Use of nanoscale devices is helping to revolutionize medical treatment and research. The Ohio State University recently became the first place in the country where coronary bypass and mitral valve repair operations are performed by a computer controlled robot, operated by a remotely located surgeon handling joysticks and considering a three-dimensional, enlarged view of the surgical field. With the invaluable help of many associates and students, researchers have focused on two classes of objects: devices that feature nanopore membranes, and multifunctional particulates for the delivery of medical therapeutic agents. Nanomembranes may also be used as flux regulators for the long-term release of biopharmaceuticals from implanted depots. This application embodies a purely passive mass transport mode of molecules in channels that are just a few nanometers larger than the effective radii of the molecules themselves.
The ability to engineer devices and their components at the nanoscale level will revolutionize medicine—and the biomedical disciplines in general. There is a simple reason for this: Whatever we put into the body, in order to be therapeutically effective, must mimic the basic biological structure it replaces or augments. And biology is intrinsically and without exception multiscale, with hierarchies of ordered components comprising complex architectures of smaller, ordered components, all the way down to the molecular or atomic level—the nano level, precisely.
Mauro Ferrari, a professor of internal medicine and mechanical engineering, is director of the Biomedical Engineering Center at The Ohio State University in Columbus, fun Liu is a doctoral candidate in his group.
EYE ON THE FUTURE: NANOTECHNOLOGY
It should not be surprising that engineering can revolutionize medicine. It may well be argued that engineering innovation has contributed perhaps the majority of recent beneficial changes in many of the most crucial sectors of the medical disciplines: The best way to beat cancer is still to catch it early, and early detection is achieved by means of engineering successes, such as computerized tomography X-ray (CT) and positron emission (PET) scans, magnetic resonance imaging (MRI), and nuclear magnetic resonance (NMR) tests, scintigraphy, and a host of technology-intensive, advanced diagnostic protocols. Surgeries that used to require extensive hospital stays, and carried significant morbidity, mortality, and cost, are now routinely performed in a minimally invasive fashion, frequently on an outpatient basis.
The Ohio State University recently became the first place in the country where coronary bypass and mitral valve repair operations are performed by a computer-controlled robot, operated by a remotely located surgeon handling joysticks and looking into a three-dimensional, enlarged view of the surgical field. Use of the small-scale robot removes the need to split open the patient’s breastbone to perform the surgery, with great reduction in risk and recovery time—just another engineering-empowered medical breakthrough to be added to a list that is already quite extensive and continues to grow rapidly, and in very diverse sectors.
Recent additions to this list are the technologies that have made mapping of the human genome possible so far ahead of schedule, and they form the technological basis for the explosion of the fields of genomics and pro-teomics. The first major breakthrough in the high-tech, very rapid sequencing of nucleic acids (that is, the determination of the order of the basic building blocks of DNA in a test sample) came from a truly interdisciplinary inspiration: the application of photolithography to the placement of single-strand DNA probes in specific known locations on a chip. The probes specifically conjugate with fluorescently labeled single strands of the DNA sample that is being tested. Thus, by observing the locations on the chip that fluoresce, it is possible to reconstruct the biochemical identity of the test DNA. The basic fabrication technology in microelectronics and MEMS, photolithography, thus provided a truly innovative path to the ultrafast investigation of nucleic acids.
Commercial applications of this core innovation are already available in fields such as the high-throughput screening of new drugs, genetic risk assessment, forensics, family law, personal identification, germ warfare detection, and a vast cohort of basic medical research sectors. Photolithography has also opened the way to the notion of a lab on a chip, or the miniaturization of diagnostic and analytical instrumentation to the point of being able to be integrated in handheld or small desktop devices.
Also referred to as “microfluidics,” this sector is developing commercial products that allow for the bedside reading of blood chemistry, of the presence and concentration of markers of disease, and even of genetic information. This, of course, allows for faster medical intervention, and a significant reduction in costs.
Technologies such as instrumentation on a chip, being based on photolithography, were traditionally referred to as biomedical microtechnology or bioMEMS. However, in the recent past, the photolithographic limit of resolution has been lowered to the hundreds-of-nanometers range, which empowers the routine entry of microelectronics-related processes into the nanotechnology arena. It is a form of top-down nanotechnology, in the sense that it starts with larger objects, such as layers of materials deposited on a silicon wafer, and proceeds to remove precisely defined sections, leaving on the substrate a large number of copies of objects that may be 200 nm in any lateral direction, and just a handful of angstroms thick.
