nanopatch^™ (which is further described in a case study in Chapter 9, Section 9.3.3.5). Dried vaccine formulations are being investigated in dissolvable microneedle technologies, such as the MicroCor^® and Vaxmat^®.

While not yet tested in humans, several groups have demonstrated the use of microneedle systems for transdermal delivery of insulin. One example is a study in rats with induced diabetes, which compared the subcutaneous infusion of insulin through a hypodermic needle with the intradermal infusion of insulin through hollow silicon microneedles. The results showed successful delivery of insulin through the microneedle patch, as indicated by a reduction in glucose levels after administration (Nordquist et al. 2007). Even though the comparison to subcutaneous infusion showed a shorter duration in suppression of glucose levels, this early work suggests that a microneedle method for insulin delivery is possible, which is not painful, does not require specialist training, and will thus improve patient compliance.

19.4.1.2 Nanostructures for Enhancing Paracellular Transport

Whereas microneedles operate by creating temporary pores in the layers of the skin to improve transdermal drug delivery, a different approach to improving permeability is by directly disrupting the structures between epithelial cells, without damaging the surrounding cells. Epithelial interfaces are associated with tight junctions between adjacent epithelial cells, which prevent drug molecules from diffusing through the paracellular space (see also Chapter 4, Figures 4.1 through 4.5 inclusive). Precisely engineered nanostructures, of the order hundreds of nanometers, could probe the space between adjacent cells, thereby facilitating paracellular transport. Several research studies have carried out preliminary investigations into the use of nanowires and nanopillars for improving paracellular permeation. Figure 19.4 shows nanowire-coated particles prepared by growing nanoscopic silicon wires from the surface of narrowly dispersed, microsized, silica beads (Uskokovic et al. 2012).

FIGURE 19.4 Silica beads without (a) and with (b) silicon nanowire coating. Nanowires enhance epithelial drug delivery. (Reprinted with permission from Uskoković, V., Lee, K., Lee, P.P. et al., Shape effect in the design of nanowire-coated microparticles as transepithelial drug delivery devices, ACS Nano, 6(9), 7832–7841. Copyright 2012 American Chemical Society.)

Nanowire-coated particles showed a twofold increase in permeability of small molecules when compared to smooth particles (Fischer et al. 2011). Two mechanisms have been proposed to explain this observed disruption in the tight junctions. One hypothesis is that the nanowires interact with the cell membrane and, through mechanotransduction, increase the transepithelial transport of the drug possibly by increasing the surface area of the cell membrane. Another hypothesis is that when epithelial cells contact the nanowires, the cells stretch out in order to adhere to the structures, thus widening the paracellular space leading to increased diffusion of drug through this space (Uskokovic et al. 2012).

Permeability-enhancing effects caused by a disruption of cell–cell junctions were also observed using arrays of nanowires and nanopillars through monolayers of epithelial cells (Kam et al. 2013). Figure 19.5 shows clearly that without the nanostructures, epithelial cells formed a cobblestone pattern with smooth cell–cell interfaces. After exposure to nanostructured surfaces, the interface between cell membranes appeared jagged or rippled, suggesting a disruption in the tight junctions between adjacent cells. This morphological change in the cells was reversible on removing the nanostructured surface. Interestingly, the disruption to the tight junctions, as well as enhanced permeability, was observed for nanopillars with an aspect ratio of 1.5 (300 nm high, 200 nm wide) but not for pillars with a larger aspect ratio of 20 (16 μm high, 800 nm wide) (Kam et al. 2013). Additional work is still needed to further investigate the effect of size, geometry, and density of nanofeatures on transepithelial transport.

19.4.1.3 Nanodevice Interactions with the Mucus Layer

In addition to penetrating through epithelial cells, many routes for drug administration (e.g., oral, nasal, pulmonary, and vaginal) first require permeation through a mucus layer. Mucus is a viscous colloid composed of glycoproteins, water, and enzymes, which overlays the epithelial cells. The viscous nature of mucus allows it to entrap particulates, and the hydrophobic nature of its lipid constituents can prevent the passage of polar drugs. In some cases, such as the gastrointestinal tract, mucous consists of two layers: a static layer adhered to the epithelium and a motile layer that actively clears captured particulates.

There are two basic approaches to design delivery systems that overcome the mucosal barrier. In the first, devices or particles are engineered for improved mucoadhesion and in the second, for enhanced mucus penetration. The primary goal of these systems is to achieve both a higher concentration and quantity of drug delivered to the underlying epithelial cells, in order to improve absorption.

FIGURE 19.5 Effect of nanostructured surfaces on epithelial cell–cell junctions. Contact with nanostructures causes a disruption to the tight junctions between epithelial cells, as visualized through immunofluorescent staining of zonula occluden (ZO-1), a tight junction protein. The nanostructures caused a ruffling of the cell–cell boundary (left), compared to the normal smooth cell–cell interface (right). (Reprinted with permission from Kam, K., Walsh, A.L., Bock, S.M. et al., Nanostructure-mediated transport of biologics across epithelial tissue: Enhancing permeability via nanotopography, Nano Letters, 13, 164–171. Copyright 2013 American Chemical Society.)

