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Biological and medical application of micro-electro-mechanical-systems (MEMS) is currently seen as an area of high potential impact. Integration of biology and microtechnology has resulted in the development of a number of platforms for improving biomedical and pharmaceutical technologies. This review provides a general overview of the applications and the opportunities presented by MEMS in medicine by classifying these platforms according to their applications in the medical field.
Keywords: BioMEMS, Microtechnology, Medical applicationsMicro-electro-mechanical-systems (MEMS) were introduced in the late 80s as an extension of the traditional semiconductor very-large-scale-integration (VLSI) technologies. MEMS technology adapted for biological and medical applications emerged into a new field of research – BioMEMS, or Bio Micro Electro Mechanical Systems. Over the last decades, this technology has led to significant advances in different fields of medicine and biology through the development of a variety of micro engineered device architectures. This acceleration is primarily due to the fact that microfabrication technology provides device miniaturization, as well as better performance, lower cost and higher reliability.
Although an exhaustive review of all the biomedical applications of MEMS technology cannot be done due to the large number of rapid advances in this field, in this review, we try to identify and discuss some of the major areas in medical industry that have profited from the development of MEMS.
In order to understand how the MEMS technique is utilized for the development of devices with potential biomedical applications, a primary understanding of the standard micro-fabrication processes is necessary. MEMS technology can basically be described as the development of device structures in the micro- or even nano-dimensions using “micromachining process” on silicon and other substrates. By utilizing the peculiar characteristics of the material silicon, complex micro 3D structures such as channels, pyramids or V-grooves can be formed. The processes used for carrying out such “micro scale” fabrication can mainly be classified into patterning process, material removal processes and material deposition processes.
This is the process of transferring the desired patterns onto a wafer surface. In MEMS lithography processes, the wafer is first coated with a photosensitive material known as photoresist and then exposed to radiation source such as UV through a mask which contains the pattern that has to be transferred on to the wafer surface. There are two types of photoresist, positive and negative. According to the type of photoresist used, the exposed area is either retained or removed after development (chemical treatment using ‘developer’). This process thus opens up ‘windows’ into the underlying material and serves as a mask for further processing. Once this mask has served its purpose the photoresist is stripped off using chemical treatment. The MEMS lithographic procedure is illustrated below ( Figure 1 ).
Illustration of MEMS photolithographic process.
Selective removal of substrate or materials is carried out by the process known as etching. The dissolving power of a chemical solution is utilized to remove the material in wet etching. Selective etching is done by covering the desired portions of the material with a mask that resist the dissolution power of the solution as shown in Figure 2a . In dry etching, the material is removed by bombardment with high energy ions. The most commonly used methods in dry etching are Reactive ion etching (RIE) and sputter etching. The material is removed as a result of chemical reaction with the ions in RIE whereas in sputter etching there is no chemical reaction involved and the material atoms are knocked off due to the high energy ions. For the purpose of etching deeper, Deep Reactive Ion Etching (DRIE) is used which is an extension of Reactive Ion Etching.
Illustration of Etching and Deposition process in MEMS technology
In this process a thin layer of material is deposited on the surface of the wafer as shown in Fig 2b . According the various physical and chemical properties of the material being deposited, different processes exists for deposition. They can be classified mainly as chemical deposition; wherein chemical reactions are required for the formation of new molecules and physical deposition where no new molecules are made but are taken from a source and deposited on the surface of the substrate. In Low pressure Chemical Vapor Deposition Processes (LPCVD) process, materials are deposited at low pressure. Although this process produces a more uniform layer of material, high temperature is required and thus this process may prove unsuitable in certain situations. Plasma Enhanced Chemical Vapor Deposition(PECVD) allows reactions to happen at lower temperature as the gases are ionized by plasma with the drawback that the thin films produced by this processes is of poorer quality. Molecular Beam Epitaxial growth is a relatively new technique where films of molecular thickness are deposited.
Having discussed briefly about the MEMS techniques, we shall now look into how devices fabricated using the above mentioned processes have revolutionized the world of Medicine. The application of MEMS technology is very broad, and it is almost impossible to discuss all the various researches done in this direction. However, we have tried to classify most of the advances under four broad titles- Diagnostics, Therapeutics, Prosthesis, and Surgery.
