Originally Published MDDI October 2001
A monthly review of new technologies and medical device innovations
THIS MONTH: |Creating Imprinted Gels for Insulin Biosensors | Clay Reduces Permeability of Implantable Polymer Components | Body Simulator Preserves Kidney for Transplant | Nanosensor Detects DNA Without Amplification
|The 20-µm wide microjaws can capture a single red blood cell.|
Microscale jaws fabricated from silicon and capable of grasping a single red blood cell have been developed at Sandia National Laboratories (Albuquerque, NM). According to Sandia researchers, the microjaws fit in a microchannel that is about 20 µm wide—about one-third the width of a human hair. As a human blood sample flows through the microchannel, the jaws close, trapping and deforming red blood cells. When the cells are released, they travel on, regaining their former shape and appearing unharmed.
Reports Sandia researcher Murat Okandan, "We've shown that we can create a micromachine that interacts at the scale of cells. We've created a demonstration tool with very flexible technology that we believe will enable many designs and concepts." The device can be mass-produced through computer-chip production techniques, according to the researchers. Ultimately it may be used to puncture cells and inject them with DNA, proteins, or pharmaceuticals.
The punctured cells do not need to be blood cells but could be, for example, stem cells, according to Okandan. He explains, "There is an incredibly wide array of applications that can be addressed using this technology—where we can operate at a cellular, even subcellular level, and be able to do it in large numbers to obtain the required statistics on cell populations. [Various] types of sensors, devices for drug discovery, drug-delivery tools, neural prostheses, and research tools are some of the immediately visible applications."
Describing some of the potential advantages of the technology, Okandan says, "This would be a continuous-flow device, which lends itself to massively parallel processing. It could reduce the cost associated with labor-intensive manual operations and enable transfection of cells or cell lines that do not survive other methods, such as electroporation or chemical methods."
|The prototype device offers the possibility of performing a considerable level of mechanical intervention at the cellular level because it operates rapidly and is small enough to allow a number of units to operate in parallel in a relatively compact area. According to Sandia, 10 complete units fit in an area smaller than a household electric plug prong, and each device can puncture 10 cells per second.|
Jay Jakubczak, deputy director of MEMS Microsystems at Sandia, describes the device as resembling a cellular Pacman. He says, "The technology used to create the microjaw prototype is an advanced surface micromachining technology developed at Sandia. It is a silicon-based technology, compatible with typical IC processes and tools, incorporating five levels of polysilicon that represent the mechanical portions of the device."
Says Jakubczak, the Sandia technology, SUMMiT, is "the most advanced surface micromachining technology in the world and is being applied to enable silicon monolithic integrated microfluidic systems." He adds that, "This technology is applicable to the manufacture and integration of silicon-based microscale pumps, valves, channels, mixers, separation columns, and sensors."
|Eight microfluidic devices fit on this tiny module, shown on a soda straw.|
The researchers are currently attempting to determine whether the captured red blood cells can absorb a fluorescent material that the group has already shown is naturally rejected by the cells. If the material is readily absorbed, it means that the Sandia scientists have succeeded in creating the first reported example of a continuous-flow, mechanical cellular-membrane disrupter.
The group also hopes to replace the microteeth with hollow silicon needles that are now in development. "The needles would rapidly inject DNA, RNA, or proteins (including drug molecules) into living cells at precise points of their anatomies and in large numbers, possibly changing the course of a disease or restoring lost functions," according to Sandia.
Current methods of cell implantation use electroporation, the application of electric fields to open cell walls for chemical absorption. A problem with this method is that it causes a significant portion of the cell populations to die. Other manual methods exist that use a very fine pipette to deliver genetic material into individual cells—a labor-intensive and specialized process. The Sandia device has the possibility of overcoming both of these problems, the researchers say.
An estimated 700,000 type-I diabetics in the United States must take insulin, either by injecting themselves with a needle at least twice a day or by using a battery-operated insulin pump. Development of sensing systems that could monitor blood glucose levels and effectively control insulin delivery would benefit many of these patients.
Now, researchers at Purdue University (West Lafayette, IN) are creating a biological sensor for glucose that could ultimately simplify diabetes treatment. The researchers suggest that the technology could help the development of intelligent drug-delivery devices that could be implanted in the body to administer medications such as insulin.
The technique entails forming a mesh-like biomimetic gel that contains glucose molecules. A slightly acidic chemical is then used to remove the glucose and create spaces where the glucose used to be. According to the researchers, if the gel is placed in a liquid such as blood, glucose in the liquid diffuses into the gel and binds to the empty spaces. In this way, the gel is imprinted for glucose molecules.
