Stanford Researchers Develop Noninvasive Method to Deliver Light Deep Inside the Body

Researchers at Stanford University have developed a noninvasive method to deliver light to specific locations deep within the body. The technique, described in a paper published on April 13 in the journal Nature Materials, uses injected nanoparticles that convert ultrasound waves into light. The work is described by the researchers as a proof of concept.

"We can noninvasively tune this emission in different brain regions to produce a variety of behavioral outcomes." — Guosong Hong, senior author.

How It Works: Ultrasound to Light

The system is based on ceramic particles that emit light in response to mechanical stress, a property known as sonoluminescence. The researchers processed these materials into nanoparticles and applied a biocompatible coating, allowing them to be suspended in a solution suitable for injection.

In experiments, the nanoparticle solution was injected into mice, where it circulated through the bloodstream via blood vessels. The nanoparticles remain inactive until activated by externally applied, focused ultrasound waves. The researchers demonstrated the ability to create light in multiple locations simultaneously and to move the point of light emission by scanning with the ultrasound's focal point.

Demonstration in Living Tissue

To test the system's ability to function in deep tissue, the researchers created a small ultrasound device for use with mice. They used it to generate light in different regions of a mouse's brain.

The emitted light stimulated specific neurons, resulting in observable behavioral changes; the mice turned either left or right depending on which brain region was activated.

Technical Scope and Future Applications

The ceramic materials used in the initial work produce blue light with a wavelength of 490 nanometers. This wavelength is applicable for exciting neurons and is used in some forms of photodynamic therapy for cancer.

The researchers state the method is general and could be adapted to produce other wavelengths using different nanomaterials. Current research directions include:

• Experimenting with materials that emit ultraviolet light, which has antimicrobial properties capable of killing bacteria and viruses.
• Working to pair the light-producing method with a light-activated gene-editing system. The goal is to use ultrasound to control gene-editing activity in localized areas of the body, which could potentially reduce off-target effects.

Safety and the Path Forward

The ceramic nanoparticles used in the study did not show adverse effects in mice during testing. However, the researchers noted a significant limitation: the materials do not break down quickly in the body and could potentially accumulate in organs like the liver.

The researchers emphasized that the current work is a proof of concept. A key next step is to replace the ceramic nanoparticles with biological materials that degrade safely in the body.

"If we can replace the material with one that is safer to be used in humans, that will start to pave the way for clinical applications." — Guosong Hong.

Senior author Guosong Hong also highlighted the method's potential scope, noting, "Ultrasound is very convenient to use, and it penetrates much deeper into the body than light. With these materials, we can produce light emission in the brain, in the gut, in the spinal cord, in the muscle—virtually anywhere—without needing a physical implant."