Diagnostics have always been an indispensible component of healthcare. As science and technology progressed over the years, diagnostics have also been subjected to continuous improvement. Currently much research and development are put into the development of personalized diagnostic tools that are highly sensitive and capable of early detection of diseases. Consumer friendly, at home pregnancy tests and hand held glucose monitoring systems are some of the substantial diagnostic advancement that has occurred during the last few years. MEMS capabilities are incorporated for developing such hand held diagnostic equipments where laboratory analysis has to be carried out in miniaturized scale.
Diagnostics can be broadly classified into five major segments as shown and MEMS technology has profited all these different areas by providing sensing platforms capable of detecting these analytes. A summary of these are given in the table.
The idea of downsizing diagnostic tools was made a reality by the engineering advances in surface and material science. In order to meet the need for more integrated and immediate clinical results, Micro-electromechanical systems (MEMS) use techniques akin to those used in the microelectronic industry to build these miniaturized devices that are capable of purifying, isolating and characterizing samples, thus putting the entire assaying operation on a single chip. These systems are also known as μTAS (micro Total Analysis Systems) or Lab-on-a-Chip devices. Such a platform integrates sample treatment modules together with separation and detection modules all brought together using the functionality of MEMS techniques. The potential for such devices are many including detection of biological weapons through protein and DNA analysis, blood analysis and drug screening systems.
For the determination of a specific type of disease, multiple multiplexing of tests might be required. Each test assay might have different stages and each stage might use a different MEMS component. We review the various MEMS-based components that could be applied to the different stages of the assaying operation such as cell separation, cell lysis, analyte molecule purification and sensing. For detection of various types of diseases, different biomedical specimen has to be used. Using well designed microfluidic processors together with other MEMS components, it is possible to isolate and analyze a particular type of biomolecule from the specimen. This requires a number of stages such as sample preparation stages and measurement stages.
Using MEMS technology it is possible to isolate cells based on their physical and chemical properties from a large population of heterogeneous cells [43]. MEMS structures based on phenomena such as hydrodynamics, optodynamics and electro kinetics have been used for this purpose. Structures using hydrodynamics take advantage of the laminar hydrodynamic focusing used in flow cytometry for the sorting of cells. Although this provides good sorting results, external activation devices such as pumps are required for high pressure fluid manipulation. In dielectrophorosis, the cells exhibit dielectrophoretic activity in the presence of an external electric field thus making it possible for them to be manipulated and separated. Using this method, MDA231 cancer cells have been separated from dilute blood by selective capture onto microelectrodes [44]. Single cell capture and manipulation has also been shown to be possible using this technique. In another technique using electric field – electrophoresis, individual cells are moved due to their intrinsic charges. Studies have shown that application of an electric field across a thin micro porous membrane, results in the trapping of cells on the membrane [45] Other techniques using ligand proteins immobilized on surface of channels and wells can also be used to trap cells in microdevices [46, 47]. Optical tweezers have also been applied for cell sorting by transferring the momentum from a focused laser beam on to cells [48-50]. Using this technique, differentiation of cancer cells from normal cells has been demonstrated [51]. Yasuda and colleagues used this method together with an on-chip microculture chip, for comparing genetically identical cells [52].
Analysis of intracellular materials for studying physiologic and pathologic condition typically requires cell lysis. Devices that perform cell lysis requires speed (to prevent further biochemical changes), selectivity (breaking down cell membranes while protecting organelle membranes), and integration with other microfluidic devices [53]. Thermal lysing method, probably due to its compatibility with PCR (polymerase chain reaction) is one of the most commonly used lysing mechanism [54]. In this method, the cell membrane breaks down as a result of high temperature provided by the micro heaters. Although this technique is well suited for DNA analysis and detection, the denaturization of proteins at elevated temperatures is a major concern. For overcoming this limitation, Marentis et al [55] used energy of sound wave from a piezoelectric minisonicator to lyse leukemia HL-60 cells and Bacillus subtilis bacterial spores in microfluidic environment. Marmottant et al introduced the novel idea of using oscillating micro bubbles to rupture cell membranes[56]. Other microscale mechanisms have also been developed such as nanoscale barbs [57] through which cells are forced causing them to rupture, and electrical lysing [58, 53, 59] wherein cells are subjected to high electric fields causing the cell membrane to destabilize and thus rupture.
