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.

An Improved Understanding of the Pathophysiology of Pelvic Organ Prolapse: A 3D In Vitro Model under Static and Mechanical Loading Conditions

The suboptimal outcomes of pelvic organ prolapse (POP) surgery illustrate the demand for improved therapies. However, their development is hampered by the limited knowledge on the cellular pathophysiology of POP. Current investigations, that are limited to tissues and 2D in vitro models, provide highly inconclusive results on how the extracellular matrix (ECM) metabolism and fibroblasts are affected in POP. This study uses a physiologically relevant 3D in vitro model to investigate the cellular pathophysiology of POP by determining the differences between POP and non-POP fibroblasts on ECM metabolism, proliferation, and fibroblast-to-myofibroblast (FMT) transition. This model, based on the synthetic and biomimetic polyisocyanide hydrogel, enables the incorporation of mechanical loading, which simulates the forces exerted on the pelvic floor. Under static conditions, 3D cultured POP fibroblasts are less proliferative, undergo FMT, and exhibit lower collagen and elastin contents compared to non-POP fibroblasts. However, under mechanical loading, the differences between POP and non-POP fibroblasts are less pronounced. This study contributes to the development of more comprehensive models that can accurately mimic the POP pathophysiology, which will aid in an enhanced understanding and may contribute to improved therapies in the future.

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 TGFβ1 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.

Injectable polyisocyanide hydrogel as healing supplement for connective tissue regeneration in an abdominal wound model

In pelvic organ prolapse (POP) patients, the uterus, bladder and/or rectum descends into vagina due to weakened support tissues. High recurrence rates after POP surgery suggest an urgent need for improved surgical outcomes. Our aim is to promote connective tissue healing that results in stimulated tissue support functions by surgically applying a hydrogel functionalized with biological cues. We used known vaginal wound healing promoting factors (basic fibroblast growth factor, β-estradiol, adipose-derived stem cells) in the biomimetic and injectable polyisocyanide (PIC) hydrogel, which in itself induces regenerative vaginal fibroblast behavior. The regenerative capacity of injected PIC hydrogel, and the additional pro-regenerative effects of these bioactive factors was evaluated in abdominal wounds in rabbits. Assessment of connective tissue healing (tensile testing, histology, immunohistochemistry) revealed that injection with all PIC formulations resulted in a statistically significant stiffness and collagen increase over time, in contrast to sham. Histological evaluation indicated new tissue growth with moderate to mild immune activity at the hydrogel - tissue interface. The results suggest that PIC injection in an abdominal wound improves healing towards regaining load-bearing capacity, which encourages us to investigate application of the hydrogel in a more translational vaginal model for POP surgery in sheep.

Growth Factor Immobilization to Synthetic Hydrogels: Bioactive bFGF-Functionalized Polyisocyanide Hydrogels

With its involvement in cell proliferation, migration and differentiation basic fibroblast growth factor (bFGF) has great potential for tissue engineering purposes. So far, however, clinical translation of soluble bFGF-based therapies is unsuccessful, because the required effective doses are often supraphysiological, which may cause adverse effects. An effective solution is growth factor immobilization, whereby bFGF retains its bioactivity at increased efficacy. Studied carriers include films, solid scaffolds, and particles, as well as natural and synthetic hydrogels. However, these synthetic hydrogels poorly resemble the characteristics of the native extracellular matrix (ECM). In this work, bFGF is covalently conjugated to the synthetic, but highly biocompatible, polyisocyanide-based hydrogel (PIC-bFGF), which closely mimics the architecture and mechanical properties of the ECM. The growth factor conjugation protocol is straightforward and readily extrapolated to other growth factors or proteins. The PIC-bFGF hydrogel shows a prolonged bioactivity up to 4 weeks although no clear effects on the ECM metabolism are observed. Beyond the future potential of the PIC-bFGF hydrogel toward various tissue engineering applications, this work underlines that simple biological conjugation procedures are a powerful strategy to induce additional bioactivity in 3D synthetic cell culture matrices.

