Tailoring Network Topology in Mechanically Robust Hydrogels for 3D Printing and Injection

Tissue engineering and regenerative medicine are confronted with a persistent challenge: the urgent demand for robust, load-bearing, and biocompatible scaffolds that can effectively endure substantial deformation. Given that inadequate mechanical performance is typically rooted in structural deficiencies─specifically, the absence of energy dissipation mechanisms and network uniformity─a crucial step toward solving this problem is generating synthetic approaches that enable exquisite control over network architecture. This work systematically explores structure-property relationships in poly(ethylene glycol)-based hydrogels constructed utilizing thiol-yne chemistry. We systematically vary polymer concentration, constituent molar mass, and cross-linking protocols to understand the impact of architecture on hydrogel mechanical properties. The network architecture was resolved within the molecular model of Rubinstein-Panyukov to obtain the densities of chemical cross-links and entanglements. We employed both nucleophilic and radical pathways, uncovering notable differences in mechanical response, which highlight a remarkable degree of versatility achievable by tuning readily accessible parameters. Our approach yielded hydrogels with good cell viability and remarkably robust tensile and compression profiles. Finally, the hydrogels are shown to be amenable to advanced processing techniques by demonstrating injection- and extrusion-based 3D printing. Tuning the mechanism and network regularity during the cell-compatible formation of hydrogels is an emerging strategy to control the properties and processability of hydrogel biomaterials by making simple and rational design choices.

Self-adhesion of uncrosslinked poly(butadiene-co-acrylonitrile), i.e. nitrile rubber, an inhomogeneous and associative polymer

Nitrile rubber (i.e., NBR) is a crosslinked copolymer of butadiene and acrylonitrile that finds widespread use in the automotive and aerospace industry as it sustains large, reversible deformations while resisting swelling by petrochemical fuels. We recently demonstrated that this material has a drift in composition due to the difference in reactivity between acrylonitrile and butadiene monomers during emulsion copolymerisation. Thus, although NBR is often thought of as a random copolymer, it does experience thermodynamic driving forces for self-assembly and kinetic barriers for processing like those of block copolymers.1 Here, we illustrate how such drift in composition hinders interdiffusion and prevents self-adhesion. The key result is that contacting uncrosslinked NBR (i) in the melt, (ii) in the presence of tackifiers, or (iii) in the presence of organic solvents promotes interdiffusion and enables self-adhesion. However, the contact times required for self-adhering, tc ∼ O(100 h), are orders of magnitude above those needed for non-polar synthetic rubbers like styrene-butadiene rubber (i.e., SBR) of comparable molecular weights and glass transition temperatures, tc ∼ O(100 s), unveiling the dramatic effect of compositional inhomogeneities and physical associations on polymer interdiffusion and large-strain mechanical properties. For example, when welded with organic solvents, the self-adhesion energy of NBR continues to increase after the solvent has evaporated because of polymer nanostructuring.

Understanding the role of crosslink density and linear viscoelasticity on the shear failure of pressure-sensitive-adhesives

Pressure-sensitive-adhesives (PSAs) are ubiquitous in electronic, automobile, packaging, and biomedical applications due to their ability to stick to numerous surfaces without undergoing chemical reactions. Although these materials date back to the 1850s with the development of surgical tapes based on natural rubber, their resistance to shear loads remains challenging to predict from molecular design. This work investigates the role of crosslink density on the shear resistance of model PSAs based on poly(2-ethylhexyl acrylate-co-acrylic acid) physically crosslinked with aluminum acetylacetonate. The key result is that crosslinking PSAs leads to notable stress concentrations ahead of the peel front, as well as a transition from cohesive to adhesive failure. The shear stress distributions, as evaluated by means of a linearly viscoelastic shear lag model, suggest that this transition is related to the evolution of the ratio of the load transfer length to the bond length as dictated by the mechanical properties of the backing and adhesive layers, and the geometry of the tape.

Digital Light Processing 3D Printing of Soft Semicrystalline Acrylates with Localized Shape Memory and Stiffness Control

Multimaterial three-dimensional (3D) printing of objects with spatially tunable thermomechanical properties and shape-memory behavior provides an attractive approach toward programmable "smart" plastics with applications in soft robotics and electronics. To date, digital light processing 3D printing has emerged as one of the fastest manufacturing methods that maintains high precision and resolution. Despite the common utility of semicrystalline polymers in stimuli-responsive materials, few reports exist whereby such polymers have been produced via digital light processing (DLP) 3D printing. Herein, two commodity long-alkyl chain acrylates (C₁₈, stearyl and C₁₂, lauryl) and mixtures therefrom are systematically examined as neat resin components for DLP 3D printing of semicrystalline polymer networks. Tailoring the stearyl/lauryl acrylate ratio results in a wide breadth of thermomechanical properties, including tensile stiffness spanning three orders of magnitude and temperatures from below room temperature (2 °C) to above body temperature (50 °C). This breadth is attributed primarily to changes in the degree of crystallinity. Favorably, the relationship between resin composition and the degree of crystallinity is quadratic, making the thermomechanical properties reproducible and easily programmable. Furthermore, the shape-memory behavior of 3D-printed objects upon thermal cycling is characterized, showing good fatigue resistance and work output. Finally, multimaterial 3D-printed structures with vertical gradation in composition are demonstrated where concomitant localization of thermomechanical properties enables multistage shape-memory and strain-selective behavior. The present platform represents a promising route toward customizable actuators for biomedical applications.