Once the cell has been disrupted, enrichment of analyte molecules (proteins and nucleic acids) might be required before analysis. MEMS based electrophoresis and isoelectric focusing [60, 61] have been used for this purpose. The intrinsic charges of the biomolecules are utilized in these methods. Specially treated bead filled columns for nucleic acid or protein purification can also be incorporated into the chip. Another alternative include polyimide membrane coated microfluidic channel capable of absorbing the specific analytes.
Amplification of nucleic acids can be done using polymerase chain reaction (PCR). The first microPCR was reported by Northrup et al. [62] in 1995. This device basically consisted of polysilicon miniature heaters. Since then, much effort had been made to improve this device and currently there are many commercial mini PCR systems. A number of literatures are available on various MEMS based PCR devices [63-67]. An excellent review of the technology for MEMS PCR is given by Zhang et al [68]. Nucleic acids and proteins after purification can be fractioned using nano-sieves[69].
Today, many molecular based diagnostics are emerging that enable identification of susceptibility to diseases long before the actual symptoms are manifested. Protein expression and genetic makeup of cells can reveal a great amount of information which can be useful in innumerable ways for medical purposes. Currently, much research has been done in this area for developing sensors capable of identifying specific protein molecules and nucleotide sequences.
Several techniques exist for the detection and quantification of proteins and nucleic acids. These detection schemes can be classified into two categories -labeled and label free methods. Although labeled methodology offers more sensitivity, the labeling procedure is both time consuming and expensive. Therefore, studies have focused more on the development of label-free detection techniques.
The following section briefly describes the basic sensing principles used in the various MEMS. based biosensing structures. Most of the biosensors are affinity-based, which uses a biorecognition layer (probe molecules) immobilized on a transducer surface to bind to the analyte molecules selectively. Microfabricated biosensors utilizing electrical, mechanical, piezoelectric and acoustic signal transduction mechanisms have been developed over the years. Along with this, biosensors based on nanotubes and nanowires have also been developed recently due to the advent of nanotechnology.
The most common electrical device architectures used for biosensing include micro fabricated capacitive electrodes and field effect transitive structures. Mechanical sensing structures such as micro-cantilevers [70-75] and diaphragms have also been shown to be extremely sensitive to biomolecular interactions and has currently been shown to weigh a single molecule [76]. The variations in mass as a result of target-probe binding are detected by these structures. Acoustic transducing mechanism is another major method that has been used for the detection of biomolecules. Changes in wave properties caused by biomolecular interactions at the surface of the sensor are detected by these sensors. Various surface acoustic waves can propagate on the surface a piezoelectric substrate, and according to them, various sensor configurations exist of biomolecular detection.
Apart from the above mentioned devices, other remarkable advances of BioMEMS include development of devices for measuring physiological parameters such as temperature, pressure, pulse rate etc. Intra-ocular [77-80], intra-cranial [81-84] and cardio-vascular pressure sensors [85-91] have been developed using MEMS techniques. Other MEMS applications include sensors embedded into smart textiles or wearable cardiovascular monitoring systems, such as wearable ECG foils [92].
Under this heading, we will review the MEMS contribution in small molecule therapeutics i. e. drug discovery, drug screening, and drug delivery. To start with, a small introduction about the conventional drug screening procedure seems necessary.
Drug discovery process is organized into different phases starting with the identification of drug targets, a process known as target identification, wherein the biomolecules that play significant role in diseases are identified. Next, from a library of innumerable number of chemical compounds, the ones that have the potential to treat the disease by interacting with the drug targets in a desirable way are identified. This process is known as lead identification. These compounds undergo optimization to become a possible clinical candidate followed by a testing phase to ensure that it is safe to be administered to the patients. The advances in drug discoveries are highly dependent on the technological advancement made in these stages. MEMS technology has penetrated into many of these stages to make it easier to find refine and test possible drug candidates.
For disease target identification, the etiology (causative factor) of disease has to be known and the molecular machinery behind the abnormal/malfunctioning biological processes has to be determined. The collective contribution of studies on genetic interaction (genomics), protein expressions and interactions (proteomics) and their relation to diseases allows for the rapid and precise discovery of these drug targets. Genomics has taken a huge leap by the advent of high throughput technologies like microarrays. Microarray generally applies to spots with diameters of 200 microns or less attached to a solid surface such as a silicon chip thus allowing larger-scale experiments using very small volumes of sample and reagents.
