Synthetic fibrous hydrogels as a platform to decipher cell-matrix mechanical interactions.

Cells continuously sense external forces from their microenvironment, the extracellular matrix (ECM). In turn, they generate contractile forces, which stiffen and remodel this matrix. Although this bidirectional mechanical exchange is crucial for many cell functions, it remains poorly understood. Key challenges are that the majority of available matrices for such studies, either natural or synthetic, are difficult to control or lack biological relevance. Here, we use a synthetic, yet highly biomimetic hydrogel based on polyisocyanide (PIC) polymers to investigate the effects of the fibrous architecture and the nonlinear mechanics on cell-matrix interactions. Live-cell rheology was combined with advanced microscopy-based approaches to understand the mechanisms behind cell-induced matrix stiffening and plastic remodeling. We demonstrate how cell-mediated fiber remodeling and the propagation of fiber displacements are modulated by adjusting the biological and mechanical properties of this material. Moreover, we validate the biological relevance of our results by demonstrating that cellular tractions in PIC gels develop analogously to those in the natural ECM. This study highlights the potential of PIC gels to disentangle complex bidirectional cell-matrix interactions and to improve the design of materials for mechanobiology studies.

Tailoring of Physical Properties in Macroporous Poly(isocyanopeptide) Cryogels.

Over the years, synthetic hydrogels have proven remarkably useful as cell culture matrixes to elucidate the role of the extracellular matrix (ECM) on cell behavior. Yet, their lack of interconnected macropores undermines the widespread use of hydrogels in biomedical applications. To overcome this limitation, cryogels, a class of macroporous hydrogels, are rapidly emerging. Here, we introduce a new, highly elastic, and tunable synthetic cryogel, based on poly(isocyanopeptides) (PIC). Introduction of methacrylate groups on PIC facilitated cryopolymerization through free-radical polymerization and afforded cryogels with an interconnected macroporous structure. We investigated which cryogelation parameters can be used to tune the architectural and mechanical properties of the PIC cryogels by systematically altering cryopolymerization temperature, polymer concentration, and polymer molecular weight. We show that for decreasing cryopolymerization temperatures, there is a correlation between cryogel pore size and stiffness. More importantly, we demonstrate that by simply varying the polymer concentration, we can selectively tune the compressive strength of PIC cryogels without affecting their architecture. This unique feature is highly useful for biomedical applications, as it facilitates decoupling of stiffness from other variables such as pore size. As such, PIC cryogels provide an interesting new biomaterial for scientists to unravel the role of the ECM in cellular functions.

The living interface between synthetic biology and biomaterial design.

Recent far-reaching advances in synthetic biology have yielded exciting tools for the creation of new materials. Conversely, advances in the fundamental understanding of soft-condensed matter, polymers and biomaterials offer new avenues to extend the reach of synthetic biology. The broad and exciting range of possible applications have substantial implications to address grand challenges in health, biotechnology and sustainability. Despite the potentially transformative impact that lies at the interface of synthetic biology and biomaterials, the two fields have, so far, progressed mostly separately. This Perspective provides a review of recent key advances in these two fields, and a roadmap for collaboration at the interface between the two communities. We highlight the near-term applications of this interface to the development of hierarchically structured biomaterials, from bioinspired building blocks to 'living' materials that sense and respond based on the reciprocal interactions between materials and embedded cells.

Magnetically Driven Hierarchical Alignment in Biomimetic Fibrous Hydrogels.

In vivo, natural biomaterials are frequently anisotropic, exhibiting directional microstructures and mechanical properties. It remains challenging to develop such anisotropy in synthetic materials. Here, a facile one-step approach for in situ fabrication of hydrogels with hierarchically anisotropic architectures and direction-dependent mechanical properties is proposed. The anisotropic hydrogels, composed of a fibrous gel network (0.1 wt%), cross-linked with magnetic nanoparticles (spheres, rods, and wires, <0.1 wt%) are readily formed in the presence of very low magnetic fields (<20 mT). The anisotropy of the nanoparticles is transduced to the polymer network, leading to macroscopic anisotropy, for instance, in mechanical properties. Electrostatic repulsion by the negatively charged nanoparticles induces an additional layer of order in the material, perpendicular to the magnetic field direction. The straightforward fabrication strategy allows for stepwise deposition of layers with different degrees or directions of anisotropy, which enables the formation of complex structures that are able to mimic some of the complex hierarchical architectures found in biology. It is anticipated that this approach of hydrogel alignment may serve as a guide for designing advanced biomaterials in tissue engineering.

