Porous scaffolds for in vitro modeling of traumatic brain injury

Aiming to create an in vitro model of the brain, a group at Tufts University led by David Kaplan has reported layered structures formed by piecing together sponge-like scaffolds seeded with neurons from rat brains and filled with collagen gel. While claims of its similarity to brain structure and function may be overblown, the work represents an important advance in the development of materials for 3D neuron culture that support long-term viability.

Much of the excitement regarding this paper in the media (and even from NIH) appears to rest on the title’s assertion of similarity to the brain, but that similarity is limited. The structures have six layers like the human cortex, but the layers in these constructs are far thicker than the thinnest in the human brain (1-2 mm vs. <100 µm), and these layers are all seeded with the same mix of cortical cells, while neurons in each layer of mammalian cortex have particular shapes, sizes, and densities. Mimicking such complexity in vitro will likely require a far more sophisticated approach, perhaps involving a combination of top-down engineering and development from stem or progenitor cells.

More deserving of attention (though less interesting to the general public) is the long-term viability enabled by the scaffold’s porous structure, created either by foaming gas through a solution of solubilized silk or by directional rapid freezing (to align pores), followed by drying or lyophilization, respectively. Previous attempts at neuron culture in 3D systems did not support neurons for more than a couple of weeks, as most gel materials collapse over time, lowering permeability to oxygen and nutrients. In contrast, the authors show their cultures maintain viability for five weeks. Such viability is surprising given the mechanical properties the Kaplan group has previously reported for silk-based scaffolds, which are far stiffer than those of the brain. Though this paper demonstrates similar compressive moduli for their spongy constructs following seeding of cells and addition of collagen to those of whole rat brains, they do not report smaller scale mechanical measures.

'Sponge' scaffold for neurons
Neurons cultured in silk fibroin (blue)/ collagen system. Neurons were immunostained for β3 tubulin (green) to mark axons and microtubule-associated protein-2 (red) to mark dendrites. Neurons establish connections across scaffold pores.

Similar mechanical properties are the rationale for examining the response of neurons cultured within this scaffold to mechanical trauma. Min Tang-Schomer et al. show that weight-drop impact transiently increases electrophysiological activity, which mimics findings in animal models. However, the extended impact-induced release of glutamate, the excitatory neurotransmitter, compared to that in animals (lasting over ten minutes vs. less than one) suggests that incorporation of astrocytes (brain cells that recycle glutamate) is necessary to make the model reflect the in vivo response.

This is the first system enabling study of neuronal responses to direct impact in vitro; previous cellular models of traumatic brain injury (TBI) involve other mechanical deformations, which may not cause the same effects. While a layered, brain-like structure is likely not essential to model TBI in vitro, the ability of the Kaplan group’s material to allow real-time recordings during impact is a distinct advantage over other materials for 3D culture such as hydrogels.

Tang-Schomer M et al., Bioengineered functional brain-like cortical tissue, Proc Nat Acad Sci USA 2014; published online Aug 11.