Nanofabrication Techniques for Controlled Drug-Release Devices

Keywords::

• drug delivery
• nanofabrication
• nanomedicine

New drugs and delivery systems have undergone a rapid development in recent years for treating cancer, HIV/AIDS, diseased organs and damaged nerves to relieve pain, preventing disease and restoring health in humans [1]. The application of nanotechnology in nanomedicine for drug delivery is a new field and has raised great attention. Compared with conventional drug-delivery mechanisms, nanoscale systems can deliver the drugs directly to diseased cells and minimize damage to healthy cells, and they can be made small enough to be implantable in the body. Alternatively, by modifying the shells of the drug, shells can release the drug automatically under certain stimuli. Nanoscale controlled-release drug systems provide highly selective and effective therapeutic modalities [2].

Controlled-release drug-delivery vehicles

The first-generation nanotechnology-based controlled-released drug products appeared around 2005. The history and projected development timeline was published by the Nanotech Project Organization [201]. From its fast growth pace, it is believed that nanotechnology will change our current drug delivery mechanisms in the near future. The research and development of nanotechnology-based drug-delivery systems started from the preparation of nanoparticles and then grew with the appearance of other nanomaterials, such as nanorods, nanotubes, nanolamellae, nanovesicles and even more complicated structures. However, most published works have dealt with the development or application of the nanomaterials, and there are only limited numbers of reports on the use of nanofabrication techniques to fabricate and control the desired nanostructures for making controlled drug-delivery systems.

Nanofabrication refers to modern fabrication technologies, which can produce structures with at least one dimension less than 100 nm. Nanofabrication is a new fabrication concept originally proposed by the electronics industry to make ultra high-density integrated circuits. However, its basic principle of molding, printing and writing can be traced to lithography concepts that are centuries old and applied to very diverse areas, including materials science and engineering, life sciences, energy and medicine. Nanofabrication techniques have been used for making 2D and 3D nanostructures.

For the purpose of controlled drug delivery, fabricated nanostructures should include, structural, mechanical and electronic features. The fabricated nanostructures can dispense drugs in the optimum dosage for long periods of time, which can reduce the toxicity and improve the efficacy of the drug. This article focuses on the fabrication of nanoparticles, micro-/nano-mechanical electronic systems and microneedle arrays.

Nanofabricated particles

Nanoparticles have demonstrated great advantages in comparison with microparticles. In drug delivery, for example, nanoscale particles can travel through the bloodstream without sedimentation and can penetrate tissues such as tumors. Nanoparticle-based encapsulating structures have been used to deliver drugs to target sites for cancer therapeutics [3]. The encapsulating particle is used to release the drugs through surface or bulk erosion and diffusion. The release can also be controlled by changing environmental conditions, such as pH, light and temperature or by the presence of analytes. In the past, controlled-release particles were normally made by chemical synthesis. The use of nanofabrication techniques to produce nanoparticles has a very short history, but it has already attracted great attention; for example, nanoscale particles have been fabricated with well-defined spheres, cubes or other shapes by using a process termed particle replication in nonwetting templates (PRINT). In the PRINT process, a mold is fabricated by conventional lithography and then filled with a liquid. The liquid in the mold cavities is then converted to a solid by either curing the liquid precursor or evaporating the solvent. Finally, the particles in the cavity can be removed from the mold and released, or transferred, to form a 2D array of free particles [4]. PRINT allows for precise control over particle size, shape, composition and surface properties. This precise control of the size and the shape, which is difficult to achieve when the particles are made by chemical synthesis, enables the study of the impact of these parameters on the mechanism of cellular internalization [5]. A number of studies have been carried out to characterize the cellular internalization mechanisms of nontargeted organic nanoparticles as a function of size, shape, composition and surface charge (i.e., cationic or anionic) in human cervical carcinoma epithelial cells [6].