Magnetic Stiffening in 3D Cell Culture Matrices.

The mechanical environment of a cell is not constant. This dynamic behavior is exceedingly difficult to capture in (synthetic) in vitro matrices. This paper describes a novel, highly adaptive hybrid hydrogel composed of magnetically sensitive magnetite nanorods and a stress-responsive synthetic matrix. Nanorod rearrangement after application of (small) magnetic fields induces strain in the network, which results in a strong (over 10-fold) stiffening even at minimal (2.5 wt %) nanorod concentrations. Moreover, the stiffening mechanism yields a fast and fully reversible response. In the manuscript, we quantitatively analyze that forces generated by the particles are comparable to cellular forces. We demonstrate the value of magnetic stiffening in a 3D MCF10A epithelial cell experiment, where simply culturing on top of a permanent magnet gives rise to changes in the cell morphology. This work shows that our hydrogels are uniquely suited as 3D cell culture systems with on-demand adaptive mechanical properties.

Structural characterization of fibrous synthetic hydrogels using fluorescence microscopy.

The structural features of the matrix surrounding the cells play a crucial role in regulating their behavior. Here, we used fluorescence microscopy and customized analysis algorithms to characterize the architecture of fibrous hydrogel networks. As a model system, we investigated a new class of synthetic biomimetic material, hydrogels prepared from polyisocyanides. Our results show that these synthetic gels present a highly heterogeneous fibrous network, with pores reaching a few micrometers in diameter. By encapsulating HeLa cells in different hydrogels, we show that a more porous structure is linked to a higher proliferation rate. The approach described here, for the characterization of the network of fibrous hydrogels, can be easily applied to other polymer-based materials and provide new insights into the influence of structural features in cell behavior. This knowledge is crucial to develop the next generation of biomimetic materials for 3D cell models and tissue engineering applications.

Biomimetic Networks with Enhanced Photodynamic Antimicrobial Activity from Conjugated Polythiophene/Polyisocyanide Hybrid Hydrogels.

Hybrid biomimetic hydrogels with enhanced reactive oxygen species (ROS)-generation efficiency under 600 nm light show high antibacterial activity. The hybrid gels are composed of helical tri(ethylene glycol)-functionalized polyisocyanides (PICs) and a conformation-sensitive conjugated polythiophene, poly(3-(3'-N,N,N-triethylammonium-1'-propyloxy)-4-methyl-2,5-thiophene chloride) (PMNT). The PIC polymer serves as a scaffold to trap and align the PMNT backbone into a highly ordered conformation, resulting in redshifted, new sharp bands in the absorption and fluorescence spectra. Similar to PIC, the hybrid closely mimics the mechanical properties of biological gels, such as collagen and fibrin, including the strain stiffening properties at low stresses. Moreover, the PMNT/PIC hybrids show much higher ROS production efficiency under red light than PMNT only, leading to an efficient photodynamic antimicrobial effect towards various pathogenic bacteria.

Synthetic Extracellular Matrices with Nonlinear Elasticity Regulate Cellular Organization.

One of the promises of synthetic materials in cell culturing is that control over their molecular structures may ultimately be used to control their biological processes. Synthetic polymer hydrogels from polyisocyanides (PIC) are a new class of minimal synthetic biomaterials for three-dimensional cell culturing. The macromolecular lengths and densities of biofunctional groups that decorate the polymer can be readily manipulated while preserving the intrinsic nonlinear mechanics, a feature commonly displayed by fibrous biological networks. In this work, we propose the use of PIC gels as cell culture platforms with decoupled mechanical inputs and biological cues. For this purpose, different types of cells were encapsulated in PIC gels of tailored compositions that systematically vary in adhesive peptide (GRGDS) density, polymer length, and concentration; with the last two parameters controlling the gel mechanics. Both cancer and smooth muscle cells grew into multicellular spheroids with proliferation rates that depend on the adhesive GRGDS density, regardless of the polymer length, suggesting that for these cells, the biological input prevails over the mechanical cues. In contrast, human adipose-derived stem cells do not form spheroids but rather spread out. We find that the morphological changes strongly depend on the adhesive ligand density and the network mechanics; gels with the highest GRGDS densities and the strongest stiffening response to stress show the strongest spreading. Our results highlight the role of the nonlinear mechanics of the extracellular matrix and its synthetic mimics in the regulation of cell functions.