Polymeric multimaterials by photochemical patterning of crystallinity

An organized combination of stiff and elastic domains within a single material can synergistically tailor bulk mechanical properties. However, synthetic methods to achieve such sophisticated architectures remain elusive. We report a rapid, facile, and environmentally benign method to pattern strong and stiff semicrystalline phases within soft and elastic matrices using stereo-controlled ring-opening metathesis polymerization of an industrial monomer, cis-cyclooctene. Dual polymerization catalysis dictates polyolefin backbone chemistry, which enables patterning of compositionally uniform materials with seamless stiff and elastic interfaces. Visible light-induced activation of a metathesis catalyst results in the formation of semicrystalline trans polyoctenamer rubber, outcompeting the formation of cis polyoctenamer rubber, which occurs at room temperature. This bottom-up approach provides a method for manufacturing polymeric materials with promising applications in soft optoelectronics and robotics.

Why is mechanical fatigue different from toughness in elastomers? The role of damage by polymer chain scission

Although elastomers often experience 10 to 100 million cycles before failure, there is now a limited understanding of their resistance to fatigue crack propagation. We tagged soft and tough double-network elastomers with mechanofluorescent probes and quantified damage by sacrificial bond scission after crack propagation under cyclic and monotonic loading. Damage along fracture surfaces and its spatial localization depend on the elastomer design, as well as on the applied load (i.e., cyclic or monotonic). The key result is that reversible elasticity and strain hardening at low and intermediate strains dictates fatigue resistance, whereas energy dissipation at high strains controls toughness. This information serves to engineer fatigue-resistant elastomers, understand fracture mechanisms, and reduce the environmental footprint of the polymer industry.

3D fluorescent mapping of invisible molecular damage after cavitation in hydrogen exposed elastomers

Elastomers saturated with gas at high pressure suffer from cavity nucleation, inflation, and deflation upon rapid or explosive decompression. Although this process often results in undetectable changes in appearance, it causes internal damage, hampers functionality (e.g., permeability), and shortens lifetime. Here, we tag a model poly(ethyl acrylate) elastomer with π-extended anthracene-maleimide adducts that fluoresce upon network chain scission, and map in 3D the internal damage present after a cycle of gas saturation and rapid decompression. Interestingly, we observe that each cavity observable during decompression results in a damaged region, the shape of which reveals a fracture locus of randomly oriented penny-shape cracks (i.e., with a flower-like morphology) that contain crack arrest lines. Thus, cavity growth likely proceeds discontinuously (i.e., non-steadily) through the stable and unstable fracture of numerous 2D crack planes. This non-destructive methodology to visualize in 3D molecular damage in polymer networks is novel and serves to understand how fracture occurs under complex 3D loads, predict mechanical aging of pristine looking elastomers, and holds potential to optimize cavitation-resistance in soft materials.

Role of Tethered Ion Placement on Polymerized Ionic Liquid Structure and Conductivity: Pendant versus Backbone Charge Placement

The role of ion placement was systematically investigated in imidazolium bis(trifluoromethane)sulfonimide (ImTFSI) polymerized ionic liquids (PILs) containing pendant charges and charges in the backbone (sometimes called ionenes). The backbone PILs were synthesized via a facile step growth route, and pendant PILs were synthesized via RAFT. Both PILs were designed to have nearly identical charge density, and the conductivity was found to be substantially enhanced in the backbone PIL systems even after accounting for differences in the glass transition temperature (T_g). Wide-angle X-ray scattering (WAXS) revealed an invariance in the location of the amorphous halo between the two systems, while the anion-anion correlation peak was shifted to lower scattering wavevector (q) in the backbone PILs. This indicates an increase in the correlation length of ions and is consistent with charge transport along a more correlated pathway following the polymer backbone. Due to the linear nature of the backbone PILs, crystallization was observed and correlated with changes in conductivity. Upon crystallization, the conductivity dropped, and eventually, two populations of mobile ions were observed and attributed to ions in the amorphous and near-crystallite regions. The present work demonstrates the important role of ion placement on local structure and conductivity as well as the ability of backbone PILs to be used as controllable optical or dielectric materials based on crystallization or processing history.

The nature of protein interactions governing globular protein-polymer block copolymer self-assembly

The effects of protein surface potential on the self-assembly of protein-polymer block copolymers are investigated in globular proteins with controlled shape through two approaches: comparison of self-assembly of mCherry-poly(N-isopropylacrylamide) (PNIPAM) bioconjugates with structurally homologous enhanced green fluorescent protein (EGFP)-PNIPAM bioconjugates, and mutants of mCherry with altered electrostatic patchiness. Despite large changes in amino acid sequence, the temperature-concentration phase diagrams of EGFP-PNIPAM and mCherry-PNIPAM conjugates have similar phase transition concentrations. Both materials form identical phases at two different coil fractions below the PNIPAM thermal transition temperature and in the bulk. However, at temperatures above the thermoresponsive transition, mCherry conjugates form hexagonal phases at high concentrations while EGFP conjugates form a disordered micellar phase. At lower concentration, mCherry shows a two-phase region while EGFP forms homogeneous disordered micellar structures, reflecting the effect of changes in micellar stability. Conjugates of four mCherry variants with changes to their electrostatic surface patchiness also showed minimal change in phase behavior, suggesting that surface patchiness has only a small effect on the self-assembly process. Measurements of protein/polymer miscibility, second virial coefficients, and zeta potential show that these coarse-grained interactions are similar between mCherry and EGFP, indicating that coarse-grained interactions largely capture the relevant physics for soluble, monomeric globular protein-polymer conjugate self-assembly.