In addition to studying the particle itself, more and more work on particle-based drug delivery is focusing on the targeting of nanoparticles to desired tissues. A number of methods have been developed, such as controlling the size, charge, hydrophobicity of the particles, and adding antibodies and peptides that recognize specific cell-surface proteins and receptors to the nanoparticle surface [7].

Furthermore, to solid particles, micro-/nano-fabrication techniques can produce 3D cubes with designed size, shape and surface patterns to encapsulate and deliver pharmaceutical agents and other molecules in vivo or in vitro[101].

Micro- or nano-electromechanical system

Micro- or nano-scale systems have been built using nanofabrication technologies to deliver drug particles and study the drug particle behavior inside the body. For example, microfluidic devices have been fabricated to mimic the body‘s vasculature, which can be used to test and optimize the interaction of targeted nanoparticles with the cells that line cancer blood vessels [7]. By using such microfluidic devices, nanoparticle characteristics, such as size and surface properties can be optimized before performing costly animal and clinical experiments.

Micro-/nano-fabricated microreservoir devices are being explored extensively. For drug-release applications, the microreservoirs are required to be made from biocompatible materials, including silicon, gold, silicon dioxide and silicon nitride, and some polymers have been proven as good biocompatible materials. Silicon is the most popular material, which is widely used in both microelectronic and biomedicine industries. The silicon microreservoirs are normally made by deep silicon etching [8]. The silicon surface can be further modified to increase biocompatibility and decrease biofouling. The reservoir size can be easily increased by changing the size of the open area and the etching depth. For drug-delivery purposes, the etched reservoir needs a membrane to cover the drug after it is loaded. The drug contained in the microreservoir is released when the membrane is opened or removed under a certain stimulus. This device can be implanted for local drug delivery. For example, a silicon device was used for the in vivo release of fluorescent dye and radiolabeled 1,3-bis(2-chloroethyl)-1-nitrosourea, a chemotherapeutic agent. [8]. Gold is one of the promising membrane candidates owing to its anticorrosive and good mechanical properties [9]. Gold membranes were found to exhibit elastic behavior up to a pressure of 4.14 × 10⁵ Pascal. Other good candidates for making the microreservoir and the membrane are polymers [10]. For example, a microreservoir made out of lactic acid and a membrane made of copolymers of polylactic acid and polyglycolic acid have been successfully fabricated by using molding and injection molding technologies, respectively. The devices range from 480 to 600 µm in thickness and each reservoir has a volume of 120–130 nl. The release was controlled by dissolving the polymer membrane. The degradation time of the reservoir membranes varies with molecular weight and lactic:glycolic ratio, thus, allowing the timing of the release from each reservoir to be engineered. Other researchers are pursuing similar polymer-based microreservoir devices [11].

In addition to the membrane-controlled drug release itself, micro-/nano- electromechanical systems technology is also emerging as the natural choice to fabricate micropumps to dose and deliver the drug. Several extensive reviews have described the state of the art up to 2009 [12,13]. Self-actuated pumps have been proposed, which respond to levels in blood pressure, lipid or glucose to treat hypertension, atherosclerosis and diabetes, using flexible membranes as nanosensors, nanoactuators and nanopumps [102]. Electrokinetic pumps that rely on direct currents are very promising to deliver pure water, pure organic solvents, inorganic buffers, biomacromolecules and hydrogels [14–16]. Another method of moving the fluid has been proposed that also relies on a small electric current propagating along a nanochannel and locally increases the wall thickness, thereby ‘pushing’ the fluid along [17]. Shape memory actuators are also a promising technique to deliver drugs [18]. Implantable piezoelectric valve-based pumps have been fabricated for insulin delivery using a 12-level silicon micromachining technology [202]. Additional references on the latest development on programmable flow pumps, typical nanofabrication sequences and embedded piezoelectric actuators can be found in [18].

In summary, micro-/nano-engineered drug-delivery systems can be used to maintain the biological activity of the drugs and facilitate the local, accurate and controlled release of potentially complex drug-release profiles [19].