Synthetic Semiflexible and Bioactive Brushes.

Polymer brushes are extensively used for the preparation of bioactive surfaces. They form a platform to attach functional (bio)molecules and control the physicochemical properties of the surface. These brushes are nearly exclusively prepared from flexible polymers, even though much stiffer brushes from semiflexible polymers are frequently found in nature, which exert bioactive functions that are out of reach for flexible brushes. Synthetic semiflexible polymers, however, are very rare. Here, we use polyisocyanopeptides (PICs) to prepare high-density semiflexible brushes on different substrate geometries. For bioconjugation, we developed routes with two orthogonal click reactions, based on the strain-promoted azide-alkyne cycloaddition reaction and the (photoactivated) tetrazole-ene cycloaddition reaction. We found that for high brush densities, multiple bonds between the polymer and the substrate are necessary, which was achieved in a block copolymer strategy. Whether the desired biomolecules are conjugated to the PIC polymer before or after brush formation depends on the dimensions and required densities of the biomolecules and the curvature of the substrate. In either case, we provide mild, aqueous, and highly modular reaction strategies, which make PICs a versatile addition to the toolbox for generating semiflexible bioactive polymer brush surfaces.

Responsive biomimetic networks from polyisocyanopeptide hydrogels.

Mechanical responsiveness is essential to all biological systems down to the level of tissues and cells. The intra- and extracellular mechanics of such systems are governed by a series of proteins, such as microtubules, actin, intermediate filaments and collagen. As a general design motif, these proteins self-assemble into helical structures and superstructures that differ in diameter and persistence length to cover the full mechanical spectrum. Gels of cytoskeletal proteins display particular mechanical responses (stress stiffening) that until now have been absent in synthetic polymeric and low-molar-mass gels. Here we present synthetic gels that mimic in nearly all aspects gels prepared from intermediate filaments. They are prepared from polyisocyanopeptides grafted with oligo(ethylene glycol) side chains. These responsive polymers possess a stiff and helical architecture, and show a tunable thermal transition where the chains bundle together to generate transparent gels at extremely low concentrations. Using characterization techniques operating at different length scales (for example, macroscopic rheology, atomic force microscopy and molecular force spectroscopy) combined with an appropriate theoretical network model, we establish the hierarchical relationship between the bulk mechanical properties and the single-molecule parameters. Our results show that to develop artificial cytoskeletal or extracellular matrix mimics, the essential design parameters are not only the molecular stiffness, but also the extent of bundling. In contrast to the peptidic materials, our polyisocyanide polymers are readily modified, giving a starting point for functional biomimetic hydrogels with potentially a wide variety of applications, in particular in the biomedical field.

Virus-like particles as crosslinkers in fibrous biomimetic hydrogels: approaches towards capsid rupture and gel repair.

Biological hydrogels can become many times stiffer under deformation. This unique ability has only recently been realised in fully synthetic gels. Typically, these networks are composed of semi-flexible polymers and bundles and show such large mechanical responses at very small strains, which makes them particularly suitable for application as strain-responsive materials. In this work, we introduced strain-responsiveness by crosslinking the architecture with a multi-functional virus-like particle. At high stresses, we find that the virus particles disintegrate, which creates an (irreversible) mechanical energy dissipation pathway, analogous to the high stress response of fibrin networks. A cooling-heating cycle allows for re-crosslinking at the damaged site, which gives rise to much stronger hydrogels. Virus particles and capsids are promising drug delivery vehicles and our approach offers an effective strategy to trigger the release mechanically without compromising the mechanical integrity of the host material.

Liquid crystal templating as an approach to spatially and temporally organise soft matter.

Chemistry quickly moves from a molecular science to a systems science. This requires spatial and temporal control over the organisation of molecules and molecular assemblies. Whilst Nature almost by default (transiently) organises her components at multiple different length scales, scientists struggle to realise even relatively straightforward patterns. In the past decades, supramolecular chemistry has taught us the rules to precisely engineer molecular assembly at the nanometre scale. At higher length scales, however, we are bound to top-down nanotechnology techniques to realise order. For soft, biological matter, many of these top-down techniques come with serious limitations since the molecules generally show low susceptibilities to the applied stimuli. A new method is based on liquid crystal templating. In this hierarchical approach, a liquid crystalline host serves as the scaffold to order polymers or assemblies. Being a liquid crystal, the host material can be ordered at many different length scales and on top of that, is highly susceptible to many external stimuli, which can even be used to manipulate the liquid crystal organisation in time. As a result, we anticipate large control over the organisation of the materials inside the liquid crystalline host. Recently, liquid crystal templating was also realised in water. This suddenly makes this tool highly applicable to start organising more delicate biological materials or even small organisms. We review the scope and limitations of liquid crystal templating and look out to where the technique may lead us.

