December 19, 2012

High-pressure sound beam with 100 times sharper focus could one day be a scalpel

A carbon-nanotube-coated lens that converts light to sound can focus high-pressure sound waves to finer points than ever before. Researchers say it could lead to an invisible knife for noninvasive surgery.

Today focused sound waves blast apart kidney stones and prostate tumors. The tools work primarily by focusing sound waves tightly enough to generate heat.

"A major drawback of current strongly focused ultrasound technology is a bulky focal spot, which is on the order of several millimeters," Baac said. "A few centimeters is typical. Therefore, it can be difficult to treat tissue objects in a high-precision manner, for targeting delicate vasculature, thin tissue layer and cellular texture. We can enhance the focal accuracy 100-fold."

The team was able to concentrate high-amplitude sound waves to a speck just 75 by 400 micrometers (a micrometer is one-thousandth of a millimeter). Their beam can blast and cut with pressure, rather than heat.

With a new technique that uses tightly-focussed sound waves for micro-surgery, University of Michigan engineering researchers drilled a 150-micrometer hole in a confetti-sized artificial kidney stone. Image credit: Hyoung Won Baac

Nature Scientific Reports - Carbon-Nanotube Optoacoustic Lens for Focused Ultrasound Generation and High-Precision Targeted Therapy

Guo's University of Michigan team took an optoacoustic approach that converts light from a pulsed laser to high-amplitude sound waves through a specially designed lens. The general technique has been around since Thomas Edison's time. It has advanced over the centuries, but for medical applications today, the process doesn't normally generate a sound signal strong enough to be useful.

The system is unique because it performs three functions:

1. it converts the light to sound
2. Focuses it to a tiny spot
3. It amplifies the sound waves.

To achieve the amplification, the researchers coated their lens with a layer of carbon nanotubes and a layer of a rubbery material called polydimethylsiloxane. The carbon nanotube layer absorbs the light and generates heat from it. Then the rubbery layer, which expands when exposed to heat, drastically boosts the signal by the rapid thermal expansion.

The resulting sound waves are 10,000 times higher frequency than humans can hear. They work in tissues by creating shockwaves and microbubbles that exert pressure toward the target, which Guo envisions could be tiny cancerous tumors, artery-clogging plaques or single cells to deliver drugs. The technique might also have applications in cosmetic surgery.

The cross-sectional views of the gold-coated CNT-PDMS composite layer are shown in (a) (scale bar = 10 μm) and (b) (scale bar = 1 μm), taken by the scanning electron microscopy.The type II lens shown in (d) was used for the SEM characterization of (a) and (b). The layer thickness is ~16 μm. The PDMS is completely infiltrated among the CNT network as shown in (b); (c) An experimental setup for the LGFU characterization. The 6-ns pulsed laser beam is expanded (×5) and then irradiated onto the transparent side of the CNT lens (detailed explanation in the method section). The LGFU was optically detected by scanning the single-mode fiber-optic hydrophone. The optical output was 3-dB coupled and transmitted to the photodetector with an electronic bandwidth of 75 MHz; (d) Two CNT lenses used in this work. The CNTs were grown on the concave side of the plano-concave fused silica lenses.

ABSTRACT - We demonstrate a new optical approach to generate high-frequency (over 15 MHz) and high-amplitude focused ultrasound, which can be used for non-invasive ultrasound therapy. A nano-composite film of carbon nanotubes (CNTs) and elastomeric polymer is formed on concave lenses, and used as an efficient optoacoustic source due to the high optical absorption of the CNTs and rapid heat transfer to the polymer upon excitation by pulsed laser irradiation. The CNT-coated lenses can generate unprecedented optoacoustic pressures of over 50 MPa in peak positive on a tight focal spot of 75 μm in lateral and 400 μm in axial widths. This pressure amplitude is remarkably high in this frequency regime, producing pronounced shock effects and non-thermal pulsed cavitation at the focal zone. We demonstrate that the optoacoustic lens can be used for micro-scale ultrasonic fragmentation of solid materials and a single-cell surgery in terms of removing the cells from substrates and neighboring cells.

SOURCES - University of Michigan, Nature Scientific Reports

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