Vaginal Fibroblast Behavior as a Function of Stiffness Changes in a Polyisocyanide Hydrogel for Prolapse Repair

There is an urgent need for improved outcomes in the treatment of pelvic organ prolapse (POP). Success of primary surgery relies on the load bearing capacity of plicated connective tissue underneath the vaginal wall, which is compromised due to an altered vaginal fibroblast function and collagen composition. There is an important factor in connective tissue repair that relates to changes in stiffness of the vaginal fibroblast microenvironment, which influences cell activity through cellular mechanosensing. The aim of this study is to investigate the effect of stiffness changes on vaginal fibroblast functions that relate to connective tissue healing in prolapse repair. The substrate stiffness was controlled by changing the polymer concentration in the fibrous and strongly biomimetic polyisocyanide (PIC) hydrogel. We analyzed stiffness during cell culture and assessed the consequential fibroblast proliferation, morphology, collagen deposition, and contraction. Our results show that increasing stiffness coincides with vaginal fibroblast alignment, promotes collagen deposition, and inhibits PIC gel contraction. These findings suggest that the matrix stiffness directly influences vaginal fibroblast functionality. Moreover, we observed a buildup in stiffness and collagen, with an enhanced fibroblast and collagen organization on the PIC-substrate, which indicate an enhanced structural integrity of the hydrogel-cell construct. An improved tissue structure during healing is relevant in the functional repair of POP. Therefore, this study encourages future research in the use of PIC gels as a supplement in prolapse surgery, whereby the hydrogel stiffness should be considered.

The Effect of Growth Factors on Vaginal Wound Healing: A Systematic Review and Meta-analysis

Surgical outcomes of pelvic organ prolapse (POP) surgery are poor, resulting in a 20% recurrence risk. Following the hypothesis that impaired wound healing is the main determinant of recurrent POP, growth factors have the potential to promote wound healing and may improve surgical outcomes. In this study, we systematically reviewed the effect of growth factors on vaginal wound healing in both in vitro and animal studies. For each independent comparison, the standardized mean difference and 95% CI were calculated using the Hedges' g correction. Of the 3858 retrieved studies, seven studies were included, of which six were included in meta-analysis (three in vitro studies and four in vivo studies). In vitro, basic fibroblast growth factor (bFGF) promotes proliferation, differentiation, and collagen types I and III production. Epidermal growth factor stimulates proliferation and connective tissue growth factor promotes Tenascin-C expression. These effects, however, are less pronounced in vivo; only bFGF slightly promotes collagen production. The review shows that growth factors, particularly bFGF, are able to promote vaginal wound healing in vitro. The uncertain in vivo findings suggest that preclinical models should be improved. The ultimate goal is to develop effective growth factor-supplemented therapies that improve surgical outcomes for POP.

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.

Polyisocyanides as a substrate to trigger vaginal fibroblast functioning in an in vitro model for prolapse repair

Pelvic organ prolapse (POP) is the descent of the bladder, uterus, and/or rectum into the vagina. POP is associated with altered vaginal fibroblast functionality and connective tissue composition in the vaginal wall. The results of surgical intervention are poor, which may be related to the lack of true restoration of the connective tissue. An innovative treatment addresses tissue repair after surgery by the introduction of a bioactive supplement that enhances the healing process through collagen and elastin deposition. As a novel strategy, we first studied the effects in an in vitro model. Here, we investigate how the presence of cell binding GRGDS (RGD) peptides on the highly biomimetic polyisocyanide (PIC) gel facilitates and promotes the function of primary vaginal fibroblasts isolated from a POP patient. Fibroblast function was analyzed in terms of morphology, proliferation, and extracellular matrix (ECM) deposition and remodeling. RGD modification of the gel facilitated cell spread and proliferation. Quantitative outcomes of the ECM content indicated increased production of collagen and elastin by fibroblasts on gels with the highest RGD density. The in vitro results suggest that PIC-RGD hydrogel application may translate into improved connective tissue healing in the pelvic floor, which is essential for its use as a regeneration promoting additive in surgery.

