Physics of engineered protein hydrogels

PhoCoil: An Injectable and Photodegradable Single-component Recombinant Protein Hydrogel for Localized Therapeutic Cell Delivery

Hydrogel biomaterials offer great promise for 3D cell culture and therapeutic delivery. Despite many successes, challenges persist in that gels formed from natural proteins are only marginally tunable while those derived from synthetic polymers lack intrinsic bioinstructivity. Towards the creation of biomaterials with both excellent biocompatibility and customizability, recombinant protein-based hydrogels have emerged as molecularly defined and user-programmable platforms that mimic the proteinaceous nature of the extracellular matrix. Here, we introduce PhoCoil, a dynamically tunable recombinant hydrogel formed from a single protein component with unique multi-stimuli responsiveness. Physical crosslinking through coiled-coil interactions promotes rapid shear-thinning and self-healing behavior, rendering the gel injectable, while an included photodegradable motif affords on-demand network dissolution via visible light. PhoCoil gel photodegradation can be spatiotemporally and lithographically controlled in a dose-dependent manner, through complex tissue, and without harm to encapsulated cells. We anticipate that PhoCoil will enable new applications in tissue engineering and regenerative medicine.

Applied Rheology as Tool for the Assessment of Chitosan Hydrogels for Regenerative Medicine

The regeneration of soft tissues that connect, support or surround other tissues is of great interest. In this sense, hydrogels have great potential as scaffolds for their regeneration. Among the different raw materials, chitosan stands out for being highly biocompatible, which, together with its biodegradability and structure, makes it a great alternative for the manufacture of hydrogels. Therefore, the aim of this work was to develop and characterize chitosan hydrogels. To this end, the most important parameters of their processing, i.e., agitation time, pH, gelation temperature and concentration of the biopolymer used were rheologically evaluated. The results show that the agitation time does not have a significant influence on hydrogels, whereas a change in pH (from 3.2 to 7) is a key factor for their formation. Furthermore, a low gelation temperature (4 °C) favors the formation of the hydrogel, showing better mechanical properties. Finally, there is a percentage of biopolymer saturation, from which the properties of the hydrogels are not further improved (1.5 wt.%). This work addresses the development of hydrogels with high thermal resistance, which allows their use as scaffolds without damaging their mechanical properties.

Protein Hydrogels: The Swiss Army Knife for Enhanced Mechanical and Bioactive Properties of Biomaterials

Biomaterials are dynamic tools with many applications: from the primitive use of bone and wood in the replacement of lost limbs and body parts, to the refined involvement of smart and responsive biomaterials in modern medicine and biomedical sciences. Hydrogels constitute a subtype of biomaterials built from water-swollen polymer networks. Their large water content and soft mechanical properties are highly similar to most biological tissues, making them ideal for tissue engineering and biomedical applications. The mechanical properties of hydrogels and their modulation have attracted a lot of attention from the field of mechanobiology. Protein-based hydrogels are becoming increasingly attractive due to their endless design options and array of functionalities, as well as their responsiveness to stimuli. Furthermore, just like the extracellular matrix, they are inherently viscoelastic in part due to mechanical unfolding/refolding transitions of folded protein domains. This review summarizes different natural and engineered protein hydrogels focusing on different strategies followed to modulate their mechanical properties. Applications of mechanically tunable protein-based hydrogels in drug delivery, tissue engineering and mechanobiology are discussed.

Evaluation of smart gelatin matrices for the development of scaffolds via 3D bioprinting

Throughout history, different techniques have been used for the development of scaffolds for Tissue Engineering. Among them, three-dimensional (3D) printing for this application has been recently enhanced due to its ease in defining the structure of the material. In this sense, a novel potential alternative could be the development of a three-part device whose leading utility is to improve the introduction of the scaffold in a bioreactor. Thus, the device consists of a polycaprolactone support on which smart gelatin (GE) matrix, and finally, on top, a collagen (C) scaffold. This gelatin matrix is included to integrate the scaffold into the support, but once both are assembled, it must be removed, leaving only the support and the scaffold. Thus, in the present work, a small gelatin matrix has been evaluated. To this end, matrices with different gelatin percentages were studied, evaluating their mechanical and morphological properties at different temperatures (22 and 37 °C) to control their deposition and elimination. The results show the high application of this smart matrix for the development of scaffolds via 3D bioprinting for Tissue Engineering.

