Neural interfaces for recording information from the brain or transmitting information to the brain are dominated by electrical signaling. However, communication between neurons in the brain primarily occurs by chemical transmission across synapses. A new generation of microfluidic devices promises to offer both electrical and chemical signaling.
Ruben Rathnasingham and colleagues from Daryl Kipke's neural engineering laboratory at the University of Michigan recently reported on the characterization of silicon-based microprobes that incorporate fluid ports to allow local chemical delivery into the brain. After using bench-top experiments to demonstrate a linear relationship between the applied pressure and the resulting flow rate, the efficacy of the devices was evaluated in vivo. The Michigan researchers injected an excitatory neurotransmitter to the inferior colliculus (IC) of the guinea pig brain while recording firing from IC neurons using five discrete recording sites on the same probes used for microinjection.
The firing rate of IC neurons increased dramatically following injection while control injections of Ringer solution did not impact the firing rate. Further, the duration over which the firing rate was increased could be manipulated by changing the injected volume.
A team headed by Steve DeWeerth at Georgia Tech's Laboratory for Neural Engineering is also working on a microfluidic/electronic neural interfacing system. They have constructed a microfabricated neuronal interfacing system to interface with a three-dimensional, in vitro neural network. DeWeerth's team is working with neural cell cultures and slices up to 2 mm thick with the aim of understanding the behavior of functional networks of neurons, neural plasticity in the networks, and the effects of injury on network behavior.
The neuronal interfacing system consists of a planar substrate onto which are grown vertical towers, up to 1 mm high, that feature connecting crossbridges, microfluidic channels, and electrical contacts. The microfluidic ports will be able to deliver nutrients, trophic factors, and neurotransmitters to stimulate localized regions of tissue. The electrical contacts on the tops and sides of the towers will interface with custom integrated circuits that perform amplification, multiplexing, and other forms of neural processing. The team hopes to create a new form of hybrid tissue-engineered implants capable of simultaneous stimulating, recording, and signal processing.
Interfaces such as these that enable local delivery could improve the efficacy of pharmaceutical treatments by allowing much greater control of the spatial and temporal pattern of delivery. For example, substances could be delivered only to that brain area required for the desired clinical effect, and not to other areas where side effects might be produced.
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