Toward Tissue-Like Material Properties: Inducing In Situ Adaptive Behavior in Fibrous Hydrogels

The materials properties of biological tissues are unique. Nature is able to spatially and temporally manipulate (mechanical) properties while maintaining responsiveness toward a variety of cues; all without majorly changing the material's composition. Artificial mimics, synthetic or biomaterial-based are far less advanced and poorly reproduce the natural cell microenvironment. A viable strategy to generate materials with advanced properties combines different materials into nanocomposites. This work describes nanocomposites of a synthetic fibrous hydrogel, based on polyisocyanide (PIC), that is noncovalently linked to a responsive cross-linker. The introduction of the cross-linker transforms the PIC gel from a static fibrous extracellular matrix mimic to a highly dynamic material that maintains biocompatibility, as demonstrated by in situ modification of the (non)linear mechanical properties and efficient self-healing properties. Key in the material design is cross-linking at the fibrillar level using nanoparticles, which, simultaneously may be used to introduce more advanced properties.

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.

Novel Synthetic Polymer-Based 3D Contraction Assay: A Versatile Preclinical Research Platform for Fibrosis

The driving factors causing fibrosis and scar formation include fibroblast differentiation into myofibroblasts and hampered myofibroblast apoptosis, which ultimately results in collagen accumulation and tissue contraction. Currently, only very few drugs are available for fibrosis treatment, and there is an urgent demand for new pharmaceutical products. High-throughput in vitro fibrosis models are necessary to develop such drugs. In this study, we developed such a novel model based on synthetic polyisocyanide (PIC-RGD) hydrogels. The model not only measures contraction but also allows for subsequent molecular and cellular analysis. Fibroblasts were seeded in small (10 μL) PIC-RGD gels in the absence or presence of TGFβ1, the latter to induce myofibroblast differentiation. The contraction model clearly differentiates fibroblasts and myofibroblasts. Besides a stronger contraction, we also observed α-smooth muscle actin (αSMA) production and higher collagen deposition for the latter. The results were supported by mRNA expression experiments of αSMA, Col1α1, P53, and Ki67. As proof of principle, the effects of FDA-approved antifibrotic drugs nintedanib and pirfenidone were tested in our newly developed fibrosis model. Both drugs clearly reduce myofibroblast-induced contraction. Moreover, both drugs significantly decrease myofibroblast viability. Our low-volume synthetic PIC-RGD hydrogel platform is an attractive tool for high-throughput in vitro antifibrotic drug screening.

A Temperature-Based Easy-Separable (TempEasy) 3D Hydrogel Coculture System

Interactions between different cell types are crucial for their behavior in tissues, but are rarely considered in 3D in vitro cell culture experiments. One reason is that such coculture experiments are sometimes difficult to perform in 3D or require specialized equipment or know-how. Here, a new 3D cell coculture system is introduced, TempEasy, which is readily applied in any cell culture lab. The matrix material is based on polyisocyanide hydrogels, which closely resemble the mechanical characteristics of the natural extracellular matrix. Gels with different gelation temperatures, seeded with different cells, are placed on top of each other to form an indirect coculture. Cooling reverses gelation, allowing cell harvesting from each layer separately, which benefits downstream analysis. To demonstrate the potential of TempEasy , human adipose stem cells (hADSCs) with vaginal epithelial fibroblasts are cocultured. The analysis of a 7-day coculture shows that hADSCs promote cell-cell interaction of fibroblasts, while fibroblasts promote proliferation and differentiation of hADSCs. TempEasy provides a straightforward operational platform for indirect cocultures of cells of different lineages in well-defined microenvironments.

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.

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.

Structure and Dynamics of a Temperature-Sensitive Hydrogel

Polyisocyanotripeptides (TriPIC) are biomimetic polymers which consist of a β-helical backbone stabilized by hydrogen bonds between amide groups. Their oligoethylene glycol side chains give aqueous TriPIC solutions a thermoresponsive behavior: at 50 °C the solution becomes a hydrogel. In this paper we study the molecular structure and water dynamics of TriPIC aqueous solutions while undergoing gelation using FT-IR spectroscopy and polarization-resolved femtosecond infrared spectroscopy (fs-IR). We find evidence that the oligoethylene glycol side chains trap part of the water molecules upon gel formation, and we propose that the interaction between the oligoethylene glycol side chains and water plays an essential role in the bundling of the polymers and thus in the formation of a hydrogel.