Dynamic Light Scattering Microrheology Reveals Multiscale Viscoelasticity of Polymer Gels and Precious Biological Materials

The development of experimental techniques capable of probing the viscoelasticity of soft materials over a broad range of time scales is essential to uncovering the physics that governs their behavior. In this work, we develop a microrheology technique that requires only 12 μL of sample and is capable of resolving dynamic behavior ranging in time scales from 10^-6 to 10 s. Our approach, based on dynamic light scattering in the single-scattering limit, enables the study of polymer gels and other soft materials over a vastly larger hierarchy of time scales than macrorheology measurements. Our technique captures the viscoelastic modulus of polymer hydrogels with a broad range of stiffnesses from 10 to 10⁴ Pa. We harness these capabilities to capture hierarchical molecular relaxations in DNA and to study the rheology of precious biological materials that are impractical for macrorheology measurements, including decellularized extracellular matrices and intestinal mucus. The use of a commercially available benchtop setup that is already available to a variety of soft matter researchers renders microrheology measurements accessible to a broader range of users than existing techniques, with the potential to reveal the physics that underlies complex polymer hydrogels and biological materials.

Bioinspired dynamic microcapsules

There is an increasing interest in bioinspired dynamic materials. Abundant illustrations of protein domains exist in nature, with remarkable ligand binding characteristics and structures that undergo conformational changes. For example, calmodulin (CaM) can have three conformational states, which are the unstructured Apo-state, Ca²⁺-bound ligand-exposed binding state, and compact ligand-bound state. CaM's mechanical response to biological cues is highly suitable for engineering dynamic materials. The distance between CaM globular terminals in the Ca²⁺-bound state is 5 nm and in the ligand-bound state is 1.5 nm. CaM's nanoscale conformational changes have been used to develop dynamic hydrogel microspheres that undergo reversible volume changes. The current work presents the fabrication and preliminary results of layer-by-layer (LbL) self-assembled Dynamic MicroCapsules (DynaMicCaps) whose multilayered shell walls are composed of polyelectrolytes and CaM. Quasi-dynamic perfusion results show that the DynaMicCaps undergo drastic volume changes, with up to ∼1500% increase, when exposed to a biochemical ligand trifluoperazine (TFP) at pH 6.3. Under similar test conditions, microcapsules without CaM also underwent volume changes, with only up to ∼290% increase, indicating that CaM's bio-responsiveness was retained within the shell walls of the DynaMicCaps. Furthermore, DynaMicCaps exposed to 0.1 M NaOH underwent volume changes, with only up to ∼580% volume increase. Therefore, DynaMicCaps represent a new class of polyelectrolyte multilayer (PEM) capsules that can potentially be used to release their payload at near physiological pH. With over 200 proteins that undergo marked, well-characterized conformational changes in response to specific biochemical triggers, several other versions of DynaMicCaps can potentially be developed.

Mechanical response of transient telechelic networks with many-part stickers

A central question in soft matter is understanding how several individual, weak bonds act together to produce collective interactions. Here, gel-forming telechelic polymers with multiple stickers at each chain end are studied through Brownian dynamics simulations to understand how collective interaction of the bonds affects mechanical response of the gels. These polymers are modeled as finitely extensible dumbbells using an explicit tau-leap algorithm and the binding energy of these associations was kept constant regardless of the number of stickers. The addition of multiple bonds to the associating ends of telechelic polymers increases or decreases the network relaxation time depending on the relative kinetics of association but increases both shear stress and extensional viscosity. The relationship between the rate of association and the Rouse time of dangling chains results in two different regimes for the equilibrium stress relaxation of associating physical networks. In case I, a dissociated dangling chain is able to fully relax before re-associating to the network, resulting in two characteristic relaxation times and a non-monotonic terminal relaxation time with increasing number of bonds per polymer endgroup. In case II, the dissociated dangling chain is only able to relax a fraction of the way before it re-attaches to the network, and increasing the number of bonds per endgroup monotonically increases the terminal relaxation time. In flow, increasing the number of stickers increases the steady-state shear and extensional viscosities even though the overall bond kinetics and equilibrium constant remain unchanged. Increased dissipation in the simulations is primarily due to higher average chain extension with increasing bond number. These results indicate that toughness and dissipation in physically associating networks can both be increased by breaking single, strong bonds into smaller components.

Assessing the Potential of Folded Globular Polyproteins As Hydrogel Building Blocks

The native states of proteins generally have stable well-defined folded structures endowing these biomolecules with specific functionality and molecular recognition abilities. Here we explore the potential of using folded globular polyproteins as building blocks for hydrogels. Photochemically cross-linked hydrogels were produced from polyproteins containing either five domains of I27 ((I27)5), protein L ((pL)5), or a 1:1 blend of these proteins. SAXS analysis showed that (I27)5 exists as a single rod-like structure, while (pL)5 shows signatures of self-aggregation in solution. SANS measurements showed that both polyprotein hydrogels have a similar nanoscopic structure, with protein L hydrogels being formed from smaller and more compact clusters. The polyprotein hydrogels showed small energy dissipation in a load/unload cycle, which significantly increased when the hydrogels were formed in the unfolded state. This study demonstrates the use of folded proteins as building blocks in hydrogels, and highlights the potential versatility that can be offered in tuning the mechanical, structural, and functional properties of polyproteins.

