Technology news
Machine Accelerates Screening of New Implant Materials
Preclinical testing of materials for orthopaedic implants typically takes about six months and requires the use of a complex and expensive joint simulator. The National Institute of Standards and Technology (NIST; Gaithersburg, MD, USA), under a Cooperative Research and Development Agreement (CRADA) with a consortium of four companies, has developed a machine that can perform accelerated screen testing of materials in a fraction of that time.
Currently any material being considered for implant applications must undergo testing in a joint simulator that subjects the material to conditions similar to those it will encounter in vivo. The cost for evaluating alternative materials can be prohibitive when they cannot be screened with confidence prior to such testing. The accelerated wear device developed by NIST would enable researchers to prescreen new materials in approximately one week; materials that show promise could then be further evaluated by means of a joint simulator.
"A previous test method that had been defined by the American Society for Testing and Materials—ASTM standard F 0732-82 (1991)—was not considered reliable enough by industry for many screening applications," says John Tesk, coordinator of NIST's Biomaterials Program, Polymers Division. "ASTM has been rewriting this standard with some defining restrictions for its applicability. The NIST technology holds the promise for accelerated evaluation of a wide range of materials under a large variety of conditions," adds Tesk.
Thus far, the machine has run tests in what Tesk calls a confirmation mode, evaluating an ultra-high molecular-weight polyethylene pin wearing against a metal counterface. Researchers achieved three results, according to Tesk. "My colleagues Steve Hsu and Ming Shen were able to develop a surface morphology of wear that resembles what can be seen from a joint simulator and, in fact, what can be seen in retrieved samples of implants," he says. "The size and shape of the particles resembled those found on retrieved samples, and the wear was actually very close to what is produced by the other commercial equipment."
Having shown that materials tested using the accelerated wear device achieve the same ranking as they do on the commercial tester, NIST and the consortium have extended the CRADA for two more years. The project's next phase is to use the device to determine how alternative implant materials might fare. They will be subjected to the effects of motion, environment, and a variety of stress-loading cycles that simulate different physical routines.
The companies participating in the Orthopaedic Accelerated Wear Resistance Consortium are Biomet Inc. (Warsaw, IN, USA), Zimmer Inc. (Warsaw, IN, USA), Johnson & Johnson Professional Inc. (Raynham, MA, USA), and Osteonics Corp. (Allendale, NJ, USA).
Modular Oxygen Sensor Developed for OEM Use
A sensor with a response time in the millisecond range measures oxygen concentrations by means of fluorescence quenching. Developed by Sentronic GmbH (Dresden, Germany), the component's modular design makes it suitable for use either as a simple sensor or a sophisticated measurement system. Application adaptation costs are minimal, according to the company.
The measurement principle involves exposing the appropriate dyes to pulses of shortwave light. The excitation causes them to emit long-wave radiation, the intensity of which reflects the concentration of oxygen in the surrounding medium. The resulting characteristic curve shows a decreasing gradient inversely proportional to the oxygen concentration. By applying the Stern-Vollmer equation, a linear curve can then be generated.
According to general manager Matthias Lau, fluorescence quenching is suitable for measuring oxygen in the 0 to 100% range, but the highest resolution is achieved at low concentrations.
In addition to its highly sensitive recognition of low oxygen concentrations, the sensor can perform measurements in high-viscosity media. It requires no electrolyte and does not consume oxygen. The company is currently seeking to further develop this technology by adapting it to specific OEM applications.
Electromechanical Polypropylene Film Holds Promise for Physiological Applications
An electromechanical film containing a permanent electric charge acts as both a sensor and actuator material. The thin foil-like plastic is coated with electrodes and can be cut to any shape or size. It has several potential uses in the medical device sector, according to VTT Automation (Tampere, Finland).
The 0.05-mm-thick film, called EMFi, is made of biaxially oriented polypropylene. This orientation produces many small flat bubbles in the material, which is suffused with a permanent electric charge during manufacture. Thin metal electrodes are then evaporated or laminated on both surfaces of the film.
When it is used in sensors, EMFi acts as a flexible charged capacitor that produces an electrical charge in proportion to the force that is applied to it. The signal can then be amplified with an instrumentation amplifier. The typical sensitivity of a single sheet is in the 30–160-pC/N range; higher sensitivities can be attained by stacking several layers of film on top of each other.
According to the company, the sensors are especially well suited for physiological applications in which sensors or sensor arrays come into contact with skin.
EMFi can also operate as an actuator. In this case, the material can be configured to work in a thickness mode—the material compresses and expands in relation to the applied voltage—or in a membrane mode—it vibrates between stators. Folded, cavity, and layered-film actuators can be constructed based on these two principles of operation. The material, which is sensitive enough to measure the breathing rate and heartbeat of a patient who is standing or lying on it, has potential patient-monitoring applications in hospitals.
