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The development of an injectable biomaterial that supports cell survival and maintains or promotes nucleus pulposus (NP) phenotype could aid delivery of cells to degenerated NPs causing low back pain. Mesenchymal cells were loaded and grown in a synthetic peptide gel, PuraMatrix ® . Cells were observed within the gels over 0–28 days, and accumulation of glycosaminoglycans were detected by histological staining. The mechanical properties of the cell-loaded constructs, and the change of the mechanical properties were studied using stress relaxation of the gels under compression and confinement. The PuraMatrix ® gel was shown to relax fast on compression indicating that the fluid could easily flow out of the gel, and thus indicating the presence of large pores/voids. The presence of these pores/voids was further supported by high mobility of dextran molecules, determined using fluorescence recovery after photo bleaching. The stress required to deform the cell-loaded constructs to a specific strain increases at day 21, at which point the presence of glycosaminoglycans within the cell-loaded constructs was also observed. The results provide evidence of changes in mechanical properties of the PuraMatrix ® matrix upon excretion of the extracellular matrix by the cells.