In a perfect world, injectable or implantable medical devices should not only be extremely small and electrically functional but they should also be very soft, similar to body tissues which they will be interacting with. Scientists from two University of Chicago labs have set out to see if they could design a material with all three of those properties rolled up into one new material.
The material they created was discussed in detail in the July 27th publication of Natural Materials journal. The material forms a basis of an ingenious light-activated injectable device that could eventually be used in order to stimulate nerve cells and manipulate the behavior of surrounding muscles and organs. Bozhi Tian, assistant professor in chemistry who collaborated with neuroscientist Francisco Bezanilla says most traditional materials for implants are very rigid and bulky, especially if you want to do electrical stimulation. However, the new material is soft and tiny. It is made up of particles that are mere micrometers in diameter (much less than the width of a single human hair) that disperse easily in a saline solution so they can be injected quickly and efficiently. The particles degrade naturally inside the body within a matter of months so there is no need for surgery to remove them.
Every particle is made up of two kinds of silicon that form a structure filled with nano-scale pores, similar to a very small sponge. The structure is about a hundred to a thousand times less rigid than the crystalline silicon that is used in transistors and solar cells. Yuanwen Jiang, Tian’s graduate student says it is very similar to the rigidity of the collagen fibers in our bodies, so what we are doing is creating a material that matches the rigidity of real tissues.
The material makes up half of an electrical device that creates itself spontaneously when one of the silicon particles is injected into a cell culture, or a human body. The particles attach to a cell making an interface with the cell’s plasma membrane. Those two elements together (cell membrane and particle) form a unit that generates current when light is shined on the silicon particle.
João L. Carvalho-de-Souza, Bezanilla’s postdoc said the single particle connection with the cell membrane allows sufficient generation of current that could be used in order 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 give the patient another injection.
Scientists built the particles with a process called nano-casting. They fabricate a silicon dioxide mold made up of tiny channels (or nano-wires) that are about seven nanometers in diameter connected by much smaller “micro-bridges”. They then inject silane gas into the mold which fills the pores and channels and decomposes into silicon. Scientists say the smaller the object is, the more the atoms on its surface dominate its reactions to what it is around. The micro-bridges are minute, so most of their atoms remain on the surface. These interact with oxygen that is present in the silicon dioxide mold which creates micro-bridges made of oxidized silicon built from materials at hand. At this point, much larger nano-wires have proportionately fewer surface atoms, and are far less interactive and remain made of mostly pure silicon.
The last step of the process is the mold dissolving. What remains left behind is a web-like structure of silicon nano-wires connected by micro-bridges of oxidized silicon that is able to absorb water and help to increase the softness of the structure. The pure silicon then retains the ability to be able to absorb light.
Next up, scientists plan 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, so near infra-red wavelength light can reach them through the skin. Tian hopes to use the light-activated devices in order to engineer human tissue and create artificial organs that will completely replace damaged ones. Scientists are currently able to make engineered organs with the right form but not with their ideal functions.
In order for an organ built within a lab to function properly, the team will need to be able to manipulate its individual cells. The injectable device would allow a scientist to do that by tweaking an individual cell using a tightly focused beam of light similar to a mechanic reaching into an engine and turning a single bolt. Tian says no one wants their genetics to be altered. 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.