Company: Stratos Product Development
Location: Seattle, WA
CONTACT: (206) 448-1388; www.stratos.com Engineering services: Mechanical engineering, electrical engineering, software engineering, biomedical engineering, optical engineering, industrial design, program management, product planning, marketing
Project: Design and prototype an instrument for Prolinx Inc. that analyzes large molecules. Instrument would be called Octave.
Design functions performed: Writing specs; mechanical, electrical, biomedical, optical, and industrial design; prototyping
WHY CONSULTANT NEEDED: Short design cycle– 9 months to beta. Prolinx needed expertise in medical instrument design and wanted one shop to produce turnkey design.
Critical issues: Prolinx Inc. required us to use very recently developed sensors that operate based on a phenomenon termed surface plasmon resonance or SPR. Existing instruments using SPR are very large and can only measure a single sample at a time. Our primary objective was to design an instrument that was the size of a household toaster, less expensive, easier to use than existing instruments, and enable eight simultaneous measurements from individual chemical samples.
The project was made feasible by the advent of a highly miniaturized SPR sensor element developed by Texas Instruments, and by proprietary surface chemistry developed by Prolinx.
One of the principal challenges was the high degree of resolution required in the refractive index measurement. Our goal was to detect refractive index changes down to as little as 1 part in 106. Thus, low noise processing of the analog sensor output signal was critical. Additionally, at this level of resolution, many environmental factors become critical, including the influence that thermal changes have on the refractive index of the sample.
The thermal environment surrounding the sample needs to be controlled to avoid masking reaction data. To accomplish this, we enclosed the sensors in an insulated aluminum thermal block whose temperature was regulated to within 0.1 degree C using thermal-electric effect devices mounted to its exterior. We refined the thermal block design using finite element analysis and a thermal-imaging system.
Fluid handling was also critical. We chose an open-well approach for holding the test fluid sample in contact with sensors. The sensor-detecting surface itself forms the bottom of the well, a disposable thin-walled plastic well liner forms the sides, and the top is open for delivery of the test sample via a pipette. The cartridges are held within the thermal block side-by-side on 9 mm centers to be compatible with standard micro-titre plate storage trays.
A robotic fluid-handling system from Cavro Scientific Instruments moves the fluids between their storage receptacle and the instrument. This robotic system can dispense and aspirate fluid between industry-standard micro-titre plates, the sensor wells, and the storage wells. It can also dispense fluid from a bulk buffer solution used for sensor cleaning and dilution of samples.
The small 7×7×7-inch volume requirement of the instrument created serious design challenges electrically and mechanically. The compressed project timeline we had to work with precluded the opportunity to build a physically larger initial prototype, so the design had to be optimized for size from the get go. We used Pro/Engineer (PTC, Needham, MA) CAD tools to model all components down to discrete component levels, even on the PCBs. Eventually, by judicious selection and manipulation of the mechanical components and by partitioning the circuit boards to optimize the use of what volume there was, we were able to squeeze everything in.