Researchers developed a system for creating nanoscale “scaffolds” that can be seeded with cells that grow into tissue.
“The current methods we have for monitoring or interacting with living systems are limited,” said Lieber. “We can use electrodes to measure activity in cells or tissue, but that damages them. With this technology, for the first time, we can work at the same scale as the unit of biological system without interrupting it. Ultimately, this is about merging tissue with electronics in a way that it becomes difficult to determine where the tissue ends and the electronics begin.”
ABSTRACT – The development of three-dimensional (3D) synthetic biomaterials as structural and bioactive scaffolds is central to fields ranging from cellular biophysics to regenerative medicine. As of yet, these scaffolds cannot electrically probe the physicochemical and biological microenvironments throughout their 3D and macroporous interior, although this capability could have a marked impact in both electronics and biomaterials. Here, we address this challenge using macroporous, flexible and free-standing nanowire nanoelectronic scaffolds (nanoES), and their hybrids with synthetic or natural biomaterials. 3D macroporous nanoES mimic the structure of natural tissue scaffolds, and they were formed by self-organization of coplanar reticular networks with built-in strain and by manipulation of 2D mesh matrices. NanoES exhibited robust electronic properties and have been used alone or combined with other biomaterials as biocompatible extracellular scaffolds for 3D culture of neurons, cardiomyocytes and smooth muscle cells. Furthermore, we show the integrated sensory capability of the nanoES by real-time monitoring of the local electrical activity within 3D nanoES/cardiomyocyte constructs, the response of 3D-nanoES-based neural and cardiac tissue models to drugs, and distinct pH changes inside and outside tubular vascular smooth muscle constructs.
The research addresses a concern that has long been associated with work on bioengineered tissue: how to create systems capable of sensing chemical or electrical changes in the tissue after it has been grown and implanted. The system might also represent a solution to researchers’ struggles in developing methods to directly stimulate engineered tissues and measure cellular reactions.
“In the body, the autonomic nervous system keeps track of pH, chemistry, oxygen, and other factors, and triggers responses as needed,” Kohane said. “We need to be able to mimic the kind of intrinsic feedback loops the body has evolved in order to maintain fine control at the cellular and tissue level.”
Using the autonomic nervous system as inspiration, Bozhi Tian, a former doctoral student under Lieber and a former postdoctoral fellow in the Kohane and Langer labs, joined with Lieber and colleagues to build meshlike networks of nanoscale silicon wires.
The process of building the networks, Lieber said, is similar to that used to etch microchips.
Beginning with a two-dimensional substrate, researchers laid out a mesh of organic polymer around nanoscale wires, which serve as the critical sensing elements. Nanoscale electrodes, which connect the nanowire elements, were then built within the mesh to enable nanowire transistors to measure the activity in cells without damaging them. Once completed, the substrate was dissolved, leaving researchers with a netlike sponge, or a mesh, that can be folded or rolled into a host of three-dimensional shapes.
Once complete, the networks were porous enough to allow the team to seed them with cells and encourage those cells to grow in 3-D cultures.
“Previous efforts to create bioengineered sensing networks have focused on two-dimensional layouts, where culture cells grow on top of electronic components, or on conformal layouts, where probes are placed on tissue surfaces,” said Tian. “It is desirable to have an accurate picture of cellular behavior within the 3-D structure of a tissue, and it is also important to have nanoscale probes to avoid disruption of either cellular or tissue architecture.”
Using heart and nerve cells, the team successfully engineered tissues containing embedded nanoscale networks without affecting the cells’ viability or activity. Using the embedded devices, the researchers were then able to detect electrical signals generated by cells deep within the tissue, and to measure changes in those signals in response to cardio- or neuro-stimulating drugs.
They were also able to construct bioengineered blood vessels, and used the embedded technology to measure pH changes — as would be seen in response to inflammation, ischemia, and other biochemical or cellular environments — both inside and outside the vessels.
Though a number of potential applications exist for the technology, the most near-term use, Lieber said, may come from the pharmaceutical industry, where researchers could use it to more precisely study how newly developed drugs act in three-dimensional tissues, rather than thin layers of cultured cells. The system might also one day be used to monitor changes inside the body and react accordingly, whether through electrical stimulation or the release of a drug.
The cyborg-like tissue supports cell growth while simultaneously monitoring the activities of those cells. It could improve in vitro drug screening by allowing researchers to track how cells in a three-dimensional environment respond to drugs in real time, the authors say. It may also be a first step toward prosthetics that communicate directly with the nervous system, and tissue implants that sense and respond to injury or disease.
The nanoelectronic scaffolds were made from a thin mesh of metal nanowires, either straight or kinked, dotted with tiny transistors that detect electrical activity. The researchers folded or rolled the mesh into a three-dimensional structure to simulate a piece of tissue or a blood vessel, respectively. The result is a scaffold that is both porous and flexible—not an easy feat for electronics. “These scaffolds are mechanically the softest electronic materials that have ever been made,” says Lieber.
Lieber says numerous pharmaceutical companies have already expressed interest in the scaffolds to monitor drug responses in different tissues. “That’s the nearest-term application,” he says—but not the ultimate goal. Someday, Lieber would like to develop tissue grafts that can report their function to doctors and provide immediate feedback to a tissue when necessary, such as releasing a drug into the skin or lungs. “We have the opportunity to merge electronics with cellular systems,” he says.
Wired scaffold: Alginate (white), a seaweed-derived material used in conventional cell scaffolds, is deposited around nanoscale metal wires (false-colored in brown) to form a three-dimensional electronic scaffold.
Charles Lieber and Daniel Kohane
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