Couplants in Acoustic Biosensing Systems

Abstract

Acoustic biosensors are widely used in physical, chemical, and biosensing applications. One of the major concerns in acoustic biosensing is the delicacy of the medium through which acoustic waves propagate and reach acoustic sensors. Even a small airgap diminishes acoustic signal strengths due to high acoustic impedance mismatch. Therefore, the presence of a coupling medium to create a pathway for an efficient propagation of acoustic waves is essential. Here, we have reviewed the chemical, physical, and acoustic characteristics of various coupling material (liquid, gel-based, semi-dry, and dry) and present a guide to determine a suitable application-specific coupling medium.

Keywords: acoustic sensors; couplant; coupling agent; dry; liquid/gel; semi-dry.

Figure 1

All-acoustic sensor versus induced-acoustic sensor with and without a couplant. (a) Acoustic wave transmission and reception in an all-acoustic sensor; (b) acoustic wave detection in an induced-acoustic sensor. Inefficient acoustic wave propagation in air is due to a high acoustic impedance mismatch between the tissue and the transducer. Acoustic coupling medium reduces the impedance mismatch between probe surface and target for improved acoustic wave transmission and reception with decreased loss of signal (thick acoustic waves-green).

Figure 2

Impact of couplant on acoustic signal propagation. (a) Experimental setup demonstrating a pair of ultrasound transducer (transmitter and receiver) coaxially placed. The distance between the transmitter and receiver was 1 mm; (b) normalized acoustic signal strength acquired by the receiver in (a) the presence of coupling medium (ultrasound gel-blue line) and no couplant (air gap-red line).

Figure 3

Attenuation of acoustic wave as a function of frequency (1–50 MHz) at different distances (5, 10, and 20 mm) in air (black) and water (red).

Figure 4

Aggregate performance of noncommercial gel alternatives compared to commercial gel. Error bars indicate adjusted 95% confidence interval (CI) from post hoc Tukey-Kramer t- tests; bars crossing dashed lines represent no statistically significant difference from commercial gel. The commercial gel mean score was 45 (ideal score). Reproduced from [102].

Figure 5

Comparison between the power generated from transducer with heavy matching layer on piezocomposite active layer, transducer without heavy matching layer on piezocomposite active layer, and transducer with conventional matching layer + piezocomposite method. Reproduced from [109].