Multivalent Sgc8c-aptamer decorated polymer scaffolds for leukemia targeting

T-cell acute lymphoblastic leukemia causes a disproportional amount of immature white blood cells in the patients' bone marrow. The significant undesired side effects associated with traditional chemotherapy treatment prompted us to study a more effective treatment strategy. We decorated polyisocyanopeptide scaffolds with the selective leukemia cell binding aptamer sgc8c and found that the polymers inhibit proliferation by G0/G1-phase arrest, serving as an opportunity for future therapeutic strategies.

Tunable Hybrid Matrices Drive Epithelial Morphogenesis and YAP Translocation

Morphogenesis is a tightly-regulated developmental process by which tissues acquire the morphology that is critical to their function. For example, epithelial cells exhibit different 2D and 3D morphologies, induced by distinct biochemical and biophysical cues from their environment. In this work, novel hybrid matrices composed of a Matrigel and synthetic oligo(ethylene glycol)-grafted polyisocyanides (PICs) hydrogels are used to form a highly tailorable environment. Through precise control of the stiffness and cell-matrix interactions, while keeping other properties constant, a broad range of morphologies induced in Madin-Darby Canine Kidney (MDCK) cells is observed. At relatively low matrix stiffness, a large morphological shift from round hollow cysts to 2D monolayers is observed, without concomitant translocation of the mechanotransduction protein Yes-associated protein (YAP). At higher stiffness levels and enhanced cell-matrix interactions, tuned by controlling the adhesive peptide density on PIC, the hybrid hydrogels induce a flattened cell morphology with simultaneous YAP translocation, suggesting activation. In 3D cultures, the latter matrices lead to the formation of tubular structures. Thus, mixed synthetic and natural gels, such as the hybrids presented here, are ideal platforms to dissect how external physical factors can be used to regulate morphogenesis in MDCK model system, and in the future, in more complex environments.

Semiflexible polymer scaffolds: an overview of conjugation strategies

Semiflexible polymers are excellent scaffolds for the presentation of a wide variety of (bio)molecules. This manuscript reviews advantages and challenges of the most common conjugation strategies for the major classes of semiflexible polymers.

Synthetic Extracellular Matrices as a Toolbox to Tune Stem Cell Secretome

The application of stem cell-derived secretome in regenerative therapies offers the key advantage that instead of the stem cells, only their effective paracrine compounds are in vivo delivered. Ideally, the secretome can be steered by the culture conditions of the stem cells. So far, most studies use stem cells cultured on stiff plastic substrates, not representative of their native 3D environment. In this study, cells are cultured inside synthetic polyisocyanide (PIC)-based hydrogels, which are minimal, tailorable, and highly reproducible biomimetic matrices. Secretome analysis of human adipose-derived stem cells (multiplex, ELISA) displays that matrix manipulation is a powerful tool to direct the secretome composition. As an example, cells in nonadherent PIC gels secrete increased levels of IL-10 and the conditioned media from 3D culture accelerate wound closure. In all, our PIC-based approach opens the door to dedicated matrix design to engineer the secretome for custom applications.

