ABOVE: Cells growing on a more aligned hydrogel will reorient themselves in alignment with the hydrogel’s fibers. Adam Farsheed
It is relatively easy to grow cells in the lab but turning them into realistic models of human tissue is harder. This requires creating an environment that closely mirrors the conditions in the body’s extracellular matrix (ECM), the molecular scaffold that supports cells. Bioengineers have sought to design materials that mimic the features of ECM, including its stiffness, density, and stickiness, and one promising material has been hydrogel, a squishy polymer network mostly filled with water.
Even though cells grow happily in hydrogels and the material can be customized to the context it is modeling, some aspects of the ECM structure are tricky to recreate. For example, in some tissues, the fibers that make up ECM are aligned in parallel to encourage cells to line up and move in a specific direction.1 In a hydrogel, the fibers typically do not display any such coordination, which makes it harder to grow tissues that require this linear structure.
In a recent study, researchers at Rice University described a new biomaterial made of peptide nanofibers that self-assemble into an aligned hydrogel.2 The team used salt to control the degree of alignment of the hydrogel’s fibers and found that this influenced the alignment of the cells that grew on the material. These findings hint that the new hydrogel could be a promising scaffold to build more realistic tissue models in the laboratory.
“The motivation is that if we put cells on an aligned material, the cells will sense the alignment and they will align themselves,” said Adam Farsheed, a bioengineer who led the work. “It's kind of an instructive material that taps into the cells’ natural way [of sensing] how to align.”
For the building blocks of this material, Farsheed turned to a peptide called K2, which his colleagues in Jeffrey Hartgerink’s lab had designed 15 years earlier.3 The chemistry of the peptide allows it to self-assemble into nanofibers that form a hydrogel when it is mixed with a salt solution. Because the hydrogel is mainly composed of water and salt, which make up 97 percent of the material, it mimics the composition of the human body.
Farsheed used a pipette to squeeze K2 into the salt solution, making long “noodles” of hydrogel fibers, but he still needed a strategy to tune the alignment of the nanofibers. He realized he could accomplish this by varying the level of salt in the solution: more salt led to more aligned fibers.
This offers a simple method to create a better lab model of how cells grow on ECM, which has historically been difficult to do, according to Darrin Pochan, a materials scientist at the University of Delaware who was not involved in the study. “To do alignment studies, most people have to come up with some completely artificial substrate that's not really transferable to a tissue engineering experiment,” he said. “This is much more natural.”
Next, Farsheed and his colleagues tested how cells grew on this material. They made versions of the hydrogel with different levels of alignment, and then added pig heart cells, which are known to reorient themselves according to the alignment of their ECM. As Farsheed expected, cells were only partially lined up in gels with less aligned fibers, which most resembled ECM in the brain. In the gels with moderately organized fibers, which mimicked the ECM found in muscle, the cells became more aligned. However, he was surprised to see that in the most aligned hydrogels, the cells did not align with the fibers at all.
“It’s a really cool, nonintuitive result,” Pochan said. “This is one of those papers that sets a standard for the field: You need to understand how aligned your nanostructure is because that has a big effect on how the cells behave.”
By inspecting the gels with an electron microscope, Farsheed realized that the fibers in the most aligned hydrogels might restrict the cells’ mobility as they tried to realign themselves. “Our material was so aligned and so closely packed together that the cells couldn't physically pull on it,” he said.
Pochan is curious about how the hydrogel will adapt to tissue engineering, especially how robust it is to the addition of other molecules required for specific cell types to grow. Farsheed’s first test will be using the gel to build better scaffolds for models of peripheral nerves. With an unaligned scaffold, nerves grown in the lab end up oriented in all directions, which do not resemble the neuronal circuits found in the body. This, in turn, makes it harder for researchers to use the model for testing the real effects of drugs, for example. Farsheed hopes that an aligned scaffold will allow for more realistic models of nerve tissue.
To do this, cooking up one-dimensional peptide noodles is not enough; Farsheed is now experimenting with using 3D printing to turn this hydrogel into more complex structures that resemble Lincoln logs or Chex.
“If we can make these more complex structures, then we can start to pattern cells in more complicated ways that start to have a structure that looks and acts like tissues in the body,” Farsheed said.
References
1. Petrie RJ, et al. Random versus directionally persistent cell migration. Nat Rev Mol Cell Biol. 2009;10(8):538-549.
2. Farsheed AC, et al. Tunable macroscopic alignment of self-assembling peptide nanofibers. ACS Nano. 2024;18(19):12477-12488.
3. Aulisa L, et al. Self-assembly of multidomain peptides: Sequence variation allows control over cross-linking and viscoelasticity. Biomacromolecules. 2009;10(9):2694-2698.