The research was presented in August at the American Chemical Society's national meeting by Purdue chemical engineering doctoral student Mark Byrne. Byrne is working on the project with Nicholas A. Peppas, Purdue's Showalter Distinguished Professor of Chemical and Biomedical Engineering, and Kinam Park, professor of pharmaceutics and biomedical engineering.
In essence, the approach attempts to mimic the manner in which certain molecules attach to binding sites on other molecules. Such binding is critical to various biological processes. Byrne explains, "Essentially, we are trying to design what nature has done so well, and that's a difficult thing to do. We are creating artificial binding sites."
The researchers suggest that medical devices implanted inside the body could eventually rely on artificial sensing mechanisms that would incorporate a meshwork containing medications. Sensing glucose in the blood would automatically trigger the meshwork to expand, opening pores and releasing insulin or a medication that would enable the body to more efficiently absorb insulin.
Says Byrne, "At this time, we have been successful in creating a polymer gel that can bind glucose molecules. This is the first step (i.e., recognition) in designing an intelligent drug-delivery device that can modulate the release of insulin or other medications in the treatment of diabetes." He adds, "What we have created so far would be involved in the recognition/release action of the overall device."
Explaining how the imprinted gels would act as the recognition/release part of the overall device, Byrne says, "By forming a polymer network in the presence of particles that can bind glucose, a sensitive network that swells with glucose in solution (and [triggers] a diffusive release of insulin from the meshlike matrix) and contracts (no insulin release) when glucose is not in solution can be prepared." In this way, the device can function as a drug-delivery mechanism. "In effect, release is modulated to give the right amount (dosage) of insulin at the correct time. Insulin would be loaded into the device by equilibrium partitioning," according to the researcher.
Byrne adds, "With this device we would expect good bioavailability of insulin. As far as the in vivo placement of the device or gel system, many factors become important, such as ease of placement in terms of surgical procedure, and the area of body where there is representative sample of true blood glucose levels."
Although the application is a few years in the future, Byrne says "we are analyzing current IDDM [insulin-dependent diabetes mellitus] treatment methods (e.g., islet cell transplantation sites) and will design and optimize our system to achieve low immunogenicity, no toxicity, good insulin bioavailability and release kinetics, and so on."
A team of researchers from Penn State University (University Park, PA) are using a compound commonly found in cosmetics and antacids to make polymer parts in artificial heart devices less permeable to air and water. The polymers used for various parts of artificial heart devices can be penetrated to some extent by air or water. Says James Runt, PhD, professor of polymer science, "We decided to look at two methods for decreasing permeability—a chemical method and a nanocomposites approach."
The researchers explain that the chemical method entailed use of a polymer similar to that already used for the pumping chamber, cannula, and compliance chamber in a left ventricular device developed by Arrow International (Reading, PA) and Penn State, and in earlier heart devices. In the current devices, air inside the compliance chamber helps in the pumping. Over time, however, air seeps through the polymer wall of the chamber into the body and dissipates. This requires the air to be replaced periodically. The polymer is also permeable to water, and additional care is required to ensure that the electronics in the devices remain dry.
|Polyisobutylene chains are attached to a standard polymer to form a comblike structure, which functions as a barrier for air and water.|
The researchers report that they took the standard polymer and attached polyisobutylene chains to it to form a comblike structure. The hope was that these structures would create a barrier for air and water. Up to 35% by volume of polyisobutylene was incorporated into the poly (urethane urea) material. The resulting polymer had good mechanical properties, but has not been tested for fatigue resistance, according to the research team.
The alternative solution to the problem was based on use of a commercially available silicate clay, Cloisite 15A, produced by Southern Clay Products. The material is an alkyl ammonium–modified montmorillonite. The researchers explain that when the constituent silica layers are mixed with the polymer in a common solvent, they disperse throughout the solvent. When the solvent is removed, the layers remain distributed to some extent.
"With an addition of 20% by weight of this modified silicate, we achieved a decrease in permeability of a factor of five," Runt explains. "This method is much more convenient than the chemical method and produces a far greater decrease in permeability."
Says Runt, "The chemical approach we took was relatively complex and resulted in about a factor of two reduction in permeability to water vapor and oxygen. Not bad, but not as low as we were looking for. Nanocomposites of rubbers and plastics with layered silicates are being investigated in both the industrial and academic sectors, and I felt that there was a natural extension to the biomaterials sector."