In order to understand the application of MEMS in this area, a basic discussion about the target identification process is necessary. For studying the gene expression of a cell using microarray, the RNA of the cell is extracted and its labeled DNA copies are made. These tagged DNA copies are washed over a microarray containing single stranded DNA with known sequence (probe DNA). Upon finding a complementary probe sequence on the microarray, the tagged single stranded DNA hybridizes with it. Scanning the microarray with a laser source causes the tagged bound DNA to florescence. Since the location and sequence of the probe DNAs are known ahead of time, a comparison of the spots on the microarray reveals the gene expression of the cell.
In microarrays, the number of features or samples on a single slide or array can exceed tens or even hundreds of thousands. Microarrays, created using photolithographic method have extended the capability of the bioassays by reducing its development time and cost thus resulting in enhanced throughput. The high density Gene Chip probe arrays of Affymetrix, developed by Steven Foder and colleagues, contains thousands of oligonucleotides in a very small area of 2cm2 developed by light directed oligonucleotide synthesis (in situ synthesis) [93]. Microelectronic Array devices developed by M.J. Heller of Nanogen is yet another example of the improvement brought about by MEMS techniques. These active electronic microarrays provide electronic addressing of probes and increased DNA hybridization rate through the application of appropriate electric field [94]. Several protein chip formats have also been developed due to the advancement of micro fabrication technologies [95-101].
MEMS technology has begun to provide unique tools that can enable earlier determination of lead compounds than what was traditionally possible. Microchip patch clamp is one of such unique tools that have revolutionized the lead identification process [102-105]. High throughput ion channel drug screening is possible using this technique. Patch clamp technique utilizes MEMS functionality to fabricate planar electrodes which are micron sized holes in an insulating layer wherein the cells are trapped and ionic current variations measured.
In order to obtain a better idea of drug behavior cellular response studies are very important. Cell based sensors can thus provide functional information about the effects of drugs on its signaling pathways. It is often quite difficult to simulate the actual condition of the environment within a living organism since cells respond to spatially and temporally organized signals in their microenvironment. With MEMS technology it is possible to create microfluidic structures mimicking the actual ‘in-vivo’ environment [106-108]. Such micro structures can help in the preclinical testing stage of the drugs as these can create cheap ‘in-vivo’ environments.
The predominant methods for drug administration such as oral delivery and injection often results in immediate or rapid drug release wherein no control over the rate of drug delivery or the target area of the drug is exercised. A variety of devices and components have been designed and fabricated using MEMS techniques which are able to release drugs of different dosages in different delivery patterns. These include transdermal patches, implants, microparticles and microencapsulation.
Microneedles were first proposed as a method for percutaneous drug delivery in the 1970s [114]. Since then, the microneedle design has been refined to provide better control over drug delivery [115]. Although transdermal drug delivery is one of the most effective modes of administration, the poor permeability of the skin had remained a major limitation for macromolecular drug delivery. In vitro experiments have shown that inserting microneedles into skin can increase permeability by orders of magnitude, thus facilitating transport of therapeutic macromolecules [116]. Needles of micron dimensions can pierce into the skin surface to create holes large enough for molecules to enter, but small enough to avoid pain or significant damage. MEMS technology has made it possible to create such micron sized needles with design considerations dependant on its strength, robustness, minimal insertion pain, and tissue damage in patients.
Various types of microneedles have been fabricated and tested for drug delivery. They exist in two basic designs, in-plane and out-of-plane. In-plane microneedles have openings at the shaft of the needle whereas out-of-plane microneedles have openings at the tip of the needle. In order to avoid the breakage of the top part of the needles inside the skin, microneedle array with biodegradable tips have been developed [117] Encapsulation of molecules within microneedles that dissolve within the skin for bolus or sustained delivery has also been studied [118]. Hollow microneedles have also been fabricated and used to flow drug solutions into the skin [119].
