A micro-thermal sensor for focal therapy applications

Abstract

There is an urgent need for sensors deployed during focal therapies to inform treatment planning and in vivo monitoring in thin tissues. Specifically, the measurement of thermal properties, cooling surface contact, tissue thickness, blood flow and phase change with mm to sub mm accuracy are needed. As a proof of principle, we demonstrate that a micro-thermal sensor based on the supported "3ω" technique can achieve this in vitro under idealized conditions in 0.5 to 2 mm thick tissues relevant to cryoablation of the pulmonary vein (PV). To begin with "3ω" sensors were microfabricated onto flat glass as an idealization of a focal probe surface. The sensor was then used to make new measurements of 'k' (W/m.K) of porcine PV, esophagus, and phrenic nerve, all needed for PV cryoabalation treatment planning. Further, by modifying the sensor use from traditional to dynamic mode new measurements related to tissue vs. fluid (i.e. water) contact, fluid flow conditions, tissue thickness, and phase change were made. In summary, the in vitro idealized system data presented is promising and warrants future work to integrate and test supported "3ω" sensors on in vivo deployed focal therapy probe surfaces (i.e. balloons or catheters).Focal energy based therapies have a long history of use in the treatment of cancer, cardiovascular and neural disease [1][2][3] . As the technique evolves, there are increasingly thin and complex tissue anatomies where focal therapies and freezing are being applied that require sub mm monitoring accuracy to avoid debilitating side effects. For instance, cryogenic approaches to treatment of atrial fibrillation are increasingly being used in the pulmonary vein (PV) which is only 1-2 mm thick 4-7 . This treatment in thin PV often suffers from over and under-freezing suggesting a need for better monitoring. Unfortunately, traditional clinical imaging has difficulty monitoring at or below the mm scale, which is of the order of thickness of the PV itself. For this reason it is not surprising that in a pilot computed tomography (CT) study, we were unable to visualize the PV or the freezing process within it 8 . So, despite the past successes of clinical image guidance for cryoablation in cm sized tissues [9][10][11]

Microfabricated sensor platform with through-glass vias for bidirectional 3-omega thermal characterization of solid and liquid samples

“…To overcome the shortcomings of the conventional 3-omega method we use an extension of the method referred to as the bidirectional 3-omega method [12], [13], [14], [15], [16]. It has been used, for example, for the investigation of liquids [15], [17], solids [18], gases [19], biological tissues [15], [16] and single cells [20]. For this method, the sensor is first fabricated on a substrate and covered by a thin passivation layer and the investigated sample is placed on top of this platform as shown schematically in Fig.…”

“…This leaves free space for any sample on the complete top side of the chip and therefore allows for an easier sample positioning and deposition as well as manipulation and optical observation of the sample during the measurements. Other platforms for bidirectional 3-omega measurements reported before usually have the contacts at the top side of the chip and a thick SiO2 or polystyrene passivation layer (200 nm -1000 nm, [15], [14], [16], [18]) on top of the sensor. Another novel feature of our platform is a very thin nanolaminate passivation layer (56 nm) obtained by atomic layer deposition (ALD) providing superior chemical resistance and reliability.…”

“…To overcome the shortcomings of the conventional 3-omega method we use an extension of the method referred to as the bidirectional 3-omega method [12], [13], [14], [15], [16]. It has been used, for example, for the investigation of liquids [15], [17], solids [18], gases [19], biological tissues [15], [16] and single cells [20].…”