Fig. 1
(a) Schematic of a traditional microfabricated 3-omega sensor of planar form factor, in top- and side-view. This requires measurement via external-contact with soft materials/biological tissues, meaning only a fraction of the sensor’s heat flows usefully into the sample, whereas much of the heat is lost through conduction into the substrate. (b) Schematic of the needle-form sensor as fabricated and implemented in this work, which enables the plunge-immersion internal contact for soft materials/biological tissues, minimizing heat lost to the sensor itself. The design uses a standard four point probe electrical geometry with a single bend near the tip of the needle. This immersion geometry is also more sensitive to the tissue properties by allowing heat to flow into the sample from all sides of the 3-omega sensor. Red lines indicate the decaying temperature waves inside the sample in response to periodic heating by the needle, with a typical thermal penetration depth \(\delta _p\) annotated. (c) Microscope image of a needle-form 3-omega sensor as received before making electrical connections. (d) Simplified circuit diagram and experimental configuration for measurements in this work, comprising a custom 3\(\omega\) circuit, a lock-in amplifier, and programmable resistor. (e) Application space for needle-form 3-omega sensor. Panels (a-c) are adapted and expanded from Alliston & Dames12.
