Fig. 1.

Modes of non-genetic cell surface engineering techniques. (1) Incorporation of cross-linkers, such as NHS, Maleimide, or pyridyldithiol, allows cell surface modification with biomaterials through chemical covalent conjugation. Cell metabolism of unnatural sugar and enzymatic reactions can be exploited to attach functional groups on the cell surface. (2) Electrostatic interactions between the cell surface and the charged polymers such as PEI, PLL, PAA, and PSS can modify cells through layer-by-layer technique. Also, charged block-co-polymers, such as PLL-PEG, can modify the cell surface through electrostatic interaction. (3) Lipid-conjugated bioactive molecules or polymers with long alkyl chains can be embedded into the cell membrane through hydrophobic interaction. Abbreviations: NHS: N-hydroxyl-succinimidyl ester; ManNAz: N-α-azidoacetylmannosamine; PAA: Poly(acrylic acid); PEG: Poly(ethylene glycol); PEI: Poly(ethyleneimine); PLL: Poly-L-lysine; PSS: Poly(styrene) sulfate; PVA: Poly(vinyl alcohol); SiaNAz: N-α-azidoacetyl sialic acid