Flexible electronics have wide applicability, ranging from development of bendable screens to personal health monitors. Scientists are particularly interested in using these materials for medical applications; they've developed three-dimensional, flexible electronics that are compatible with the human body, and these show promise for integration into various tissues.

However, there are still significant limitations that must be addressed before these bioelectronics can be effectively used in a clinical setting. For example, it is very difficult to deliver soft bioelectronics to diseased regions in a patient-friendly manner. Recently, an international team of scientists demonstrated that flexible mesh electronics can be compacted and delivered using syringe injection.

After being manufactured, the mesh electronics are loaded into a needle that is inserted into an internal cavity, and the mesh is injected while the needle is withdrawn, placing the electronics in the targeted region. The mesh consists of longitudinal polymer/metal/polymer elements that interconnect electronics embedded in the polymer. The scientists found that the transverse and longitudinal stiffness of the material could be optimized to enable the mesh to “roll up” when passing through the needle.

The scientists demonstrated they could inject a 2mm wide sample of the mesh through a glass needle with an inner diameter of only 95μm. During injection, the mesh structure continuously unfolds as it exits the needle. Injection of the mesh through a needle with a 600μm inner diameter produced similar results.

Once it's injected, does it actually work? The electrical performance was investigated after the mesh was inserted into phosphate buffer saline solutions through 100-600μm inner diameter needles. Over 94 percent of the devices survived. The change in impedance, which measures the opposition a circuit presents to a current while a voltage is being applied, is an important characteristic for certain applications and was found to have a low value of 7 percent post-injection, which is pretty good.

They also tested different mesh structures. The scientists found that devices that had an angle of 45° at each junction in the mesh could be smoothly injected even when the widths were substantially larger than the inner diameter of the needles. For example, a mesh electronic 1.5cm wide could be injected smoothly through a needle with an inner diameter 33 times smaller. The 45° angle appears to limit the impact of the stresses the mesh is subjected to during injection.

The scientists explored the use of their devices for delivery of electronics to internal regions of both man-made structures and live animals. Mesh electronics that incorporated strain sensors (made of piezoresistive silicon nanowires) were injected with polymer precursors into polydimethylsiloxane (PDMS) cavities. The researchers monitored the response of the sensors to deformation of the PDMS structures. The results from this study suggest that these sensors could be used to monitor and map internal strains within structural components in a way that is currently not possible. The scientists expect that this can be extended to measure other types of chemical changes such as pH.

The team also explored how the mesh would respond after injection to a gel that mimics actual tissues. They created a cavity-free one made of Matrigel, which is often used for tissue-engineering applications. These studies revealed that the mesh unfolds approximately 80 percent within the gel in the radial direction over a three-week period when maintained at body temperature. They found that the extent of unfolding of the mesh was highly dependent on gel concentration and the mesh's mechanical properties.

Finally, the scientists injected the electronics into the brain of live rodents. The lateral ventrical (a relatively open space) and hippocampus were selected because delivery of soft biomaterials to these areas typically requires more invasive surgeries. These studies revealed that the electronic mesh could be injected into the lateral ventrical with little chronic tissue response. After injection, the mesh expanded, integrating within the local extracellular matrix. Cells were able to bind to the mesh, forming tight junctions—neural cells were even able to migrate hundreds of micrometers through the mesh. These results indicate that the mesh has the potential for use to mobilize and monitor neural cells in lateral ventrical following brain injury.

The scientists then injected the mesh into the dense tissue of the hippocampus. In this case, the mesh fully extended in the longitudinal direction. Healthy neurons were found to have surrounded certain parts of the mesh electronics. In contrast, astrocytes, cells that are involved in repair following traumatic brain injury, had limited or no proliferation around the mesh electronics, indicating that implantation of the mesh causes minimal damage to the brain tissue. The team also verified that the injected mesh electronics were able to record brain activity in the hippocampus of anaesthetized mice.

The scientists think that two main factors contribute to the biocompatibility of their mesh electronics. Their ultra-small bending stiffness is similar to that of the tissues and their micrometer-scale features are similar to the size of the cells; both of these factors have been previously demonstrated to reduce damage. These mesh electronics have outperformed most previous soft bioelectronics, and show promise as a basis for development of new biomedical monitors.

Nature Nanotechnology, 2015. DOI: 10.1038/NNANO.2015.115 (About DOIs).