Building Neural Interfaces with Colloidal Nanocrystals

We report the first application of colloidal aluminum antimonide nanocrystals being the interfacial layer of the optoelectronic neural interface. Our experiments indicate the potential of semiconductor nanocrystals for generating safe and efficient neural stimulation.

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Neural interfaces are widely used tools for a wide range of applications including therapeutic neural stimulation, regeneration, and building the bidirectional communication between electronic devices and the human nervous system. Over the four generations1 of neural interfaces, implantable and integrated systems have progressed rapidly with advances in materials science, and several programs such as the BRAIN Initiative and Neuralink have further increased this interest both for the researchers and society. Many researchers around the world have created effective neural stimulation platforms with newly developed material systems, especially with nanomaterial-based designs. Moreover, unlike conventional electrical stimulation devices, photovoltaic designs can minimize the electrical artifacts on the recording devices as well as detrimental effects on the target cells. Although nanoparticles are widely used for imaging, labeling, and cancer treatment, optoelectronic application areas have not been explored in detail. In our group, we build nanoparticle-based/assisted systems owing to tunable optoelectronic properties of nanoparticles, particularly semiconductor nanocrystals (NCs). Depending on the energy levels, these nanomaterials can be directly used as single-material systems, blend with photoactive polymers, or utilized as electron/hole transport layers. Moreover, well-studied synthesis methods enable tunable absorption and photoluminescence spectrums of these colloidal nanoparticles. For photovoltaic systems, the former is crucial since the optical absorption profile determines the working window of these devices. Although strong responsivity in the optical region covering UV-to-NIR wavelengths can be preferred for generic devices, wavelength selective responsivity brings new opportunities. 

Fig. 1 Biointerface design and operation. a The structure of the photovoltaic biointerface using ITO, ZnO, P3HT, AlSb NCs as substrate, electron transport layer (ETL), donor component and hole transport layer (HTL), respectively. Inset: Cross-sectional scanning electron microscopy (SEM) image of the biointerface (Scale bar: 200 nm). b Energy levels of bulk ZnO, P3HT, and AlSb.

In our recent study2 led by Prof. Sedat Nizamoğlu of Koç University, Turkey, published in Communications Materials adopted this idea for enhanced optoelectronic performance in the blue region of the visible spectrum. For this purpose, we utilized colloidal aluminum antimonide (AlSb) NCs3 which has dominant absorption from UV to 500 nm. When used as a hole transfer layer, the enhancement due to AlSb NCs is effective under blue light illumination, whereas it is negligible for other wavelengths. This unique feature enables our design to selectively operate under different illumination windows. However, the choice of convenient architecture was an initial problem due to unknown energy levels of the recently synthesized colloidal AlSb NCs, which have not been used in any photovoltaic device in this form. Moreover, aqueous stability was another unknown to be discovered. AlSb NCs are very stable in the aqueous medium, proven by the accelerated stress tests, without any additional stabilizers or surface coatings. Advantageously, cell viability tests and immunofluorescence images indicate high biocompatibility for primary hippocampal neurons. However, one major challenge for using blue light illumination for neural stimulation is the potentially dangerous photothermal and photochemical impact on the target cells. This illumination intensity threshold requires an optimized device that generates sufficient depolarization on the cell membrane under illumination within the ocular safety limits. This was particularly important for our future research in retinal implants. Moreover, since we target and fabricate a device producing capacitive currents – a safe stimulation mechanism of cells, this enables the use of pulsed rather than continuous illumination. One major benefit of pulsed illumination is the reduced thermal and chemical effects such that higher intensities can be used with an increased ocular safety threshold. The optimized biointerface in this study can induce successful neural firing of primary hippocampal neurons up to 20 Hz stimulation frequency, which may further be enhanced to achieve 50 Hz stimulation via new nanomaterial assisted systems. 

For now, semiconductor NC assisted systems show their potential in vitro and the results indicate the future use for in vivo experiments. Although the long-term biocompatibility of the designed biointerfaces needs to be carefully investigated, our results hold great potential for nanomaterial-based neural interfaces with new material systems to be discovered.  

For more information, please refer to the full article in Communications Materials:

https://www.nature.com/articles/s43246-021-00123-4

  1. Zhang, M., Tang, Z., Liu, X. & Van der Spiegel, J. Electronic neural interfaces. Nat. Electron. 1–10 (2020).
  2. Han, M. et al. Photovoltaic neurointerface based on aluminum antimonide nanocrystals. Commun. Mater. 2, 19 (2021).
  3. Bahmani Jalali, H. et al. Colloidal aluminum antimonide quantum dots. Chem. Mater. 31, 4743–4747 (2019).

 

Mertcan Han

MSc Student, IDEALAB, Koç University