Dynamic thermal sensor for biofilm monitoring

• We present a sensor using AC thermal excitation to monitor biofilm development.
• The electrically passivated sensor is not affected by changes in ionic strength.
• The measuring system exhibits repeatable measurements and very low signal drift.
• Ampicillin-inhibition test shows the viability of the sensor as monitoring tool.
• Thermal wave theory may explain signal evolution as the biofilm grows.

A novel sensor principle to continuously monitor biofilm formation by Enterococcus faecalis is presented. The sensor consists of a small heater and a temperature probe (thermistor); both are located on a thermally insulating, silicon nitride membrane. Biofilm is allowed to develop in M17-medium at 37 °C on the surface of the membrane. Electrical current applied to the heater produces a steady sinusoidal power signal at 40 Hz. The resulting temperature oscillations are captured by the temperature probe. Their amplitude and phase shift are influenced by the thickness and composition of the biofilm. Results show that the sensor system is able to monitor the evolution of biofilms for large time frames (hours). It also features very stable baseline measurements that outperform other types of sensor systems. A proof-of-concept by means of an antibiotics-growth-inhibition test is achieved.