When tethered to the surfaces of particles or microdevices, nanowires were shown to improve mucoadhesion in vitro. Improved mucoadhesion is thought to be because the nanowire features are able to entangle with the proteins and fibers in the mucosal layer, thereby anchoring the attached particle or device (Fischer et al. 2009). The resulting increased contact time of the device with the mucosal surface improves bioavailability.

These systems are being designed for sustained release of the drug during the time that the device is adhered to the mucus. To further increase the percentage of drug available for absorption through the epithelium, devices are being developed to provide unidirectional release. In these planar devices, one surface is modified with nanotopography to improve mucoadhesion, and asymmetric layering allows for drug release from only this side of the device (Figure 19.6). For oral delivery, this is intended to focus drug release in the vicinity of the epithelial cells for absorption and to prevent release of drug into the intestinal lumen. Additionally, the uptake of many drugs is limited by efflux transporter proteins and metabolizing enzymes, and increased localized drug concentrations may result in the saturation of efflux transporters and enzymes, in turn improving the bioavailability of the released drug (International Transporter Consortium 2010). Unidirectional drug release is typically achieved by using an outer drug-impermeable layer, followed by a drug-permeable layer, such as a hydrogel, for drug loading. The drug-permeable layer is often contained within a reservoir in the drug-impermeable layer to prevent escape of drug from the sides of the device (Figure 19.6).

Most of the work discussed in this section is still in the early stages of development and has not yet been proven in vivo. As knowledge and understanding of the interaction between nanostructures and cellular systems advances, we can hope to see similar features integrated into more devices in development as well as to see in vivo and clinical results demonstrating the efficacy of such systems. These systems are of particular interest as delivery systems for macromolecules, such as protein and peptide drugs. Due to their size and complexity, these therapeutics have low permeability and can be sensitive to the harsh conditions of mucosal environments. Microdevices have potential to simultaneously protect therapeutics from degradation and provide sustained, unidirectional release directed toward the epithelium, for improved absorption.

FIGURE 19.6 Advantages of a planar, asymmetric microdevice design for oral drug delivery. A planar device geometry minimizes the amount of shear force per mass on microdevices, improving device adhesion to mucosal and epithelial surfaces. A drug reservoir on one side of the device allows for unidirectional drug release, and asymmetric surface modifications allow for selective binding of the side of the device releasing the drug.

19.4.1.4 Gecko-Inspired Nanotopography for Improved Adhesion

There is a need and interest in the medical field for improving adhesion in a variety of applications such as wound dressings, drug delivery patches, or mucus-targeted delivery vehicles, among others. A novel approach inspired by a naturally occurring phenomenon is a gecko-inspired surface. One of the unique properties of the nanoscale is the drastic increase in surface area that accompanies the presence of nanostructures on a surface. Taking advantage of this, gecko-inspired surfaces are modeled after the microscale angled fibers and nanotopography found on gecko toes (Figure 19.7). By creating a large number of nanoscale surface contact points, gecko toes and gecko-inspired surfaces increase adhesion through an increase in intermolecular forces. For geckos, this allows them to cling to a vertical wall or even upside down from a ceiling. In medical applications, gecko-inspired surfaces show promise in both wet and dry environments. Similar to the microneedles and nanopillars previously discussed, the adhesion characteristics of gecko-inspired nanostructures depend on geometry, density, and material properties.

Nanostructured poly(glycerol-sebacate-acrylate) (PGSA) polymer surfaces were tested both in vitro using porcine intestine tissue and in vivo in a rat model to look at the use of these gecko-inspired materials as a biodegradable adhesive. With PGSA polymer structures, nanopillars with larger tip-to-base diameter ratios show higher adhesion; independently, larger tip diameters to pillar length ratios were also found to produce stronger adhesion (Mahdavi et al. 2008). While these and many other early studies on gecko-inspired materials do not focus on a drug delivery applications, there is potential for translating these results to improve adhesion of microneedle arrays or muco-adhesion of drug delivery devices.

19.4.2 NANOTECHNOLOGY IN CONTROLLED-RELEASE DRUG DELIVERY

Nanofabrication techniques are also used to create nanopores and nanochannels for controlled drug release. Demand is growing for the development and commercialization of controlled- and sustained-release drug delivery systems. Controlled-release systems aim to eliminate frequent dosing, provide more consistent blood levels, improve the efficacy and efficiency of therapeutics, and reduce adverse reactions. As the biopharmaceutical industry continues to grow, there is a strong emphasis on developing controlled-release delivery systems for peptides and proteins in particular.

FIGURE 19.7 Hierarchical gecko-inspired topography: three-level hierarchical polyurethane fibers. (a) Curved base-level fibers. (b, c) Midlevel fibers on top surface of base fibers. (d) Third-level fibers on top surface of midlevel fibers. (Reprinted with permission from Murphy, M.P., Kim, S., Sitti, M., Enhanced adhesion by gecko-inspired hierarchical fibrillar adhesives, ACS Appl. Mater. Interfaces, 1(4), 849–855. Copyright 2009 American Chemical Society.)

