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Sound waves have been used in healing for centuries. Music is said to "soothe the savage beast," and music therapy has been used for anxiety and psychological disorders since time immemorial.

Sound waves have been used in healing for centuries

Ultrasound, low-energy pulses of sound vibrating at 1 to 5 million vibrations per second, is used to align skin cells into channels so that medications are easily absorbed. And more recently, according to an article published in the Proceedings of the National Academy of Sciences of the USA, "sound lenses" can produce precise frequencies precisely focused on a target that act like "bullets" of sounds.

A New Way to Focus Sound Waves

Bullets of sound are not made by making the sound louder and louder or by making their frequency higher and higher. Bullets of sound are made by arranging sound waves so that their crests are higher and their troughs are deeper.

The idea behind focusing sound waves is a little like tossing two stones into a pond at the same time. Each of the stones generates waves that proceed from it in all directions. When waves from the two stones meet, if the crest from the wavefront of the first stone coincides with the crest from the wavefront of the second stone, the result is a single, higher wave.

If the trough of the wavefront from the first stone meets the trough from the wavefront of the second stone, the result is a deeper trough. And if the trough of one wavefront meets the crest of the other, they cancel each other out.

The process of making "bullets" of sound requires multiple modifications to the waves of sound put out by the source. The acoustic lens is half of a sphere, lined with chains of spherical particles. When sound strikes the spherical particles in the lens, it proceeds in a curve rather than in a straight line.

Wave fronts come off the spheres within the lens in ways that highs get higher and lows get lower. The net effect of the entire acoustic lens is to produce concentrated bursts of sound with considerable power.

Sound Scalpels the Opposite of Therapeutic Ultrasound

Sound scalpels are the opposite of therapeutic ultrasound. The usefulness of therapeutic ultrasound depends on the limited power of the sound. Sound waves are generated very fast so that each wave has very little power. The changes in the skin induced by ultrasound machines that make help it absorb nutrients or mediations take place in milliseconds and only over a distance of microns (millionths of a meter).

A sound scalpel, on the other hand, has the power not only to realign skin cells but also to open a space between them. And it has the ability to move tissues beneath the skin without ever cutting the skin itself.

Surgical Scalpels that Work Like Newton's Cradle

Dr. Chiara Daraio, who is an assistant professor of aeronautics and applied physics at the California Institute of Technology (also known as Cal Tech), likens a sound scalpel to a popular toy called Newton's cradle.

This toy consists of a series of balls attached to strings hanging from a bar. If you pull back a ball on one end of the cradle, the ball at the other end of the cradle moves, but the balls in between seem to stay still. Actually, they are moving, but in a circular motion. Only the ball at the end of the row moves in a linear motion.

Dr. Daraio and her assistant Dr. Alessandro Spadoni explain that their surgical scalpel works in the same way. The two Cal Tech scientists made their acoustic lens by welding 21 parallel chains of stainless steel spheres into an array. Each of the 21 chains consisted of 21 stainless steel spheres a little less than half an inch wide.

The scientists "wound up" their acoustic lens with an easily available material that happened to have been fishing line. By compressing all 21 chains and releasing them at the same time, a series of sound waves was created. The sound waves traveled around the lens to meet in such way that they created a single, high-powered bullet in the center of the lens. The spheres themselves appear not to move (they actually move in a circular motion), but anything at the focus of the sound lens experiences a high dose of sound energy.

Sonic Scalpel Not Ready for the Operating Room

Drs. Daraio and Spadoni do not claim that their prototype is at all close to commercial application. Their ingenious arrangement of stainless steel and fishing line was intended as a demonstration of the concept, nothing more. The eventual surgical acoustic lens undoubtedly will not be controlled with fishing line. The pre-compression of the chains of spheres would be regulated with electronics, for example.

The possibilities, however, are substantial. Dr. Daraio believes their device can increase the safety and clarity of ultrasound imaging. Each pulse produced by the lens—which is 10 to 100 times more powerful than conventional ultrasound—would produce a clearer image deeper inside tissue. The "sound bullets" can travel farther and deeper in the body that low-amplitude pulses.

Possibly, as a surgical scalpel, the device could zero in on cancerous tissue so that it could be removed without a wide incision. Creating a cleaner edge around a cancerous tumor increases the possibilities for removing the cancer without spreading it.

The Cal Tech scientists also believe their device would be a useful way of looking inside physical structures that could have structural defects, oil pipelines, airplane wings, and hulls of ship, for example. Non-linear sound will generate clearer pictures because it is nearly free of noise, it can reach deeper into denser materials, and, like Newton's cradle, it only "moves" the last tissue or material the sound bullet strikes.

The incision-free surgical scalpel is still a tool of the future. If Chiara Daraio and Alessandro Spadoni have their way, however, in five to ten years bloody, painful incisions to remove cancerous tumors may be a thing of the past.

  • Spadoni A, Daraio C. Generation and control of sound bullets with a nonlinear acoustic lens. Proc Natl Acad Sci U S A. 10 Apr 20107(16):7230-4. Epub 2010 Apr 5.