Table 2.

Classification of scaffolds, examples, clinical comments, and current modifications.

Functional sites	Natural			Synthesized polymer			References
	Examples	Clinical comments	Modifications	Examples	Clinical comments	Modifications
Hydrogels	Agarose	Support chondrogenesis, facilitate function of biomechanical stimuli	Not indicated yet	PEG	Biocompatible, suitable for chondrocytes and MSCs cultivation	Lactic acid, RGD residue, combinedly used with other natural materials	Kim et al. (2012) [42] Dewan et al. (2014) [32] Stevens and George (2005) [45] Wang et al. (2011) [47]
	Alginate	Low stability and degradation rate	RGD peptides (improve adhesion)
	Hyaluronic acid	Facilitate chondrogenic differentiation	MMP-sensitive peptide (controllable degradation)
	Collagen	Low biomechanical stability, easily contracted during expansion, great biocompatibility	Nanoscale detail addition via electrospinning
	Fibrin	Support chondrogenesis, compromised biomechanical property	Not indicated yet
Membrane	Periosteum	Immune response Hypertrophy	Not indicated yet	Aliphatic polyesters	Toxic degradation by-product	Matching degradation with local metabolic clearance
	Collagen membrane	No hypertrophy Efficiently seal lesion site	Coculture with chondrocytes during cell expansion

PEG = polyethylene glycol; RGD peptide = repeated sequence of arginine, glycine, and aspartame.