A soft, stimulating scaffold supports brain cell development outside the body

Brain-computer interfaces (BCIs) are a hot topic these days, with companies like Neuralink racing to create devices
that connect the human brain to machines through tiny implanted electrodes.
Potential benefits of brain-computer interfaces include improved monitoring of brain activity in patients with neurological disorders, restoring vision to the blind, and allowing humans to control machines
with our minds alone.
But the main obstacle to the development of these devices is the electrodes themselves – they must conduct electricity, so almost all electrodes are made
of metal.
Metals aren't the brain-friendly materials because they're hard and hard and can't replicate the physical environment
in which brain cells normally grow.

Now, by Harvard Wyss Institute, Harvard University John A.
A new conductive hydrogel holder developed by the Paulson School of Engineering and Applied Sciences (SEAS) and MIT addresses this problem
.
The scaffold not only mimics the soft, porous brain tissue environment, but also supports the growth and differentiation of human neural progenitor cells (NPCs) into a variety of different brain cell types
over a period of up to 12 weeks.
The results were published in the journal Advanced Medical Materials
.

"This conductive hydrogel-based scaffold has a lot of potential
.
It can be used not only to study the formation of human neural networks in vitro, but also to create implantable biohybrid BCIs that bind more seamlessly to a patient's brain tissue, improving its performance and reducing the risk of injury," said first author Christina Tringides, PhD, a former graduate student at Wyss and SEAS and now a postdoctoral researcher
at ETH Zürich.

Tringides and her team created their first hydrogel-based electrode in 2021, and they hope to make softer electrodes that can "flow" into the brain's natural curves, nooks and crannies
.
While the team demonstrated that their electrodes are highly compatible with brain tissue, they know that the substances most compatible with living cells are other cells
.
They decided to try to integrate living brain cells into the electrode itself, which could allow the implanted electrode to transmit electrical impulses to the patient's brain
through more natural cell-cell contact.

To make their conductive hydrogel a more comfortable place for cells to live, they added a freeze-drying step
to the manufacturing process.
The ice crystals formed during the freeze-drying process force the hydrogel material to concentrate in the space around the
crystals.
When the ice crystals evaporate, they leave pores surrounded by conductive hydrogels, forming porous scaffolds
.
This structure ensures that the cells have enough surface area to grow, and the conductive components form a continuous channel in the hydrogel that delivers pulses
to all cells.

The researchers changed the formulation of the hydrogel to create scaffolds
that are either viscoelastic (like jelly), elastic (like rubber bands), soft or hard.
They then cultured human neural progenitor cells (NPCs) on these scaffolds to see which combination of physical properties best supported nerve cell growth and development
.

Cells grown on softer gels with viscoelasticity form a lattice-like structural network on scaffolds and differentiate into a variety of other cell types
after five weeks.
In contrast, cells cultured on elastomeric gels formed clumps
consisting mainly of undifferentiated NPCs.
The team also varied the amount of conductive material in the hydrogel material to see how it affected nerve growth and development
.
The more conductive the scaffold, the easier it is for cells to form a network of branches, just like in the body, rather than clumping
.

The researchers then analyzed different types of cells
that developed in the hydrogel scaffold.
They found that astrocytes, which physically and metabolically support neurons, formed their characteristic long projections when grown on viscoelastic gels, and significantly more in number when they contained more conductive material in viscoelastic
gels.
Oligodendrocytes are also present in scaffolds, which produce myelin sheaths, the axons
that isolate neurons.
The total amount of myelin on the viscoelastic gel is greater than that of the elastic gel, the myelin sheath segment is longer, and the higher the content of conductive substances in the gel, the thicker
the myelin thickness.

Finally, the team electrically stimulated living human cells through conductive material inside the hydrogel scaffold to see how this affected cell
growth.
These cells are electrically impulse for 15 minutes
every day or every other day.
After 8 days, there were very few viable cells on the scaffold pulsed daily, while the scaffold pulsed every other day was filled with live cells
.

After this stimulation period, cells are left in the scaffold for a total of 51 days
.
The few remaining cells in the scaffolds stimulated daily did not differentiate into other types of cells, while the scaffolds stimulated every other day had highly differentiated neurons and astrocytes with long protrusions
.
The electrical impulse changes tested did not appear to have an effect
on the amount of myelin sheath in the gel.

"In our scaffold, human NPCs successfully differentiated into multiple types of brain cells, confirming that conductive hydrogels provide them with a suitable environment for in vitro growth," said
Dave Mooney, Ph.
D.
, a core faculty member and senior author at the Wyss Institute.
"It's especially exciting to see myelination on neuronal axons, as this has been a challenge
to replicate in vivo models of the brain.
"

Tringides is continuing to study conductive hydrogel scaffolds and plans to further investigate how different types of electrical stimulation affect different types of cells and develop more comprehensive in vitro models
.
She hopes the technology will one day create devices
that help human patients with neurological and physiological problems restore function.

"This work represents a major advance in creating an in vitro microenvironment with the right physical, chemical, and electrical properties to support the growth and specialization
of human brain cells.
" In addition to opening up an entirely new approach to creating more efficient electrodes and brain-computer interfaces that integrate seamlessly with neuronal tissue, the model could potentially be used to accelerate the search for effective treatments
for neurological diseases.
We are excited to see where this innovative convergence of materials science, biomechanics and tissue engineering heads in the future," said
Dr.
Don Ingber, founding director of the Wyss Institute.

Journal Reference:

1. Christina M.
   Tringides, Marjolaine Boulingre, Andrew Khalil, Tenzin Lungjangwa, Rudolf Jaenisch, David J.
   Mooney.
   Tunable Conductive Hydrogel Scaffolds for Neural Cell Differentiation.
   Advanced Healthcare Materials, 2022; 2202221 DOI: 10.
   1002/adhm.
   202202221