Bioorthogonal Click Chemistry: An Indispensable Tool to Create Multifaceted Cell Culture Scaffolds

Abstract

Over the past decade, bioorthogonal click chemistry has led the field of biomaterial science into a new era of diversity and complexity by its extremely selective, versatile, and biocompatible nature. In this viewpoint, we seek to emphasize recent endeavors of exploiting this versatile chemistry toward the development of poly(ethylene glycol) hydrogels as cell culture scaffolds. In these cell-laden materials, the orthogonality of these reactions has played an effective role in allowing the creation of diverse biochemical patterns in complex biological environments that provide new found opportunities for researchers to delineate and control cellular phenotypes more precisely than ever.

Figure 1

Examples of various click reactions that are commonly used in bioconjugation or hydrogel cross-linking: (a) copper-catalyzed Huisgen cycloaddition, (b) strain-promoted azide–alkyne cycloaddition (SPAAC), (c) base-catalyzed thiol-vinyl sulfone, (d) base-catalyzed thiol-maleimide Michael addition, (e) photoinitiated thiol–ene photocoupling.

Figure 2

(left) Structures of multiarm and linear PEG precursors and (right) schematic of an idealistic step-growth hydrogel. The molecular weight of the precursors, their functionality, and their stoichiometric ratio can all influence the final network cross-linking density and ultimate material properties, including equilibrium water content, elasticity, and diffusion coefficients.

Figure 3

Schematic of a Michael addition driven step-growth hydrogel formed using thiol-reactive 4-arm PEG tetravinyl sulfone, cysteine-flanked MMP degradable peptides (↓ shows cleavage site), and simultaneous tethering of cysteine containing RGDS peptides.

Figure 4

Thiol–ene hydrogel chemistry: (a) structure of 4-arm PEG tetranorbornene; (b) dicysteine-terminated chymotrypsin cleavable peptide; (c) schematic of spatial photopatterning throughout hydrogel networks created via thiol–ene by off-stoichiometrically reacting hydrogel precursors; (d) predictable relationship between photopatterning concentration and dosage of exposed light (● = constant intensity, varied exposure; ■ = constant exposure, varied intensity). The graph qualitatively shows that the extent of photopatterning can be varied by the alteration of light dosage, which in turn can be varied by exposure time/intensity. The graph also depicts the effect of photoinitiator concentration (C_low–C_high) on photopatterning.

Figure 5

Sequential click approach for dynamically tuning extracellular microenvironments: (a) step-growth network formation via SPAAC that enables thiol–ene photopatterning without altering the original network structure by employing 4-arm PEG tetra azide and dicyclooctyne MMP cleavable (↓ shows cleavage site) peptide; (b) spatial thiol–ene photopatterning of a first biochemical unit; (c) patterning of a second biochemical unit at a different time point and location.