(265a) Allyl-Sulfide Modified Hydrogels with Switchable Properties As Dynamic Cellular Niches

allyl-sulfide modified HYDROGELS with
switchable properties

as dynamic cellular niches

Tobin Brown, Joseph Grim, Ian Marozas, and Kristi
Anseth

Department of Chemical and Biological Engineering and
the BioFrontiers Institute

University of Colorado, Boulder, CO, USA  80303

Hydrogels
possess a number of properties that render them useful as synthetic
extracellular matrices for 3D cell culture and/or cell delivery systems for
regenerative medicine; these properties include cytocompatibility, ease of functionalization,
and physical properties similar to many soft tissues. Most synthetic hydrogel
systems are highly tunable, both in terms of their mechanical and biochemical
properties, and when combined with advanced manufacturing methods (e.g.,
photochemical reactions, 3D printing, microfluidics), experimenters can create
hydrogels with gradients, patterned ligands or other hierarchical structures
found in native tissues. While the ability to control and manipulate hydrogel
properties across many size scales has been powerful, the native extracellular
matrix (ECM) is a dynamic environment, particularly during development, wound
healing, and disease, and biological and mechanical signals can change
dramatically with time. The ability to dynamically tune hydrogel properties can
be lost in many synthetic ECM mimics, especially if they are crosslinked and
functionalized by irreversible covalent bonds.

In an effort to capture the
inconstant nature of native cellular niches, we have investigated hydrogels
containing allyl sulfide functionalities. In the presence of photogenerated
radicals, allyl sulfides undergo reversible addition and fragmentation. When
incorporated as a pendant group within a polymer network, this allows for
reversible photopatterning, which we have demonstrated with thiol-containing
peptides [1]. More recently, we have expanded this approach to achieve
reversible protein immobilization in hydrogels (Figure 1a). Thiolated proteins were
covalently tethered to hydrogels through a photoinitiated thiol-ene reaction
with the alkene of the allyl sulfide. To release the protein, a subsequent
thiol-ene reaction was performed using a small molecule thiol. Importantly,
this process is cytocompatible and can be repeated numerous times as the allyl
sulfide is regenerated with each thiol-ene reaction.  Next, to achieve dynamic
control over matrix mechanical properties, allyl sulfides were incorporated
directly into the network crosslinks. During photoinitiated radical generation,
crosslinks where exchanged with pendant thiols or soluble monofunctional thiols
to achieve rapid stress relaxation or photodegradation, respectively. Figure 1b
shows temporal regulation of the network viscoelasticity in the presence of
photogenerated radicals. During light exposure, pendant thiyl radicals exchange
with network allyl sulfides, resulting in deformation under an applied stress.
Similarly, soluble monofunctional thiyl radicals can exchange with network
allyl sulfides, resulting in rapid photodegradation [2] (Figure 1c). Together,
these materials constitute a class of hydrogels with dynamic chemical and
mechanical properties, and this presentation include recent progress in using
these materials as a dynamic culture platform for regulating the functional
properties of human mesenchymal stem cells.

Figure 1: Hydrogels with
dynamic biochemical and mechanical properties. a) A full-length protein is
first tethered to the hydrogel in a stripe pattern. The protein is then
released in defined regions in the presence of soluble thiol and photoinitiator
LAP. b) A creep experiment demonstrates light-mediated changes in
viscoelasticity. During UV exposure (shaded regions) the gel deforms in
response to an applied stress, and the gel reverts to purely elastic behavior
when the light is shuttered and when the photoinitiator is consumed. c)
Photodegradation in the presence of soluble thiol. Increased concentration of free
thiol leads to faster photodegradation.

References  ­–

(1)    N. R. Gandavaparu, M. A. Azagarsamy and K.S.
Anseth, Advanced Materials, 26, 2521-26 (2014).

(2)    T.E.
Brown, I.A. Marozas and K.S. Anseth, Advanced Materials, 10.1002/adma.201605001 (2017)