A MEMS-based mass flow controller was instrumental in the successful development of a plasma-based surgery system.
|The PlasmaJet system, developed by Plasma Surgical, uses a high-energy plasma beam to cut and coagulate tissue.|
The PlasmaJet system, developed by Plasma Surgical Ltd (Abingdon, UK), cuts and coagulates tissue by means of a fine beam of electrically neutral, high-energy plasma. The beam is generated by ionising a low flow of inert argon gas, which is excited into a plasma state and shot from the tip of a handpiece. For the device to work safely and effectively, the gas flow must be precisely controlled, and the device's mass flow controller must operate in a predictable and repeatable manner. In the early stages of product development, this proved to be a challenge that could have scuttled the project.
Controlling the controller
Inconsistent settling times plagued the first MFCs that Plasma Surgical tested. The MFC has to provide the high pressure and high flow rate necessary to ignite the plasma, after which it must ramp down to mere tenths of standard litres per minute (slpm). Overshoot must be prevented, or the plasma beam is lost. Unable to make the MFC—and, hence, the PlasmaJet—work properly, the company turned to Bürkert Fluid Control Systems (Ingel-fingen, Germany).
Bürkert’s brief was to provide an MFC that would control the extremely low flow rates with a repeatable accuracy of ±0.01 slpm. The specification also required that the MFC be suitable for use in EMC noisy environments—each unit is sited below a large 3.5-kV power supply and has two fans working at low frequency—that it be stable between 25° and 40°C and that manufacturing tolerances ensure consistent settling times. Bürkert delivered a prototype MFC just nine days after its first meeting with the client. The device tested successfully at the company’s R&D facility in Sweden, and a preproduction order was placed, just three months after the initial contact.
“The main advantage of the MFC developed by Bürkert resides in the gas flow of the bypass channel,” says Tony Brennan, Corporate Account Manager at Bürkert. “In a conventional system, the bypass channel is heated externally. The thermal lag associated with having to heat up the bypass tube creates the delay in the settling time and speed of response.” Bürkert also engineered a solution to the repeatability issues. “Each MFC from the previous supplier behaved differently, making it impossible for Plasma Surgical to filter out rogue measurements in the software of their device. With our strict manufacturing process, the repeatability of the units supplied is much better,” explains Brennan.
The MEMS advantage
MEMS technology plays a key role in the successful operation of the device, because it allows the delivery of mass flow without pressure or temperature corrections. The actual flow rate is detected by a sensor embedded in the wall of a specifically designed bypass channel, into which a small part of the total gas stream is diverted, ensuring laminar flow conditions.
The CMOS sensing element contains a heating resistor and two temperature sensors (thermopiles) that are arranged symmetrically upstream and downstream of the heater. The differential voltage of the thermopiles is a measure of the mass flow rate passing this bypass channel; the calibration procedure ensures a unique assignment of the sensor signal to the total flow rate passing the device.
Given the lightning speed at which this MFC was developed, my assumption was that chunks of the technology must have been borrowed from other projects. Think again, says Brennan.
“The technology employed by the PlasmaJet is unique, and we were working from scratch without any previous project experience. We could see the potential for the product in the market and pulled out all the stops to get a working prototype to site as fast as we could,” says Brennan. Nikolay Suslov, Chief Technology Officer and inventor of the PlasmaJet technology, is glad they did. “It is true that the ability of Bürkert’s mass flow controller to control very low gas flows precisely, and with continuous repeatability, has made our unique technology possible,” says Suslov.