19.4.2.1 Nanochannels in Membrane-Controlled Drug Delivery Devices

Porosity has been utilized as a tool in drug delivery for a long time, but it is traditionally through the use of naturally porous materials, leaching techniques, or self-assembly, that porosity has been introduced into a system. Micro- and nanofabrication approaches to create porosity allow for greater precision in design and control of pore size, distribution, and material in delivery systems. These fabrication approaches not only allow for more precise control of pore features but also make it possible to create monodisperse pores throughout a substrate. There are a number of examples of continuous-release implantable devices in development and on the market that use micro- and nanochannels or pores in a membrane in order to control diffusion of a therapeutic out of a reservoir (see also Chapter 14, Section 14.5.2).

Release rates and profiles for small- and large-molecule therapeutics are controlled in these systems through the size of the pore relative to the drug molecule, pore or channel length, and properties of the membrane material or surface coating such as surface charge, polarity, and hydrophobicity. When the pore size is large compared with the molecule, release of drug from a reservoir through the porous membrane is described by Fickian diffusion (see also Chapter 2).

However, when pore size is on the order of the hydrodynamic radius of the molecule, diffusion of the drug molecule through the pore is constrained, resulting in “single-file” diffusion and a linear release rate. Being able to fabricate devices with nanoscale channels and pores facilitates this single-file diffusion for macromolecules such as peptides and proteins. For example, Nanopore^™ technology comprises a small subcutaneously implantable reservoir, fitted at each end with membranes that are microfabricated to contain pores or channels that are 1–5 times the hydrodynamic diameter of the selected drug molecules. Pore diameter is empirically “tuned” with molecular size, to enable sustained release of drug molecules held within the reservoir, via a constrained passive diffusion mechanism.

These systems can be made from silicon, metals, polymers, and biopolymers. For silicon and metal membranes, lithographic techniques such as electron-beam lithography are powerful tools for precise fabrication of pores. To create nanochannels in a polymeric material, a series of techniques including solvent casting and template molding can be leveraged, as shown in Figure 19.8 (Bernards et al. 2012).

FIGURE 19.8 Fabrication of nanoporous polymer films. (a) A clean silicon substrate. (b) Zinc oxide nanorods are grown on the substrate from a zinc-oxide seed layer deposited using spin-coating techniques. (c) Using solvent-casting and spin-coating, the template is coated with a layer of polycaprolactone thin enough to not cover the nanorods. (d) An additional support layer of a mixture of polycaprolactone and PEG is added. (e) Deionized water dissolves the PEG-phase from the supporting layer and sulfuric acid etches the zinc-oxide template to generate a supported, nanostructured, polycaprolactone thin film. (f) SEM image of a typical nanostructured polycaptrolactone film. (g) The supporting nanochannel layer. (Reproduced with permission from Bernards, D.A., Lance, K.D., Ciaccio, N.A. et al., Nanostructured thin film polymer devices for constantrate protein delivery, Nano Letters, 12, 5355–5361. Copyright 2012 American Chemical Society.)

19.4.2.2 Nanochannels in Cell Encapsulation

Another important feature of nanoporous reservoir devices is their capacity for immunoisolation. Nanoporous membranes can allow for the diffusion of small-molecule therapeutics and/or nutrients, while limiting the flux of large molecules and cells. This feature is particularly important in the development of biocapsules in cell or tissue transplants, which aim to prevent immune rejection (Desai et al. 1999). Much ongoing research focuses on the use of these technologies and approaches to deliver pancreatic islet cells for the treatment of diabetes. A further application is in ophthalmic drug delivery: Chapter 13 (Section 13.7.2.1) describes how genetically engineered cells loaded into polymeric microcapsules can be implanted directly to the back of the eye and subsequently produce therapeutic proteins. The microcapsules provide immunoprotection for the implanted cells while simultaneously allowing efflux of the therapeutic proteins.

19.5 CONCLUSIONS

Micro- and nanofabrication techniques allow for precise control and patterning at micro- and nanometer scales for an increasing variety of materials. These techniques are being used for the top-down fabrication of nanoparticles, nanopillars, and devices with specific features including size, geometry, surface topography, and porosity. Micro- and nanofabrication techniques are being applied through numerous avenues to improve drug delivery and targeting. In particular, nanosystems are being leveraged to improve permeability and transport across biological membranes, to control drug release, to improve pharmacokinetics, to target therapeutics, and to achieve stimuli-responsive drug delivery.

As the complexity of these systems and the breadth of their features increases, there is a growing need for additional studies investigating the mechanistic and functional impact of these features on biological processes. Many of the systems referenced in this chapter, as well as others not mentioned, combine multiple features and approaches to achieve their drug delivery goal. A better understanding of how each individual component and the combined effects of material properties and micro-and nanofeatures affect the biology and the drug delivery process is needed. Additionally, only a few of the systems under study have been tested in vivo or commercialized. Additional studies focusing on in vivo considerations are needed to ascertain the impact of system features on biodistribution, toxicity, and immunogenicity. This rapidly expanding field of nanomedicine offers exciting opportunities to optimize drug delivery and targeting.

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