Rational design of contact guiding, neurotrophic matrices for peripheral nerve regeneration

Nerve guides filled with magnetically aligned hydrated gels of type I collagen have been shown to impart strong contact guidance cues to elongating neurites in vitro and to increase the number of regenerating axons in vivo relative to an isotropic collagen gel. We have formulated and analyzed a model to determine the conditions under which the target concentration of nerve growth factor (NGF) to support axonal growth can be sustained by entrapping either NGF-secreting cells or NGF-releasing polymer microspheres in the aligned gel. The equation describing NGF concentration with a distributed source term was solved after experimental determination of (1) the rate of NGF release from PLGA 85/15 microspheres, (2) the NGF diffusion coefficient in the gel and nerve guide membrane containing the gel, and (3) the maximum microsphere loading that does not compromise the magnetic alignment of collagen fibrils. We find that for a rat sciatic nerve, when using a 1 mm diameter nerve guide within a commercially available collagen membrane, the microsphere loading limit will prevent the construct's capacity to sustain the target NGF concentration of 1 ng/ml at two months when either wild type Schwann cells or PLGA 85/15 microspheres are used as the NGF source. This target concentration, however, will be maintained when transfected cells described in the literature to hypersecrete NGF are used, or when the microspheres are used if the permeability of the nerve guide membrane can be moderately decreased. For a human median nerve, when using a 5 mm diameter nerve guide within a commercially available membrane, the microspheres are capable of sustaining NGF concentrations above 1 ng/ml to at least 75 days without the need to decrease membrane permeability.