Copyright © 2014 David I. Shrieber - Rutgers, The State University of New Jersey - All Rights Reserved.
Tissue Engineering & Biomaterials
Gradients to guide axon growth
During development, a myriad of chemical and mechanical signals guide axon growth to the specific
motor or sensory target. In our lab, we aim to recapitulate these signals in vitro to identify combinations of bioactive and mechanical
signals that increase neurite outgrowth, and/or bias directionality of outgrowth. Control of neurite outgrowth and direction of growth
using identified biochemical and/or mechanical patterns may translate to an increased potential for a successful biomaterial containing
these signals. This biomaterial can be utilized for conditions such as peripheral nerve and spinal cord injuries, where it is imperative
that axons accurately reconnect to their previous targets.
We developed an in vitro microfluidic system with two inlets which we can inject combinations of modified or unmodified type-I collagen hydrogels. These hydrogels may contain gradients of covalently grafted bioactive peptides, including laminin fragments such as IKVAV and YIGSR, or of mechanical stiffness using a natural crosslinker such as genipin. In vitro results using dorsal root ganglia explants from E8 chick embryos have demonstrated that neurite outgrowth is biased down a gradient of stiffness compared to its converse. Neurites also grow significantly longer up steep gradients of YIGSR, shallow gradients of IKVAV, and in combination compared to unmodified collagen controls.
Using these cost-effective, simple in vitro microfluidic techniques, we have identified combinations of bioactive and mechanical elements that are advantageous to test in vivo to control neurite outgrowth.
Type-I collagen hydrogels are often used for soft tissue engineering applications as they self-assemble into a fibrillar gel in vivo
and are biocompatible, biodegradable, and naturally support cell adhesion and growth. While these hydrogels are easily amenable to
chemical modifications, they have been criticized for their weak mechanical properties and lack of spatial control of bioactive/mechanical
patterns. Our lab covalently grafted methacrylic acid to the free amines on the lysine residues of type-I collagen to synthesize collagen
methacrylamide (CMA). CMA can be photocrosslinked in the presence of long-wave UV light and a photoinitiator for specific spatiotemporal
control of chemical and mechanical patterns.
Photo-active Type-I Collagen
Additionally, CMA can thermo-reversibly self-assemble, forming a liquid suspension at cool temperatures, and re-assembling into a hydrogel at physiological temperature. Currently, we are using CMA for tissue engineering strategies, such as the design of specific 3-D cell microenvironments and methods for free form fabrication for controlled tissue architectures. For example, on the right are pictured two photo-masks next to opaque CMA scaffolds generated by exposing CMA gels to UV light through the masks, and then "cold-melting" the unreacted CMA away.
Biomaterials that Induce Preferential Motor Reinnervation
Peripheral nerve injury (PNI) is a common outcome following physical trauma
such as motor vehicle accidents, resulting in debilitating loss of motor and sensory function. Direct reconnection of nerve stumps
through suturing is preferred in small gaps; however the current gold standard for large gap nerve repair, an autologous nerve graft,
is limited by the availability of nerve tissue and donor site morbidity. Nerve guidance conduits present a promising tissue engineered
solution, however, currently available commercial nerve guidance conduits are ineffective in regenerating nerve defects larger than
3cm, due in large part to their hollow lumens which serve only to confine axonal growth. Polysialic acid (PSA) and a factor first
found on natural killer cells (HNK-1) are two naturally occurring glycans which are naturally upregulated following nerve injury.
PSA enhances neurite outgrowth and Schwaan cell migration, whereas HNK-1 preferentially enhances growth of motor neurons. Research
in our lab and with our collaborators has identified small glycomimetic peptides which mimic the activity of PSA and HNK-1. Together
these glycomimetic peptides enhance the speed and the quality of regeneration by preferentially targeting the regeneration of motor
neurons, a phenomenon known as preferential motor reinnervation. Our lab has demonstrated increased efficacy of these compounds by
covalently grafting them to collagen hydrogels. Peptide grafted hydrogels enhanced neurite outgrowth in sensory and motor neurons
as well as increased proliferation and extension of Schwaan cells in vitro. Further, these peptide grafted hydrogels promoted functional
motor recovery in vivo. Our ongoing efforts are to optimize the presentation of these glycomimetic peptide cues along with structural
guidance cues to design a functional conduit filling which serve to accelerate regeneration across a nerve gap.
Collagen is an attractive biomaterial for a number of tissue engineering strategies because it is highly biocompatible, supports
cellular infiltration and growth, and can be degraded through natural enzymatic pathways. Further, collagen has the ability to direct
regeneration when alignment of collagen fibers present oriented contact guidance cues. The ability to control alignment and orientation
of cells seeded into tissue engineered constructs is a critical design factor for a number of different tissues, including nerve,
tendon or ligament, and smooth muscle. Current methods for inducing alignment of collagen fibers occurs following self-assembly through
mechanical or cellular traction forces or occurs during self-assembly in the presence of electrochemical or sheer flow gradients.
These require specialized equipment or apparatuses and are difficult to scale. Aligned collagen fiber scaffolds can also be fabricated
through electrospinning, however the harsh processing conditions characteristic of electrospinning can irreversibly alter the native
structure of collagen. Our lab has developed a novel method for fabricating highly aligned collagen sponge-like scaffolds through
freezing sublimation. These scaffolds exhibit ling range uniformity and feature a highly porous interior. Aligned collagen fiber scaffolds
fabricated through this simple, rapid technique have the potential to serve as building blocks for regenerative medicine strategies
across a variety of tissue types where tissue anisotropy is critical to functional outcome, such as engineering tendons and ligaments
or nerve guidance conduits.
Highly Aligned Collagen Sponge-like Scaffolds