Bottom-up fabrication nanotechnologies, by contrast, manipulate single atoms into atomic clusters or molecules, and these into larger structures, like building blocks of exceedingly small dimensions. Methods for the manipulation of individual atoms and molecules include the families of atomic force and scanning tunneling microscopies.
Extraordinarily exciting as the notion of atomic Lego may be, it, of course, runs into the practical limit of having to assemble atom-by-atom multiple copies of relatively large devices in a finite period of time. Thus, most current approaches to nanofabrication combine top-down and bottom-up components. Molecular self-assembly may be .guided by lithographically prepared substrates, and nanosize objects, such as buckyballs, nanotubes, and atomic clusters, may be prepared by conventional synthetic processes and used as primers for further assembly.
For several years, the chief research focus of our group has been the use of nano- and microtechnologies to develop multifunctional medical therapeutic devices. One conceptual model is of a smart drug-delivery implant with the ability to localize in areas of need, and modulate release in harmony with biological or external signals.
With the invaluable help of many associates and students, we have focused on two classes of objects: devices that feature nanopore membranes, and multifunctional particulates for the delivery of medical therapeutic agents. To date, the smallest particulates we have micro-machined are dimensioned at about 1 micron, with prospective intravascular use in the anti-angiogenesis therapy of metastatic cancer. Larger, multifunctional variants, of complexity that increases with size, have been developed with the objective of administering biopharmaceutical drugs orally, or by inhalation.
These ideas illustrate the basic concept: To be effective in the body it is necessary to integrate, in very small spaces, a host of multiscale substructures with different functions, and subcomponents as small as individual molecular monolayers. A second major concept is intimately related to this: As we reach deeper into the small dimensions, physical phenomena appear that are really counter to our macroscale physical and engineering intuition. The point is perhaps best illustrated by reference to our first class of medical therapeutic nanotechnologies; that is, devices that feature nanoscale pores with minimal dimensions as small as 6 nanometers, and variations confined to within 10 percent of the nominal pore size.
Over the last seven years, we have manufactured many variants of such membranes in silicon, employing a variety of proprietary manufacturing protocols, which have in common the use of a spacer layer—a sacrificial silicon dioxide interlayer between two structural silicon layers, to be exact. A selective chemical etching of the sacrificial layer is performed to leave the structural layers essentially intact. Thus, the pore geometry exactly reproduces the geometry of the sacrificial layer, which in turn can be controlled with no less than nanometer precision—et voilà le nanopore.
Of course, our sacrificial layer technology draws inspiration from the “gate oxide” of microprocessors, and is reminiscent of the lost wax technique of sculpting that dates back to the great artists of ancient Greece.
The first applications of our nanopore technology are in molecular-scale separation, as in the extraction of viruses from biological fluids, and the protection of implantable biosensors from albumin and other biofouling molecules. Our main application has been in implantable cell bioreactors for the physiologically regulated release of insulin in the therapy of type I diabetes mellitus.
In this context, we have pioneered the cross-species transplant of insulin-secreting cells into diabetic animals. The role of the nanopore membranes is that of impeding or retarding the rejection of the cell implant, which is caused by molecules that are large enough to be selectively excluded.
Tejal Desai of the University of Illinois in Chicago recently demonstrated that “stealth drug factory” biocapsule implants actually do cure diabetes in laboratory animals, at least for periods of several weeks, without the need for injections of insulin or anti-rejection drugs.
Nanomembranes may also be used as flux regulators for the long-term release of biopharmaceuticals from implanted depots. This application embodies a purely passive mass transport mode of molecules in channels that are just a few nanometers larger than the effective radii of the molecules themselves. The process is related to diffusion, but in reality involves much more complex phenomena, such as the continuous interaction of the molecules with the pore walls and each other, which has been described as “single-file diffusion.”
Active mass transport by convection—as would be required for a remotely controlled, or self-regulating, implantable drug dispenser—is practically impossible, since the pressures needed to push fluid through nano-size channels are many orders of magnitude in excess of the realistic load-carrying capacities of the materials and structures involved.
WITH LATERAL DIMENSIONS of one micron or less, the depicted particle is smaller than any blood cell. Once a concentration of these particles is safely injected in the bloodstream, they travel freely through the circulatory system. In order to direct these drug-delivery microparticles to cancer sites, their external surfaces are chemically modified to carry molecules that have lock-and-key binding specificity with molecules that are expressed preferentially on the blood vessels that support a growing cancer mass.
Here is how these microparticles may provide a revolutionary new approach to the fight against cancer: As soon as the particles “dock” on the cells that line the internal surface of these blood vessels, a compound is released that forms a pore on the membrane of these cells. This leads to cell death, the consequent collapse of the entire blood vessel, and ultimately to the death of the cancer mass that was nourished by the blood vessel.
