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1. Highly entangled hydrogels with degradable crosslinks

   https://doi.org/10.1016/j.eml.2022.101953  

   Shi, Meixuanzi; Kim, Junsoo; Nian, Guodong; Suo, Zhigang (March 2023, Extreme Mechanics Letters)

   This paper studies a polymer network in which crosslinks are degradable but polymer chains are not. We show that entanglements markedly enhance the mechanical properties of the polymer network before degradation and slow down degradation. We synthesize polyacrylamide hydrogels with disulfide crosslinks. In a precursor of a low water-to-monomer molar ratio and low crosslinker-to-monomer molar ratio, the monomers are crowded and the resulting polymer chains are long, so that the entanglements greatly outnumber crosslinks. The as-synthesized hydrogels are submerged in pure water to swell to equilibrium. We show that entanglements enhance the swell resistance of the hydrogel, as well as stiffen and toughen the hydrogel. We further show that entanglements slow down degradation when the hydrogel is submerged in an aqueous solution of cysteine. This work demonstrates that entanglements substantially expand the properties space of degradable polymers. 

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2. Orthogonal Crosslinking: A Strategy to Generate Novel Protein Topology and Function

   https://doi.org/10.1002/chem.202202828  

   Wang, Zheng; Rabb, Johnathan_D; Lin, Qing (November 2022, Chemistry – A European Journal)

   Abstract Compared to the disulfide bond, other naturally occurring intramolecular crosslinks have received little attention, presumably due to their rarity in the vast protein space. Here we presented examples of natural non‐disulfide crosslinks, which we refer to as orthogonal crosslinks, emphasizing their effect on protein topology and function. We summarize recent efforts on expanding orthogonal crosslinks by using either the enzymes that catalyze protein circularization or the genetic code expansion strategy to add electrophilic amino acids site‐specifically in proteins. The advantages and disadvantages of each method are discussed, along with their applications to generate novel protein topology and function. In particular, we highlight our recent work on spontaneous orthogonal crosslinking, in which a carbamate‐based crosslink was generated in situ, and its applications in designing orthogonally crosslinked domain antibodies with their topology‐mimicking bacterial adhesins. 

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3. A design framework for actively crosslinked filament networks

   https://doi.org/10.1088/1367-2630/abd2e4  

   Fürthauer, Sebastian; Needleman, Daniel J.; Shelley, Michael J. (January 2021, New Journal of Physics)

   Abstract Living matter moves, deforms, and organizes itself. In cells this is made possible by networks of polymer filaments and crosslinking molecules that connect filaments to each other and that act as motors to do mechanical work on the network. For the case of highly cross-linked filament networks, we discuss how the material properties of assemblies emerge from the forces exerted by microscopic agents. First, we introduce a phenomenological model that characterizes the forces that crosslink populations exert between filaments. Second, we derive a theory that predicts the material properties of highly crosslinked filament networks, given the crosslinks present. Third, we discuss which properties of crosslinks set the material properties and behavior of highly crosslinked cytoskeletal networks. The work presented here, will enable the better understanding of cytoskeletal mechanics and its molecular underpinnings. This theory is also a first step toward a theory of how molecular perturbations impact cytoskeletal organization, and provides a framework for designing cytoskeletal networks with desirable properties in the lab. 

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4. Sliding on Slide-Ring Gels

   https://doi.org/10.1007/s11249-024-01920-x  

   Rhode, Andrew R; Montes_de_Oca, Iván; Chabinyc, Michael L; Bates, Christopher M; Pitenis, Angela A (December 2024, Tribology Letters)

   Abstract Recent investigations have pointed to physical entanglements that greatly outnumber chemical crosslinks as key sources of energy dissipation and low friction in hydrogel networks. Slide-ring gels are an emerging class of hydrogels described by their mobile crosslinks, which are formed by rings topologically constrained to slide along linear polymer chains within the network. These materials have enjoyed decades of study by polymer chemists but have been underexplored by the tribology community. In this work, we synthesized a pseudo-rotaxane crosslinker from poly(ethylene glycol) diacrylate (PEG-diacrylate) andα-cyclodextrin-acrylate followed by hydrogel networks by connecting the sliding crosslinks with polyacrylamide chains. The mechanical and tribological properties of slide-ring hydrogels were investigated using a custom-built microtribometer. Slide-ring hydrogels exhibit unique behavior compared to conventional covalently crosslinked polyacrylamide hydrogels and offer a vast design space for future investigations. Graphical Abstract 

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5. Selective electrochemical degradation of bottlebrush elastomers

   https://doi.org/10.1002/pol.20240393  

   Albanese, Kaitlin R; Morris, Parker T; Roehrich, Brian; Read_de_Alaniz, Javier; Hawker, Craig J; Bates, Christopher M (July 2024, Journal of Polymer Science)

   <sc>A</sc>bstract We introduce a simple synthetic strategy to selectively degrade bottlebrush networks derived from well‐defined poly(4‐methylcaprolactone) (P4MCL) bottlebrush polymers. Functionalization of the hydroxyl groups present at the terminal ends of P4MCL side chains withα‐lipoic acid resulted in bottlebrush polymers having a range of molecular weights (M_n = 45–2200 kg mol⁻¹) and a tunable number of reactive dithiolane chain ends. These functionalized chain ends act as efficient crosslinkers due to radical ring‐opening of the dithiolane rings under UV light. The resulting redox‐active disulfide crosslinks enable mild electrochemical or chemical degradation of the SS crosslinks to regenerate the starting bottlebrush polymer. P4MCL side chains and the disulfides can be degraded simultaneously using harsher reducing conditions. This combination of bottlebrush architecture with facile disulfide crosslinking presents a versatile platform for preparing highly tunable elastomers that undergo controlled degradation under mild conditions. 

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