Automatic inline sensor calibration station for highly viscous media

Bioprinting requires precise control of the amount of extruded material. The sensors usually have to be recalibrated for each fluid. A new process now allows printing systems to be calibrated directly on the platform.

Factsheet

  • Lead school School of Engineering and Computer Science
  • Institute Institut für Drucktechnologie IDT
  • Funding organisation Innosuisse
  • Duration 01.05.2018 - 31.12.2021
  • Project management Karl-Heinz Selbmann
  • Head of project Karl-Heinz Selbmann
  • Partner regenHU Ltd
    CSEM SA
  • Keywords Micro flow rates, thermal sensor calibration, optical flow, bioprinting, inline flow rate measurement, time-pressure dispensing, single-use flow sensor

Situation

In bioprinting applications, precise and clean control of the extrusion is important to be able to generate stable and precise structures. The materials used for extrusion are often highly viscous and have elastic flow properties. This makes it difficult for conventional flow sensors to measure the flow rate of such media. Measurement is particularly difficult at low flow rates, such as 0.5 µl/min. The goal of the project was to develop a measuring method as well as the appropriate set-up to enable a wide range of flow rates of highly viscous media (0.5 µl/min to 66 µl/min) to be determined. Because the procedure is used in the medical field, there must be no risk of contamination. This means the system must either be sterilisable or a single-use product.

Course of action

The first step involved compiling a list of existing flow sensors and technologies, which provided an initial overview of the current market. From this list, any sensors that appeared suited to the purpose at hand were acquired and evaluated. However, none of the existing sensors were found to meet all the requirements. The individual technologies were therefore examined in greater detail and a new list drafted that classifies these technologies according to their capacities, strengths and weaknesses. Finally, the researchers made the decision to develop all prototypes of mechanical, thermal and optical sensors themselves and to manufacture the most suitable sensor.

Result

The new method consists of a small thermal sensor as an in-line measuring system. The thermal sensor is calibrated using a camera system. For the calibration, a programme was written that makes it possible to automatically run through multiple flow rates one after the other. In each case, the signal of the thermal sensor is measured and stored. At the same time, the camera system records the nozzle outflow. The camera recordings are used to calculate the exact flow rate of the medium at the nozzle outlet using an algorithm. The software then calculates a calibration function for the thermal sensor. After calibration, the actual printing process can begin. Now the flow rate is measured by the thermal sensor at high frequency. The camera system is no longer needed during the printing process.