In view of the targeting specificity of the molecules on the surface of the microdevice, very little or no collateral damage is done to healthy tissue, and thus the treatment may be repeated multiple times, as needed.
The project is being conducted at iMEDD Inc. of Columbus, Ohio, and The Ohio State University, with funding from the National Cancer Institute.
That brings us to a discussion of the all-important role that mechanical engineers must play in the development of truly innovative biomedical nanotechnology. The main lesson from biology is that the transfer, equilibrium, and dynamic balance of entities such as mass, momentum, moment of momentum, heat, and all forms of energy are achieved by active mechanisms. By contrast, in engineering we almost exclusively consider completely passive mechanisms, which are nonreactive, nonadaptive, nonself-regulated, or, in a word, pretty dumb.
To get from the passive to the active mode, even for completely mechanical concerns such as load bearing and mass transport, we will have to incorporate modes of action that couple the mechanical with the nonmechanical—such as electrical, magnetic, hygrothermal, or other modes of excitation.
The need to understand and employ these multifield, coupled phenomena represents a great call to arms for the mechanical engineers of current and future generations. Technological breakthroughs based on these advances will require the incorporation of nanoscale phenomena into engineering devices in a manner somewhat analogous to the retooling that electrical engineers have had to do with the advent of quantum phenomena in their trade.
An equally important mission for the mechanics professionals will be the incorporation of multiscale modeling into the analysis and design of novel devices. By “multiscale,” we refer to the notion that many physical phenomena are scale-dependent; that is, they manifest themselves only at a specific dimensional scale. Thus, in order to model them effectively, it is necessary to have a theory that is viable at different scales.
For instance, the physical property of isotropy (identical mechanical properties in all directions) is scale-dependent: A large piece of steel typically is isotropic, but the micron-scale crystals of which it is made most certainly are not.
Why is multiscale modeling of importance in biomedicine? Philosophically speaking, it must be, since biological entities typically comprise a nested hierarchy of structures. A theory that can reproduce and accurately describe such complexity will yield invaluable scientific insights on how biology works—and will, at the same time, grant us novel powers in the fight against disease.
To illustrate the point, reference may be made to the analysis of biological tissue samples, also known as biopsies, which are currently required for the positive diagnosis of cancer. The diagnosis is typically made by a pathologist analyzing the shape, size, and physical presentation of the cells in the sample, sometimes supplemented by additional biochemical information. These methods are not quantitative and lend themselves to the subjectivity of interpretation.
On these foundations, we have started performing ultrasound characterization of tissue samples from normal and cancerous breast biopsies, of both human and animal origin. In short, this involves sending a mechanical wave of known frequency into the tissue, in a special multilayer configuration, and detecting the reflected part of the wave from a separate transducer.
A plot depicting the reflection coefficient versus the excitation frequency is known as a reflection spectrum. The comparative analysis of reflection spectra from normal and diseased tissue is then the tool through which novel cancer diagnostic protocols may be established.
Based on work started with Vladimir Granik at the University of California, Berkeley, in the 1990s, we have developed a multiscale model that reproduces with extreme precision the reflection spectra of normal and diseased tissues. The central part of this potentially revolutionary approach is the use of discrete (as opposed to continuum) mechanics, according to the method that has been called “doublet mechanics” or “nanomechanics.”
simulated and experimental reflection spectra of human breast tissue specimens are reported in this graph. To date, research has found the spectra of healthy tissues to differ substantially from those of human biopsies with three types of cancer: adenocarcinoma, invasive ductal carcinoma, and lobular carcinoma.
The scale-accounting, discrete-based model employed is capable of reproducing the experimental data with great precision. The accuracy diminishes if the nonscale variant of the theory is used, but a discrete representation of matter is still used. The fit is worse yet, if the methods of conventional continuum elasticity are used.
Greater precision in the reproduction of the experimental spectra is expected to translate into greater confidence in the identification of cancer, especially at the early stages of the disease.
Familiar examples of discrete mechanics include finite element analysis and truss analysis.
Simply put, the use of conventional continuum mechanics does not allow for the mechanical signature of the tissues to be reproduced with the precision that is possible with the new approach. The precision in the reproduction of the experimental spectra, in turn, is greater with a multiscale discrete-based variant of the theory than with a simple nonscale discrete one, which is better than the continuum approach.
This may lead to a very precise quantitative assessment tool, through which the presence of cancer may be ascertained with far fewer incorrect diagnoses than is now possible. It’s an example of how the development of a better mechanical theory may hopefully be used not only for scientific advancement, but directly to save lives.