Antimicrobial and anti-inflammatory thermo-reversible hydrogel for periodontal delivery

In periodontal treatment, topical adjunctive therapy with antimicrobials or anti-inflammatory agents is frequently applied. However, currently available drug carrier biomaterials often exhibit poor perfusion into small crevices, such as the deep and irregular periodontal pockets, due to relatively high viscosity. Moreover, high polymer concentrations of the polymer can potentially be cytotoxic upon confined local administration. This study aimed to formulate an antimicrobial and anti-inflammatory treatment option, by incorporating doxycycline (DOX) and/or lipoxin A₄ (LXA₄) into 0.5 wt% thermo-reversible polyisocyanopeptide (PIC). PIC can form hydrogels upon low polymer concentration, and we hypothesized that the thermo-reversible nature of the material would allow for application into the periodontal pocket. The formulations were characterized in vitro and finally tested in dogs with naturally occurring periodontitis, which were not euthanized afterward. Results showed that PIC/DOX/LXA₄ hydrogel could be easily prepared and injected into periodontal pockets. The PIC hydrogel facilitated the release of DOX or LXA₄ for around 4 days in vitro. When applied in dogs, the hydrogel exerted no local or systemic adverse effects. Gels loaded with LXA₄ and/or DOX reduced the subgingival bacterial load and pro-inflammatory interleukin-8 level. In addition, PIC-DOX and PIC-DOX+LXA₄ improved gingival clinical attachment by 0.6 mm compared with conventional periodontal treatment alone (i.e. mechanical debridement). In conclusion, the thermo-reversible PIC hydrogel is a safe and effective vehicle for periodontal drug delivery.

Polyisocyanide Hydrogels as a Tunable Platform for Mammary Gland Organoid Formation

In the last decade, organoid technology has developed as a primary research tool in basic biological and clinical research. The reliance on poorly defined animal-derived extracellular matrix, however, severely limits its application in regenerative and translational medicine. Here, a well-defined, synthetic biomimetic matrix based on polyisocyanide (PIC) hydrogels that support efficient and reproducible formation of mammary gland organoids (MGOs) in vitro is presented. Only decorated with the adhesive peptide RGD for cell binding, PIC hydrogels allow MGO formation from mammary fragments or from purified single mammary epithelial cells. The cystic organoids maintain their capacity to branch for over two months, which is a fundamental and complex feature during mammary gland development. It is found that small variations in the 3D matrix give rise to large changes in the MGO: the ratio of the main cell types in the MGO is controlled by the cell-gel interactions via the cell binding peptide density, whereas gel stiffness controls colony formation efficiency, which is indicative of the progenitor density. Simple hydrogel modifications will allow for future introduction and customization of new biophysical and biochemical parameters, making the PIC platform an ideal matrix for in depth studies into organ development and for application in disease models.

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.

Polyisocyanopeptide Hydrogels Are Effectively Sterilized Using Supercritical Carbon Dioxide

Adequate sterilization procedures for soft biomaterials such as hydrogels are known to be challenging. These materials are delicate in structure, making them sensitive to harsh conditions and prone to damage. In this study, a suitable sterilization method for hydrogels composed of tri(ethylene glycol)-functionalized polyisocyanopeptides (PIC) was explored. These high biomimetic hydrogels are temperature and strain sensitive and have been presented as novel cell culturing matrices, wound dressings, and drug carriers. The methods that were investigated include autoclaving, γ-irradiation, ultraviolet (UV) light irradiation, and supercritical CO₂ (scCO₂) treatment. The results show that autoclaving and γ-irradiation have deleterious effects on the gelation behavior and mechanical characteristics of PIC. For γ-irradiation, cooling the gels on dry ice alleviated this negative impact, but not sufficiently enough to make the method viable. In contrast, UV light and scCO₂ treatment do not affect the mechanical properties of the PIC gels. Studies with gels inoculated with 10⁷ CFU/mL Gram-positive bacteria Staphylococcus aureus show that only scCO₂ is capable of successfully sterilizing PIC hydrogels by achieving a 6-log reduction in bacterial load. It was concluded that, within the range of tested techniques, the sterilization of PIC is limited to scCO₂.

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.

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.

Thermosensitive biomimetic polyisocyanopeptide hydrogels may facilitate wound repair

Changing wound dressings inflicts pain and may disrupt wound repair. Novel synthetic thermosensitive hydrogels based on polyisocyanopeptide (PIC) offer a solution. These gels are liquid below 16 °C and form gels beyond room temperature. The architecture and mechanical properties of PIC gels closely resemble collagen and fibrin, and include the characteristic stiffening response at high strains. