A Soft, Stimulating Scaffold Supports Brain Cell Development Ex Vivo

A Soft, Stimulating Scaffold Supports Brain Cell Development Ex Vivo

The brain-computer interface (BCI) is a hot topic these days, and companies like Neuralink are trying to develop devices that connect the human brain to machines via tiny implanted electrodes. The potential benefits of BCI range from improving monitoring of brain activity in patients with neurological disease, to restoring vision in the blind, and enabling people to drive with their thoughts alone. However, the main obstacle to the development of this device was the electrode itself. They must conduct electricity, which is why almost all of them are metal. Metal is not the hardest material for the brain because it is hard, rigid, and does not mimic the physical environment in which brain cells normally grow.

John A. from Harvard University. A new type of electrically conductive hydrogel scaffold developed at Wyss Institute's Paulson School of Engineering and Applied Sciences (SEAS) and MIT now offers a solution to this problem. The grafts not only mimic the soft porous state of brain tissue, but also support the growth and differentiation of human neural progenitor cells (NPCs) into different types of brain cells for up to 12 weeks. The acquisition is reported in Advanced Healthcare Materials .

“This hydrogel-based conductive scaffold has great potential. “They can be used not only to study the structure of human neural networks in the laboratory, but also allow the creation of implantable biohybrid BCIs that integrate more easily with the patient’s brain tissues, increasing their performance and reducing the risk of injury,” the report says. first author. Christina Tringidis, PhD, is a former Wyss and SEAS PhD student who is currently a postdoctoral fellow at ETH Zurich.

one to many

Tringides and his team developed their first hydrogel-based electrode in 2021, driven by a desire to create soft electrodes that could "flow" to cover the natural crevices, nooks and crannies of the brain. While the team showed that their electrode matched brain tissue perfectly, they knew that other cells would be the most suitable material for living cells. They decided to try integrating living brain cells with electrodes, potentially allowing the implanted electrodes to transmit electrical impulses to the patient's brain through a more natural cell-to-cell connection.

To make the electrically conductive hydrogel safer for cells, they added a freeze-drying step to the manufacturing process. Ice crystals formed during freezing cause condensation of the hydrogel material in the space around the crystals. When ice crystals evaporate, they leave pores surrounded by a conductive hydrogel, creating a porous structure. This structure ensures that the cells have a large surface area for growth, and the electrically conductive material creates a continuous path through the hydrogel, transmitting impulses to all cells.

The researchers modified their hydrogel recipe to create a viscous (jelly-like) or elastic (rubber-like) structure that was either soft or hard. They then grow human neuronal progenitor cells (NPCs) on these scaffolds to find out which combination of physical properties best promotes the growth and development of neuronal cells.

Cells grown in viscoelastic and soft gel formed a network of mesh structures on the scaffold and differentiated into many other cells after five weeks. In contrast, cells cultured on elastic gels formed aggregates consisting mainly of undifferentiated NPCs. The team also varied the amount of transmitters in the hydrogel material to see how this affects the growth and development of neurons. The more conductive the scaffolds were, the more cells formed branched networks, and did not stick together (as happened in vivo ).

The researchers then analyzed the different types of cells that formed inside their hydrogel scaffold. They found that astrocytes, which support neurons both physically and metabolically, formed characteristic long ridges when grown in viscoelastic gels rather than elastic gels, and to a greater extent when viscous gels contained more electrically conductive material. The structure also contains oligodendrocytes, which form a myelin sheath that insulates the axons of neurons. Viscoelastic gels contain more total myelin and longer myelin segments than elastic gels, and myelin thickness increases when the gel contains more conductive material.

Piece D (electrical) resistance.

Finally, the team applied electrical stimulation to live human cells using an electrically conductive material embedded in a hydrogel scaffold to see how it affects cell growth. Cells were shocked for 15 consecutive minutes daily or every other day. After eight days, there were very few viable cells in the daily pulse scaffolds, while the daily pulse scaffolds were full of viable cells throughout the scaffold.

After this stimulation period, cells remain in frame for a total of 51 days. The few remaining cells in the diurnal stimulated scaffolds did not differentiate into other cell types, while the long-processed neurons and astrocytes were highly differentiated in the diurnal scaffolds. Changing the tested electrical impulses did not affect the amount of myelin present in the gel.

"Successful differentiation of human NPCs into multiple brain cells on our scaffold supports the fact that electrically conductive hydrogels provide a suitable environment for their growth in vitro ," said senior author Dave Mooney, Ph.D. Wyss Institute Lecturer. "Seeing myelination in the axons of neurons is exciting because replication in a living brain model is an ongoing challenge." Mooney Pinkus family professor Robert P.

Tringides continues to work on electrically conductive hydrogel scaffolds, planning further research into how different types of electrical stimulation can affect different cell types and developing more complete in vitro models. He hopes the technology will one day enable devices to help people with neurological and physiological problems regain function.

“This work represents a major step forward in creating an in vitro microenvironment with appropriate physical, chemical and electrical properties to support the growth and specialization of human brain cells. This model can be used to speed up the process of finding effective treatments for neurological diseases with more efficient electrodes. and opens up completely new approaches to creating brain-machine interfaces that are easily integrated with neural networks. We are excited to see where this innovative combination of materials science, biomechanics and tissue engineering will lead in the future,” said the Wyss Institute. Founding Director Don Ingber, Ph.D. Ingber is Professor of Vascular Biology at Harvard Medical School and Boston Children's Hospital, Judah Folkman Professor of Biology, and Hansjörg Wiese Professor of Bioengineering at SEAS.

Additional contributors include Marzalin Bullingray of SEAS, Andrew Khalil of the Weiss Institute and Whitehead Institute of the Massachusetts Institute of Technology, Tenjin Longzhongwu of the Whitehead Institute, and Rudolf Yenish of the Whitehead Institute and the Massachusetts Institute of Technology.

This work was supported by the National Science Foundation in the framework of the named award. 1541959 and DMR-1420570, and Graduate Scholarship Program Grants, NIH Award RO1DE013033 5R01DE013349, NSF-MRSEC DMR-2011754 for Wyss Foundation Biology-Inspired Engineering Program.

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Professor Gordon Wallace at the 2018 Research Breakfast Seminar