Bundle Formation in Biomimetic Hydrogels.

Bundling of single polymer chains is a crucial process in the formation of biopolymer network gels that make up the extracellular matrix and the cytoskeleton. This bundled architecture leads to gels with distinctive properties, including a large-pore-size gel formation at very low concentrations and mechanical responsiveness through nonlinear mechanics, properties that are rarely observed in synthetic hydrogels. Using small-angle X-ray scattering (SAXS), we study the bundle formation and hydrogelation process of polyisocyanide gels, a synthetic material that uniquely mimics the structure and mechanics of biogels. We show how the structure of the material changes at the (thermally induced) gelation point and how factors such as concentration and polymer length determine the architecture, and with that, the mechanical properties. The correlation of the gel mechanics and the structural parameters obtained from SAXS experiments is essential in the design of future (synthetic) mimics of biopolymer networks.

Directed peptide amphiphile assembly using aqueous liquid crystal templates in magnetic fields.

An alignment technique based on the combination of magnetic fields and a liquid crystal (LC) template uses the advantages of both approaches: the magnetic fields offer non-contact methods that apply to all sample sizes and shapes, whilst the LC templates offer high susceptibilities. The combination introduces a route to control the spatial organization of materials with low intrinsic susceptibilities. We demonstrate that we can unidirectionally align one such material, peptide amphiphiles in water, on a centimeter scale at a tenfold lower magnetic field by using a lyotropic chromonic liquid crystal as a template. We can transform the aligned supramolecular assemblies into optically active π-conjugated polymers after photopolymerization. Lastly, by reducing the magnetic field strength needed for addressing these assemblies, we are able to create more complex structures by initiating self-assembly of our supramolecular materials under competing alignment forces between the magnetically induced alignment of the assemblies (with a positive diamagnetic anisotropy) and the elastic force dominated alignment of the template (with a negative diamagnetic anisotropy), which is directed orthogonally. Although the approach is still in its infancy and many critical parameters need optimization, we believe that it is a very promising technique to create tailor-made complex structures of (aqueous) functional soft matter.

Key Developments in Ionic Liquid Crystals.

Ionic liquid crystals are materials that combine the classes of liquid crystals and ionic liquids. The first one is based on the multi-billion-dollar flat panel display industry, whilst the latter quickly developed in the past decades into a family of highly-tunable non-volatile solvents. The combination yields materials with a unique set of properties, but also with many challenges ahead. In this review, we provide an overview of the key concepts in ionic liquid crystals, particularly from a molecular perspective. What are the important molecular parameters that determine the phase behavior? How should they be introduced into the molecules? Finally, which other tools does one have to realize specific properties in the material?

Stabilisation of 2D colloidal assemblies by polymerisation of liquid crystalline matrices for photonic applications.

Colloidal crystals in anisotropic matrices are extremely stable and versatile, but disassemble as soon as the anisotropy of the matrix disappears. We present an approach to first custom-assemble colloidal structures and subsequently stabilize them through photo-polymerisation of the liquid crystalline matrix. The resulting 2D colloidal assemblies are stable at high temperatures and can even be obtained as free-standing films without a decrease in the degree of organization. This approach could be used to stabilize and extract recently proposed soft-matter photonic microcircuits based on liquid crystal optical microresonators, microlasers and microfibers, and opens up routes towards real soft matter photonic devices that are stable over extended time and temperatures.

Antifibrotic properties of hyaluronic acid crosslinked polyisocyanide hydrogels.

