These images show the mesostructured silicon particle. Left: transmission X-ray microscopy 3D data set of one region of a particle, suggesting a spongy structure. Right: transmission electron microscopy image showing an ordered nanowire array. Images: Tian Lab.
These images show the mesostructured silicon particle. Left: transmission X-ray microscopy 3D data set of one region of a particle, suggesting a spongy structure. Right: transmission electron microscopy image showing an ordered nanowire array. Images: Tian Lab.

Ideally, injectable or implantable medical devices should not only be small and electrically functional, they should be soft, just like the body tissues with which they will interact. Scientists at the University of Chicago set out to see if they could design a material with all three of these properties. The nanomaterial they came up with, reported in Nature Materials, forms the basis for an ingenious light-activated injectable device that could eventually be used to stimulate individual nerve cells and manipulate the behavior of muscles and organs.

"Most traditional materials for implants are very rigid and bulky, especially if you want to do electrical stimulation," said Bozhi Tian, an assistant professor in chemistry, whose lab collaborated with that of neuroscientist Francisco Bezanilla on the research.

The new nanomaterial, in contrast, is soft and tiny – comprising particles just a few micrometers in diameter that disperse easily in a saline solution, allowing them to be injected. The particles also degrade naturally inside the body after a few months, so no surgery would be needed to remove them.

Each particle is built of two types of silicon that together form a structure full of nano-scale pores, like a tiny sponge. And like a sponge, it is squishy – between 100 and 1000 times less rigid than the familiar crystalline silicon used in transistors and solar cells. "It is comparable to the rigidity of the collagen fibers in our bodies," said Yuanwen Jiang, Tian's graduate student. "So we're creating a material that matches the rigidity of real tissue."

The nanomaterial forms one half of an electrical device that creates itself spontaneously when one of the silicon particles is injected into a cell culture, or, eventually, the human body. The particle attaches to a cell, making an interface with the cell's plasma membrane, and these two elements together – cell membrane plus particle – form a unit that generates current when the silicon particle is irradiated with light.

"You don't need to inject the entire device; you just need to inject one component," said João Carvalho-de-Souza, a postdoc in Bezanilla's group. "This single particle connection with the cell membrane allows sufficient generation of current that could be used to stimulate the cell and change its activity. After you achieve your therapeutic goal, the material degrades naturally. And if you want to do therapy again, you do another injection."

The scientists built the particles using a process they call nano-casting. This involves fabricating a silicon dioxide mold composed of tiny channels – "nano-wires" – about 7nm in diameter connected by much smaller ‘micro-bridges’. Into this mold, the scientists inject silane gas, which fills the pores and channels and decomposes into silicon.

And this is where things get particularly cunning. The scientists exploit the fact that the smaller an object is, the more the atoms on its surface dominate its reactions with the external environment. The micro-bridges are tiny, so most of their atoms are on the surface. These atoms interact with the oxygen present in the silicon dioxide mold, creating micro-bridges made of oxidized silicon. In contrast, the much larger nano-wires have proportionately fewer surface atoms, are much less interactive and so remain mostly pure silicon.

"This is the beauty of nanoscience," Jiang said. "It allows you to engineer chemical compositions just by manipulating the size of things."

Finally, the mold is dissolved, leaving behind a web-like structure of silicon nano-wires connected by micro-bridges of oxidized silicon that can absorb water and help increase the structure's softness. Meanwhile, the pure silicon retains its ability to absorb light.

The scientists have added these particles to neurons in culture in the lab, shone a light on the particles, and seen current flow into the neurons, activating them. The next step is to see what happens in living animals. They are particularly interested in stimulating nerves in the peripheral nervous system that connect to organs. These nerves are relatively close to the surface of the body, allowing near-infrared wavelengths of light to reach them through the skin.

Tian imagines using the light-activated devices to engineer human tissue and create artificial organs to replace damaged ones. Currently, scientists can make engineered organs with the correct form, but not the ideal function.

To get a lab-built organ to function properly will require manipulating individual cells in the engineered tissue. The injectable device would allow scientists to do that, tweaking an individual cell using a tightly focused beam of light like a mechanic reaching into an engine and turning a single bolt. The possibility of doing this kind of synthetic biology without genetic engineering is enticing.

"No one wants their genetics to be altered," Tian said. "It can be risky. There's a need for a non-genetic system that can still manipulate cell behavior. This could be that kind of system."

This story is adapted from material from the University of Chicago, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.