Table 5.

Example Studies Involving Concentration/Surface Density Gradients of Peptides, Proteins, or Other Model Factors for Tissue Engineering

		Gradient type
Factorsa	2D/3D	Profile	Soluble/immobilized	Materials used to incorporate the factorb	Notes	Application	Cell type investigatedc	References
CAMs/peptides
RGD-containing peptides	2Dd	Linear	Immobilized (covalently bound)	Poly(acrylic acid), poly(methacrylic acid) (photoiniferter SAM), poly(ethylene glycol)	Acryloyl derivatization to induce photopolymerization	Controlled cell adhesion, alignment, and motility (temporally and spatially)	Fibroblasts, endothelial cells	42, 67, 154, 156
	3D	Nonlinear	Soluble	Collagen or fibrin gel	–	Studying chemotaxis	Fibroblasts	157
IKVAV	2D	Linear	Immobilized (covalently bound)	Polystyrene substrate	Benzophenone derivatization to induce photopolymerization	Probing axon guidance	DRG neurons	164
Laminin	2D	Linear	Immobilized (adsorbed or covalently bound)	Glass/poly-L-lysine–coated substrate, carboxy-terminated alkanethiol monolayer on gold	–	Probing neuronal development and axon guidance, cell migration	Hippocampal neurons, intestinal IEC-6 cells	146, 147, 170
	2D	Sigmoidal	Immobilized	Polyethylene terephthalate	Carbodiimide chemistry	Probing axon guidance	PC-12 cells	46
	3D	Quadratic	Immobilized (covalently bound)	Agarose	SANPAH (heterobifunctional crosslinker), photoreactive perfluoro arylazide group	Improved neurite outgrowth and nerve regeneration in 3D environment	DRG neurons, sciatic nerve regeneration (in vivo)	145, 153
Fibronectin	2D	(Function of alkyl chain length)/sigmoidal	Immobilized (adsorbed)	ω-Methacryloyloxyalkyl phosphorylcholine–grafted PE/poly(2-hydroxyethyl methacrylate)	–	Manipulating cell adhesion and spreading	Fibroblasts	73, 78
Growth factors
EGF	2D	Custom	Immobilized (covalently bound)	Azidophenyl-derivatized poly(allylamine)-coated polystyrene	Azidophenyl derivatization	Concentration-dependent effects of immobilized biosignals on cellular mobility and localization, artificial juxtracrine simulation	Chinese hamster ovary cells	158
bFGF	2Dc	Linear	Immobilized (covalently bound)	Poly(ethylene glycol)	Acryloyl derivatization to induce photopolymerization	Directed cellular migration and alignment	VSMCs	155
NGF	3D	Custom/linear	Soluble	Collagen gel/agarose gel	–	Axonal guidance, studying chemotaxis and morphogenesis	DRG neurons/PC-12 cells	148, 150
		Linear	Immobilized (entrapped)	Poly(2-hydroxyethyl methacrylate)	–	Axonal guidance	PC-12 cells	152
NGF and NT-3	3D	Linear	Soluble	Agarose	–	Axonal guidance	DRG neurons	151
	3D	Linear	Immobilized (entrapped)	Poly(2-hydroxyethyl methacrylate)	–	Axonal guidance	DRG neurons	149
Model proteins and others
Human serum	2D	Nonlinear/linear/linear	Soluble	Culture medium	–	Studying chemotaxis	Polymorphouclear leukocytes/neutrophils	159, 161, 162
cAMP	3D	Linear	Soluble	Agarose gel	–	Studying chemotaxis	Dictyostelium discoideum amoebae	163
ZAS	3D	Linear	Soluble	Agarose gel	–	Studying chemotaxis	Leukocytes	160
Casein-b	3D	Custom	Soluble	Collagen gel	–	Axonal guidance, studying chemotaxis and morphogenesis	–	166
HRP	3D	Linear	Immobilized	Silk fibroin	Carbodiimide chemistry	Chemotaxis, tissue engineering, biosensors	–	174
R-phycoerythrin	2D	Linear	Immobilized	Polystyrene substrate	Carbodiimide chemistry, benzophenone derivatization	Probing biological responses	–	66
Fluorescent dye, antigens and/or antibodies	2D	Custom	Soluble/surface adsorbed	PDMS	–	Chemotaxis, haptotaxis, etching, nucleation and growth, Marangoni effects, antibody-antigen binding	–	168, 175

^aRGD: arginine-glycine-aspartic acid; SANPAH: sulfosuccinimidyl-6-[4′-azido-2′-nitrophenylamino] hexanoate; EGF: epidermal growth factor; bFGF: basic fibroblast growth factor; NGF: nerve growth factor; NT-3: neurotrophin-3; ZAS: zymosan-activated serum; HRP: horseradish peroxidase; CAMs: cell adhesion molecules; cAMP: 3′-5′-cyclic adenosine monophosphate.

^bPDMS: polydimethylsiloxane; SAM: self-assembled monolayer; PE: polyethylene.

^cDRG: dorsal root ganglion; PC-12: pheochromocytoma; VSMCs: vascular smooth muscle cells.

^dSome of these studies (e.g., Delong et al.¹⁵⁶) employed the techniques that were capable of generating gradients of immobilized factors in 3D. However, cellular interactions were monitored on the surfaces of the gels (i.e., in 2D).