Microfabricated needles

Another way to deliver drugs into the body is through skin. Traditional needles are large in size and cause pain in the drug injection process. Micro-/nano-fabrication techniques enables the needle size to be reduced to micrometer or even nanometer scales [20]. Owing to the shrinkage of needle size, microneedles can be used to deliver compounds to cells in cultures or into localized regions of tissues. The microneedles can penetrate deeply enough for the therapeutic compounds to enter the systemic circulation [21]. A needle array has more than one needle, so the delivery dose is higher than the single one. Besides improving drug-delivery efficiency, these microfabricated needles can reduce the pain for the patient. The size of the microneedles is small enough to avoid activating sensory nerves in the tissue [22]. At present, microneedles are being fabricated from silicon, glass (silicon dioxide) and metal.

Micro-/nano-fabrication techniques

Conventional lithography processes

Most nanoparticles, micro-/nano-electromechanical systems and micro-/nano-needles are fabricated by conventional nanofabrication techniques, which are based on radiation beams (e.g., optical, laser, ion and electron) and radiation sensitive materials to define the patterns on the surface.

Ultraviolet is the most popular beam for nanofabrication, which is termed photolithography. In photolithography, a photomask is required [23]. The photomask is an opaque material on a transparent background, which contains features of desired shapes and sizes. Ultraviolet light passes through the transparent area to expose a light-sensitive material (photoresist), which is then ‘developed’ in solvents to dissolve the exposed (positive photoresist) or unexposed (negative photoresist) regions and provide a patterned surface. The resolution of optical lithography is light wavelength dependent [24]. Although the resolution of optical lithography has been pushed down to below 50 nm [25], the cost of the tools, mask fabrication and photoresists is extremely high and has become a barrier for most researchers and small businesses.

Electron is another popular beam for making even smaller structures. Unlike photolithography, electron beam lithography (EBL) is a maskless direct writing technology. The designed nanopatterns are generated by scanning a focused electron beam across an electron-sensitive resist coated on a substrate. The electron beam-exposed resist is then developed to remove the exposed area (or unexposed area, depending on the tone of the resist). Finally, the surface pattern can be further transferred into the underlying substrate by plasma or ion etching. Compared with photolithography, the advantage of EBL is its high resolution; 3–5 nm isolated lines in polymethylmethacrylate [26] and 1–2 nm resolution in metal halide resists have been demonstrated [27]. The drawback of EBL is its low throughput owing to the low sensitivity of the resist and sequential writing nature of the process. Similar to EBL, a laser beam can also used for direct pattern writing [28].

Unlike the above radiation beams, an ion beam does not need a radiation-sensitive media to generate the pattern. A focused ion beam can write patterns on a substrate directly (e.g., semiconductors, metals or ceramics) without major forward- and back-scattering. Ion sources can focus on spots in the order of 10 nm [29]. However, 3–6 nm features have been demonstrated [30].

Nonconventional lithography processes

In addition to the above radiation-based nanofabrication techniques, a number of nonradiation technologies have been proposed recently to improve the pattern resolution and/or reduce the cost. Many newly developed techniques, such as stamping, molding, scan printing and self-assembly, are already or potentially good candidates for fabricating nanomedicine devices.

Stamping-based nanofabrication is termed soft lithography, which uses topographically patterned flexible polydimethylsiloxane as a stamp to print chemically normal self-assembled monolayers on a substrate. The self-assembled monolayers then react with the desired chemical or biochemical molecules to form micropatterns of various materials [31]. Different procedures, such as microcontact printing, replica molding, microtransfer molding, micromolding in capillaries, solvent-assisted micromolding and patterning by etching at the nanoscale, have been developed [32].

