Photoacoustic Technologies beyond Medical Imaging-PASA and LGFU

Abstract

The Photoacoustic (PA) effect has been extensively studied as a direct and efficient light-to-sound conversion process. The majority of previous applications of the PA effect are focused on photoacoustic imaging, where the combination of optics and acoustics ensures both optical level resolution and large penetration depth into bio-samples. This thesis aims to explore the possibilities to applying the photoacoustic effects on other fields including non-imaging PA signal spectrum analysis, therapeutic treatment and nozzle-free jetting for printing.

First, in the case of fast and quantitative analysis of bio-samples, a non-imaging approach is preferable to analyze the spectrum of the PA signals. This is referred to as photoacoustic spectrum analysis. We have found a solid relationship between the morphological characteristics of the objects generating PA signals, and the polynomial fit of the measured spectrum of these PA signals. By measuring the signal with an ultra-broad-bandwidth ultrasound detector, we are able to characterize the size and shape of bio-samples in single cell level, paving the way for applications such as flow cytometry, cell counting, disease detection including blood diseases and blood freshness detection.

Second, with the development of highly-efficient photoacoustic generation materials, PA transmitters that generate large amplitude, broadband and complex ultrasounds waveforms have been fabricated, extending the PA applications from pure imaging to therapeutic treatment and other areas. Our PA transmitters are fabricated with simple and inexpensive ways compared with piezoelectric ceramic film fabrication used in conventional transducers or arrays. In this thesis, a self-focusing PA lens, made from candle soot (CS)/polydimethylsiloxane (PDMS) composite has been developed and applied in laser-generated-focused-ultrasound (LGFU). A Pulsed laser illuminates the PA lens to generate a tightly focused PA wave within an ellipsoid of 90μm (minor axis) * 200μm (major axis). Compared with previously developed PA generation layers made from carbon nanotubes (CNT) and metals, candle soot can be deposited with much lower cost and simplicity. Within the focal region, a negative pressure over 27MPa is achieved, which ensures steady bubble cavitation. We demonstrate one way to enhance this cavitation with the help of a superimposed low-frequency, low-amplitude ultrasound field, and get ~30% generation rate enhancement and ~60% cavitation bubble size enhancement. The energy released from bubbles collapse are used for ablation of bio-tissues or generation of thin and high-speed streams.

Taking advantages of these properties, LGFU equipped with CS/PDMS PA lens has been applied in two major areas: 1) Selective treatment, where the LGFU is used for direct ablation and dysfunction of the soft tissues less than 1mm. This treatment enjoys both selectiveness and accuracy and can treat each individual with his/her own need; 2) Nozzle-free jet printing, where the LGFU is used to generate ~10μm jets of materials with various viscosity and density. It has been demonstrated that the 2D materials like graphene and MoS2 can be printed with resolution of ~200μm. This can be applied as a potential nozzle-free high-resolution patterning modality in flexible electronics.