Hydrogels Constructed from Engineered Proteins

Due to their various potential biomedical applications, hydrogels based on engineered proteins have attracted considerable interest. Benefitting from significant progress in recombinant DNA technology and protein engineering/design techniques, the field of protein hydrogels has made amazing progress. The latest progress of hydrogels constructed from engineered recombinant proteins are presented, mainly focused on biorecognition-driven physical hydrogels as well as chemically crosslinked hydrogels. The various bio-recognition based physical crosslinking strategies are discussed, as well as chemical crosslinking chemistries used to engineer protein hydrogels, and protein hydrogels' various biomedical applications. The future perspectives of this fast evolving field of biomaterials are also discussed.

Thermoresponsive and Mechanical Properties of Poly(L-proline) Gels

Gelation of the left helical N-substituted homopolypeptide poly(L-proline) (PLP) in water was explored, employing rheological and small-angle scattering studies at different temperatures and concentrations in order to investigate the network structure and its mechanical properties. Stiff gels were obtained at 10 wt % or higher at 5 °C, the first time gelation has been observed for homopolypeptides. The secondary structure and helical rigidity of PLP has large structural similarities to gelatin but as gels the two materials show contrasting trends with temperature. With increasing temperature in D2O, the network stiffens, with broad scattering features of similar correlation length for all concentrations and molar masses of PLP. A thermoresponsive transition was also achieved between 5 and 35 °C, with moduli at 35 °C higher than gelatin at 5 °C. The brittle gels could tolerate strains of 1% before yielding with a frequency-independent modulus over the observed range, similar to natural proline-rich proteins, suggesting the potential for thermoresponsive or biomaterial-based applications.

Disulfide-crosslinked albumin hydrogels

Albumin hydrogels crosslinked by disulfide bonds between the protein's own thiol groups.

Varying the counter ion changes the kinetics, but not the final structure of colloidal gels

We show that, while the gelation of colloidal silica proceeds much faster in the presence of added KCl than NaCl, the final gels are very similar in structure and properties. We have studied the gelation process by visual inspection and by small angle X-ray scattering for a range of salt and silica particle concentrations. The characteristic times of the early aggregation process and the formation of a stress-bearing structure with both salts are shown to collapse onto master curves with single multiplicative constants, linked to the stability ratio of the colloidal suspensions. The influence of the salt type and concentration is confirmed to be mainly kinetic, as the static structure factors and viscoelastic moduli of the gels are shown to be equivalent at normalized times. While there is strong variation in the kinetics, the structure and properties of the gel at long-times are shown to be mainly controlled by the concentration of particles, and hardly influenced by the type or the concentration of salt. This suggests that the differences between gels generated by different salts are only transient in time.

Celebrating Soft Matter's 10th Anniversary: chain configuration and rate-dependent mechanical properties in transient networks

Numerical solution of a coupled set of Smoluchowski convection-diffusion equations of associating polymers modelled as finitely extensible dumbbells enables computation of time-dependent end-to-end distributions for bridged, dangling, and looped chains in three dimensions as a function of associating end-group kinetics. Non-monotonic flow curves which can lead to flow instabilities during shear flow result at low equilibrium constant and high association rate from two complementary phenomena: a decrease in the fraction of elastically active chains with increasing shear rate and non-monotonic extension in the population of elastically active chains. Chain tumbling leads to reformation of bridges, resulting in an increased fraction of bridged chains at high Deborah number and significant reduction in the average bridge chain extension. In the start-up of steady shear, force-activated chain dissociation and chain tumbling cause both stress overshoot and stress ringing behaviour prior to reaching steady state stress values. During stress relaxation following steady shear, chain kinetics and extension mediate both the number of relaxations and the length of time required for system relaxation. While at low association rate relaxation is limited by the relaxation of dangling chains and the rate of dangling chain formation, at high association rate coupling of dangling and bridged chains leads to simultaneous relaxation of all chains due to a dynamic equilibrium between dangling and bridged states.