Runt says the initial research was done on nonoptimized compositions and preparation methods. "There is real work to be done in optimizing these, and hopefully improving performance even more. Also, biocompatibility and flex fatigue for materials that are being considered for blood pumps are still open issues."
Researchers at the University of Chicago Hospitals are developing an organ preservation method that could help improve transplantation procedures. Currently solid organs removed from donors must be placed on ice and maintained at 4ºC until they can be transplanted. Cooling the organs slows cellular activity and reduces metabolic function. However, exposure to cold can damage the organs and they must be used quickly to minimize such effects. In addition, as an organ is reperfused with warm blood, it often sustains further damage.
Using the new technique, the university researchers report, a kidney has been preserved—functioning in the same manner it did in the donor's body—for almost 24 hours without exposing the organ to cold. The machine, called the Portable Organ Preservation System (POPS) is being developed to enable organs to be kept in a warm, blood-based, oxygenated, nutrient solution.
"This could transform the way transplants are performed," says David Cronin, MD, assistant professor of surgery at the University of Chicago. "POPS could make it possible to keep organs undamaged for much longer periods of time. We would have more time to properly prepare both the patient and the organ for transplant surgery," he adds. Cronin and a team of university researchers are working with TransMedics (Woburn, MA) to develop the device.
The POPS device is intended for use with all organs that are currently used in transplantation procedures, including kidneys, lungs, and hearts. Cronin has been involved in the design of the POPS for the past three years. According to the researcher, more than 500 animal organs, including hearts, kidneys, and livers, have been tested on the machine in the last 18 months.
The first human organ storage in the POPS was reported on August 25, when the researchers removed a human kidney from the machine at the end of 24 hours. Data on the functioning of the kidney while it was in the POPS are being analyzed, and the group is examining the kidney itself in detail to see how it was affected.
The organ appears to have behaved exactly like a kidney in a human body. As blood is pumped through it in a way that mimics the heart's pumping, the blood was filtered and the kidney made urine normally. Animal organs that have been tested have all behaved normally.
"We are currently forced to maintain a very high bar for organs used in transplant," says Cronin. "A patient receiving a heart or liver that doesn't function may not get a second chance. This system, in addition to giving us time, allows us to see that the organ is functioning and perhaps even repair it. This technology could expand the range of usable organs and save lives." Cronin explains that the POPS could conceivably be used to help "weed out" organs that are unfit for transplant.
A sensitive and flexible molecular sensing system that detects a broad array of analytes from small ions to nucleic acids to proteins could have a significant impact on pharmaceutical research and clinical diagnostics. The system is being developed by Charles Lieber, PhD, and Hongkun Park, PhD, who are the cofounders of Nanosys Inc. (Cambridge, MA) and professors at Harvard University (Cambridge, MA).
The researchers explain that the sensor uses a nanowire—one ten-thousandth the width of a human hair—to which specific capture molecules have been affixed. These molecules detect the presence of minute quantities of analytes, such as ions, nucleic acids, and proteins.
Detection is performed using inexpensive and portable low-voltage measurement equipment. This eliminates the need for cumbersome and expensive chemical labels, such as fluorescent dyes, and sophisticated optical equipment, such as lasers, according to the researchers.
There are a number of potential advantages that the new technology may make possible. According to Lieber, "The unique features of the nanosensor could reduce the cost of existing types of detection tasks, but more exciting to us is the ability to do things simply not possible with other technology."
Lieber explains that the nanosensor could be used to carry out DNA detection without amplification. "This has been somewhat of a holy grail and enables direct analysis of genomic DNA samples," he says, adding that this would be useful in large-scale disease association studies. "Of course, this type of analysis could be carried out in an array format, but the key is doing so without the need for PCR [polymeric chain reaction] amplification or costly organic labels."
Lieber also says the nanosensor could be used to perform single-cell analysis of large numbers of species—both on a given cell and on each cell in an array. It could also simultaneously monitor expression levels of a large number of nucleic acids and proteins in real time and study the influence of exogenous species, such as potential drugs, on these species. "This could be useful in understanding pathways involved in disease as well as for evaluating therapeutics," he says.
The technology could also provide powerful diagnostics. "In an integrated, cheap package, one could have a home test for a suite of cancer markers in the same way that a glucose test is done by diabetics," Lieber says. "Being able to make such measurements with regularity. . . with a minimally invasive real-time device would enable cancer detection at its earliest stages when it is most treatable."
Lieber further notes that "the same type of technology can be implemented in cancer marker discovery. So this sensor readily lends itself to basic biology research and subsequent transition to diagnostic [applications]."
Copyright ©2001 Medical Device & Diagnostic Industry