Transparent graphene implant for deep brain imaging and optogenetics
(Nanowerk Spotlight) Notwithstanding the progress neuroscientists have made in understanding the microscale function of single neurons and the macroscale activity of the human brain – a comprehensive understanding of the brain still remains an elusive goal. Over the past several years, nanoscale analysis tools and in the design and synthesis of nanomaterials have generated optical, electrical, and chemical methods that can readily be adapted for use in neuroscience and brain activity mapping (read more: "Nanotechnology for neuroscience").
"Although advances in optical technologies such as multi-photon microscopy and optogenetics have revolutionized our ability to record and manipulate neuronal activity, integration of optical modalities with electrical recordings is challenging due to generation of light-induced artifacts," Duygu Kuzum, an Assistant Professor of Electrical and Computer Engineering at the University of California, San Diego, tells Nanowerk.
In new work, scientists from the labs of Kuzum and Anna Devor report a transparent graphene microelectrode neural implant that eliminates light-induced artifacts to enable crosstalk-free integration of 2-photon microscopy, optogenetic stimulation, and cortical recordings in the same in vivo experiment. The new class of transparent brain implant is based on monolayer graphene. It offers a practical pathway to investigate neuronal activity over multiple spatial scales extending from single neurons to large neuronal populations.
The photograph shows graphene array placed under glass imaging window in a mouse. The Top view figure shows 16 electrode transparent graphene implanted over somatosensory cortex. 3D view image shows 3D reconstruction of brain tissue directly beneath the graphene array from a calcium imaging experiment. Cellular imaging figure shows Z-stacks acquired at four different depths directly below one electrode pad (black square trace). OGB-1 loaded neuron bodies are clearly visible at 120 µm depth and beyond. (Image: Duygu Kuzum, University of California, San Diego)
Conventional metal-based microelectrodes cannot be used for simultaneous measurements of multiple optical and electrical parameters, which are essential for comprehensive investigation of brain function across spatio-temporal scales. Since they are opaque, they block the field of view of the microscopes and generate optical shadows impeding imaging.
More importantly, they cause light induced artifacts in electrical recordings, which can significantly interfere with neural signals. Transparent graphene electrode technology presented in this paper addresses these problems and allow seamless and crosstalk-free integration of optical and electrical sensing and manipulation technologies.
In their work, the scientists demonstrate that by careful design of key steps in the fabrication process for transparent graphene electrodes, the light-induced artifact problem can be mitigated and virtually artifact-free local field potential (LFP) recordings can be achieved within operating light intensities.
"Optical transparency of graphene enables seamless integration of imaging, optogenetic stimulation and electrical recording of brain activity in the same experiment with animal models," Kuzum explains. "Different from conventional implants based on metal electrodes, graphene-based electrodes do not generate any electrical artifacts upon interacting with light used for imaging or optogenetics. That enables crosstalk free integration of three modalities: imaging, stimulation and recording to investigate brain activity over multiple spatial scales extending from single neurons to large populations of neurons in the same experiment."
The team's new fabrication process avoids any crack formation in the transfer process, resulting in a 95-100% yield for the electrode arrays. This fabrication quality is important for expanding this technology to high-density large area transparent arrays to monitor brain-scale cortical activity in large animal models or humans.
"High optical transmittance of these graphene arrays supports simultaneous 2-photon imaging down to > 1 mm directly beneath the transparent microelectrodes," notes Kuzum. "For the first time, we show that transparent graphene electrodes can be employed for crosstalk-free integration of three different modalities, 2-photon imaging, optogenetics and electrical recordings of cortical potentials at the same time in the same in vivo experiment."
Transparent graphene electrode technology could be directly applied to investigate numerous different questions related to basic neuroscience research and neurological disorders.
This technology can also provide a viable complementary alternative to invasive micro-needle arrays (sharp penetrating electrodes) for multimodal measurements/manipulations within the penetration depth of multiphoton microscopy.
In cases where depth-resolved electrical recordings are not required, optically transparent graphene surface arrays allow seamless integration with depth-resolved optical imaging modalities, while circumventing the need to insert invasive probes into brain tissue.
"Our technology is also well-suited for neurovascular and neurometabolic studies, providing a 'gold standard' neuronal correlate for optical measurements of vascular, hemodynamic, and metabolic activity," Kuzum points out. "It will find application in multiple areas, advancing our understanding of how microscopic neural activity at the cellular scale translates into macroscopic activity of large neuron populations."
"Combining optical techniques with electrical recordings using graphene electrodes will allow to connect the large body of neuroscience knowledge obtained from animal models to human studies mainly relying on electrophysiological recordings of brain-scale activity," she adds.
Next steps for the team involve employing this technology to investigate coupling and information transfer between different brain regions.
Advancements in measurement technology play a critical role in neuroscience enabling scientific inquiry and powering discovery. This is also the goal of the ongoing BRAIN Initiative. Close collaborations and interactions between engineers and neuroscientists is very important to come up with innovative technologies like the transparent graphene electrodes to overcome fundamental limitations of neuroscience research.