Doubling the cross-linking interface of a rationally designed beta roll peptide for calcium-dependent proteinaceous hydrogel formation

We have rationally engineered a stimulus-responsive cross-linking domain based on a repeats-in-toxin (RTX) peptide to enable calcium-dependent formation of supramolecular hydrogel networks. The peptide isolated from the RTX domain is intrinsically disordered in the absence of calcium. In calcium rich environments, the peptide binds Ca(2+) ions and folds into a beta roll (β-roll) secondary structure composed to two parallel β-sheet faces. Previously, we mutated one of the faces to contain solvent exposed leucine side chains which are localized only in the calcium-bound β-roll conformation. We demonstrated the ability of this mutant peptide to self-assemble into hydrogels in the presence of calcium with the aid of additional peptide-based cross-linking moieties. Here, we have expanded this approach by engineering both β-roll faces to contain leucine residues, thereby doubling the cross-linking interface for each monomeric building block. These leucine rich surfaces impart a hydrophobic driving force for self-assembly. Extensive characterization was performed on this double-faced mutant to ensure the retention of calcium affinity and subsequent structural rearrangement similar to that of the wild type domain. We genetically fused an α-helical leucine zipper capable of forming tetrameric coiled-coil bundles to the peptide and the resulting chimeric protein self-assembles into a hydrogel only in calcium rich environments. Since this new mutant peptide enables cross-linking on both surfaces simultaneously, a higher oligomerization state was achieved. To further investigate the cross-linking capability, we constructed concatemers of the β-roll with maltose binding protein (MBP), a monomeric globular protein, without the leucine zipper domains. These concatemers show a similar sol-gel transition in response to calcium. Several biophysical techniques were used to probe the structural and mechanical properties of the mutant β-roll domain and the resulting supramolecular networks including circular dichroism, fluorescence resonance energy transfer, bis-ANS binding, and microrheology. These results demonstrate that the engineered β-roll peptides can mediate calcium-dependent cross-linking for protein hydrogel formation without the need for any other cross-linking moieties.

Controlling topological entanglement in engineered protein hydrogels with a variety of thiol coupling chemistries

Topological entanglements between polymer chains are achieved in associating protein hydrogels through the synthesis of high molecular weight proteins via chain extension using a variety of thiol coupling chemistries, including disulfide formation, thiol-maleimide, thiol-bromomaleimide and thiol-ene. Coupling of cysteines via disulfide formation results in the most pronounced entanglement effect in hydrogels, while other chemistries provide versatile means of changing the extent of entanglement, achieving faster chain extension, and providing a facile method of controlling the network hierarchy and incorporating stimuli responsivities. The addition of trifunctional coupling agents causes incomplete crosslinking and introduces branching architecture to the protein molecules. The high-frequency plateau modulus and the entanglement plateau modulus can be tuned by changing the ratio of difunctional chain extender to the trifunctional branching unit. Therefore, these chain extension reactions show promise in delicately controlling the relaxation and mechanical properties of engineered protein hydrogels in ways that complement their design through genetic engineering.

Oxidatively Responsive Chain Extension to Entangle Engineered Protein Hydrogels

Engineering artificial protein hydrogels for medical applications requires precise control over their mechanical properties, including stiffness, toughness, extensibility and stability in the physiological environment. Here we demonstrate topological entanglement as an effective strategy to robustly increase the mechanical tunability of a transient hydrogel network based on coiled-coil interactions. Chain extension and entanglement are achieved by coupling the cysteine residues near the N- and C- termini, and the resulting chain distribution is found to agree with the Jacobson-Stockmayer theory. By exploiting the reversible nature of the disulfide bonds, the entanglement effect can be switched on and off by redox stimuli. With the presence of entanglements, hydrogels exhibit a 7.2-fold enhanced creep resistance and a suppressed erosion rate by a factor of 5.8, making the gels more mechanically stable in a physiologically relevant open system. While hardly affecting material stiffness (only resulting in a 1.5-fold increase in the plateau modulus), the entanglements remarkably lead to hydrogels with a toughness of 65,000 J m^-3 and extensibility to approximately 3,000% engineering strain, which enables the preparation of tough yet soft tissue simulants. This improvement in mechanical properties resembles that from double-network hydrogels, but is achieved with the use of a single associating network and topological entanglement. Therefore, redox-triggered chain entanglement offers an effective approach for constructing mechanically enhanced and responsive injectable hydrogels.

Facile and rapid ruthenium mediated photo-crosslinking of Bombyx mori silk fibroin

We report a unique and facile way of preparing silk fibroin gel by ruthenium-mediated photocrosslinking of silk solution. Compared to existing methods, this approach is faster, taking only a few minutes to form the gel with tunable modulus. Hydrogels demonstrate their potential suitability as biomaterials for tissue engineering applications.