An assessment of the factors affecting the commercialization of cell-based therapeutics: a systematic review protocol by Pettitt, D, Arshaad, Z, Davies, B, Smith, J, French, A, Cole, D, Bure, K, Dopson, S, DiGiusto, D, Karp, J, Reeve, B, Barker, R, Holländer, G, Brindley, D