Fibrosis is characterized by the formation of fibrous connective tissue in response to primary injury. As a result, an affected organ may lose part of its functionality due to chronic, organ-specific tissue damage. Since fibrosis is a leading cause of death worldwide, targeting fibrotic diseases with antifibrotic hydrogels can be a lifesaving therapeutic strategy. This study developed a novel hybrid antifibrotic hydrogel by combining the synthetic polyisocyanide (PIC) with hyaluronic acid (HA). Gels of PIC are highly tailorable, thermosensitive, and strongly biomimetic in architecture and mechanical properties, whereas HA is known to promote non-fibrotic fetal wound healing and inhibits inflammatory signaling. The developed HA-PIC hybrids were biocompatible with physical properties comparable to those of the PIC gels. The antifibrotic nature of the gels was assessed by 3D cultures of human foreskin fibroblasts in the presence (or absence as control) of TGFß1 that promotes differentiation into myofibroblasts, a critical step in fibrosis. Proliferation and macroscopic contraction assays and studies on the formation of stress fibers and characteristic fibrosis markers all indicate a strong antifibrotic nature of HA-PIC hydrogel. We showed that these effects originate from both the lightly crosslinked architecture and the presence of HA itself. The hybrid displaying both these effects shows the strongest antifibrotic nature and is a promising candidate for use as in vivo treatment for skin fibrosis.

TempEasy 3D Hydrogel Coculture System Provides Mechanistic Insights into Prostate Cancer Bone Metastasis.

Patients diagnosed with advanced prostate cancer (PCa) often experience incurable bone metastases; however, a lack of relevant experimental models has hampered the study of disease mechanisms and the development of therapeutic strategies. In this study, we employed the recently established Temperature-based Easy-separable (TempEasy) 3D cell coculture system to investigate PCa bone metastasis. Through coculturing PCa and bone cells for 7 days, our results showed a reduction in PCa cell proliferation, an increase in neovascularization, and an enhanced metastasis potential when cocultured with bone cells. Additionally, we observed increased cell proliferation, higher stemness, and decreased bone matrix protein expression in bone cells when cocultured with PCa cells. Furthermore, we demonstrated that the stiffness of the extracellular matrix had a negligible impact on molecular responses in both primary (PCa cells) and distant malignant (bone cells) sites. The TempEasy 3D hydrogel coculture system is an easy-to-use and versatile coculture system that provides valuable insights into the mechanisms of cell-cell communication and interaction in cancer metastasis.

Conductive Polyisocyanide Hydrogels Inhibit Fibrosis and Promote Myogenesis.

Reliable in vitro models closely resembling native tissue are urgently needed for disease modeling and drug screening applications. Recently, conductive biomaterials have received increasing attention in the development of in vitro models as they permit exogenous electrical signals to guide cells toward a desired cellular response. Interestingly, they have demonstrated that they promote cellular proliferation and adhesion even without external electrical stimulation. This paper describes the development of a conductive, fully synthetic hydrogel based on hybrids of the peptide-modified polyisocyanide (PIC-RGD) and the relatively conductive poly(aniline-co-N-(4-sulfophenyl)aniline) (PASA) and its suitability as the in vitro matrix. We demonstrate that incorporating PASA enhances the PIC-RGD hydrogel's electroactive nature without significantly altering the fibrous architecture and nonlinear mechanics of the PIC-RGD network. The biocompatibility of our model was assessed through phenotyping cultured human foreskin fibroblasts (HFF) and murine C2C12 myoblasts. Immunofluorescence analysis revealed that PIC-PASA hydrogels inhibit the fibrotic behavior of HFFs while promoting myogenesis in C2C12 cells without electrical stimulation. The composite PIC-PASA hydrogel can actively change the cell fate of different cell types, providing an attractive tool to improve skin and muscle repair.

Gradient matters via filament diameter-adjustable 3D printing.

Gradient matters with hierarchical structures endow the natural world with excellent integrity and diversity. Currently, direct ink writing 3D printing is attracting tremendous interest, and has been used to explore the fabrication of 1D and 2D hierarchical structures by adjusting the diameter, spacing, and angle between filaments. However, it is difficult to generate complex 3D gradient matters owing to the inherent limitations of existing methods in terms of available gradient dimension, gradient resolution, and shape fidelity. Here, we report a filament diameter-adjustable 3D printing strategy that enables conventional extrusion 3D printers to produce 1D, 2D, and 3D gradient matters with tunable heterogeneous structures by continuously varying the volume of deposited ink on the printing trajectory. In detail, we develop diameter-programmable filaments by customizing the printing velocity and height. To achieve high shape fidelity, we specially add supporting layers at needed locations. Finally, we showcase multi-disciplinary applications of our strategy in creating horizontal, radial, and axial gradient structures, letter-embedded structures, metastructures, tissue-mimicking scaffolds, flexible electronics, and time-driven devices. By showing the potential of this strategy, we anticipate that it could be easily extended to a variety of filament-based additive manufacturing technologies and facilitate the development of functionally graded structures.