Molding-based nanofabrication is termed nanoimprint lithography [33]. Nanoimprint lithography uses a hard mold to form nanostructures by pressing into a deformable polymer layer deposited on the substrate surface. Features of 5 nm or smaller in size, [34] over a large area [35] have been demonstrated. Pressure and temperature are critical for pressing the mold into the polymer layer and a mold-release layer coating is needed for separating the mold from the polymer layer after the imprint [103]. The patterned polymer layer can be an inert pattern transfer layer or a bioactive material. Solid polymer thin films, as well as liquid precursors, can be printed and then cured by ultraviolet exposure [36,37]. In addition to replicate nanostructures on a surface, nanoimprint lithography can also be used to replicate nanoscale particles by using PRINT [38], as mentioned previously in the nanofabricated particles section.

Scan printing-based nanofabrication is another useful technique for the fabrication of biomedical devices. Dip pen nanolithography (DPN) is one of the well-known techniques in this category [39]. In DPN, an atomic force microscopy tip is dipped into the ink and then scanned across the surface. At present, DPN is able to write patterns as small as 15 nm [40]. Self-assembled monolayers, small organic molecules, macromolecules, nanoparticles and metal ions can be used as ink materials to write on a variety of surfaces [41]. A massively parallel tip scanning approach (termed 2D DPN) has been developed to improve the throughput [42]. Electrohydrodynamic jet printing is another high-resolution surface pattern writing technology. It uses glass microcapillary nozzles to deliver liquid ‘inks’ on a surface. Sol-gels, DNA and other liquid molecules can be printed [43]. Another ‘pen‘-based technology termed polymer pen lithography, combines the scanning probe contact printing concept and microcontact printing process to create a massively parallel elastomeric array of pyramids to write a surface pattern over large areas [44]. The discussed scan printing nanofabrication technologies enable direct writing of organic, polymer, biomolecules and sol-gel materials on surfaces and have been used to fabricate nanoarrays of proteins, nucleic acids and other soft-materials for biomedicine research [45].

Self-assembly-based nanofabrication is a low-cost process for making regular patterns over a large area. The colloidal surface self-assembly process was the first of these to be explored. Physical interactions can align colloids into regular patterns on the surface [46]. These patterned colloids can be functionalized directly to produce a device, or used as a mask to pattern the underlying substrate [47] to achieve desired charge, roughness and chemistry on different substrates for biomedicine and other applications [48]. Phase separation is a molecular level self-assembly, which is being extensively studied to pattern nanoscale features. The phase structures exhibit interesting morphologies, such as sphere, rods, lamellae and bicontinuous structures [49,50]. In addition to forming surface nanopatterns, polymer phase separation structures can also be used to produce a wide range of sponge-like scaffolds by selectively removing one phase from the film. The fabricated nanoporous structures are very useful in nanomedicine, especially for controlled drug delivery [51]. Templated self-assembly is a process that uses the topographical and/or chemical templates to guide the self-assembly of colloidal particles or molecular phase separation [52,53] in a more regular shape. Both patterned or blank templates can guide a regular pattern formation. Lithographically induced self-assembly is a newly discovered polymer thin film self-assembly pattern formation process, which uses a blank template on top [54]. A variety of self-assembly patterns (e.g., concentric rings, rods and pillars), in polymer thin films has been achieved [55]. In addition to surface patterning, the self-assembly concept has also been applied to produce 3D structures. This new process uses surface forces to fold a precisely fabricated 2D surface pattern into a 3D structure [101]. Since 2D lithographic patterning is very well developed, it is very useful to transform the patterned 2D templates into 3D objects. 3D structures, such as rings, tubes and polyhedrons, have been demonstrated. It is clear that components with complex surface patterns can result in more complex assemblies [56]. The fabricated 3D cage will be very useful to encapsulate the drugs for controlled-release applications.

Conclusion

The convergence of three major advances in technology, biomaterials and conventional lithography in the last 40 years, and emerging lithography in the last 15 years, has enabled the development of completely new drug-delivery schemes and devices. As these three technologies continue to ‘push the envelope‘, along with the other associated nanofabrication processes, and as economies of scale come into play, we can be confident that nanorobots, and nanomedicine in general, will become a mainstay in human healthcare.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.

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Funding

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.