For every drug delivery system, a drug depot or supply is required. In the case of in vivo drug delivery system, this drug depot has to protect the drug from the body until needed and when needed has to allow delivery in a controlled manner. Ingestion is widely accepted as the ideal form of drug delivery, but it presents difficulties for a number of newly developed drugs such as proteins and peptides as they are unable to survive the stomach's acidic environment and have reduced bioavailability. In order to overcome this limitation, microparticles and nanoparticles capable releasing drugs at specific targeted areas have been developed [121-125]. T. A Desai and team developed such reservoir-containing silicon micro particles capable of delivering therapeutics to the targeted sites in the gastrointestinal system [124]. The surfaces of these devices were designed to adhere onto specific cells in the digestive tract to deliver drugs by functionalizing them with avidin linked to biotinylated lectins. Various implantable delivery devices have also been developed for targeted and controlled drug delivery. Such a structure was designed and developed by Santini and colleagues, containing an array of individually addressable microreservoirs containing gold membrane, each of which contained a dosage of drug that could be released separately [126, 127]. The implantable drug delivery device developed by MicroCHIPS Inc. is shown below. Other non-traditional MEMS fabrication techniques have also been explored to form reservoirs with greater biocompatibility [128]. Although research on microfabricated drug delivery devices has rapidly expanded in recent years, in order to achieve improved patient compliance, much research still has to be done to optimize the size, shape, number, volume, and surface characteristics of the drug delivery systems.
The trends in modern medicine are to use less invasive methods that significantly reduces body trauma by performing surgery through smaller incisions using specialized tools. The main advantages of which includes lesser trauma, reduced post-operative pain, and quicker recovery time. MEMS technology has played an important role in the evolution of these minimally invasive procedures. These techniques allow the surgical tools to reach down to the size scale of individual cells and provide access for manipulation in previously inaccessible areas of the body. The development and integration of sensors, actuators and associated electronics required for augmenting the surgeon's dexterity and perception at micron level is possible due to this technology.
Conventional surgical tools have limited capability when it comes to manipulation of small structures such as nerves and vessels of small diameters. In order to achieve higher spatial resolution, researchers have developed tools such as micro-grippers, micro-tweezers, micro-forceps and micro-scissors with the aid of microfabrication techniques [130,131]. Recently, thermally actuated grippers capable of grasping micron sized objects have been developed for use in ophthalmic surgery wherein they could be used within the small volume of the eye [132]. Other development in this area include biocompatible polymer-metal bimorph microforceps made from a sandwich of gold film and SU-8 capable of grasping micron size object such as a single cell [130].
In MIS procedures, a major challenge faced by surgeons is the lack of sense of touch. In order to overcome this limitation microfabricated devices capable of restoring and enhancing tactile sensation is being looked into [133, 134]. MEMS based tactile sensors equipped with the ability to continuously monitor the type of tissue being handled and the force exerted on the tissue has been developed to enhance the surgeon's haptic perception. A miniature tactile sensor array consisting of an array of capacitive sensor cells mounted at the tip of their laparoscopic tool was developed by Gray and Fearing [135]. Deformation of this system by the application of pressure causes a detectable change in the capacitance of the affected cells. In the design for a micro-endoscopic tactile sensor, Rao et al. [136] describes a tactile sensor developed using PVDF films for better flexibility and sensitivity. Force sensors typically consisting of piezoelectric or piezoresistive elements embedded at critical locations along the structure of a mechanical device have been developed to provide three-dimensional mapping of the mechanical deformations in the device. In addition to these sensing mechanisms, various other methods have also been investigated. MEMS sensors for monitoring mechanical properties of tissues such as palpitation have also been developed [137]. One of the main research issues to be resolved for the tactile sensor design is the packaging to protect tissue and the sensor and cabling to bring signals out of the body without interfering with its range of motion [138].
The principle of miniaturization brings forth ultra small cutting tools which makes smaller incision that causes lesser bleeding. Research has been looking into the development of such nano-knives which are made sharper by etching silicon precisely along its crystal planes [132]. Utilization of vibratory mechanism for cutting tissues have also been demonstrated [139]. A surgical device called data knife developed by Verimetra. Inc (Pittsburg, PA, U.S.A.) is shown in Figure 6 . It consists of a scalpel outfitted with different strain sensors along the edges of the blade to sense the amount of force being applied. Vibratory mechanism by piezoelectric actuation is used in this device for cutting through tissues. Other sensing mechanisms included in this device are pressure sensors, temperature sensors as well as impedance sensors for measuring tissue impedance [140].
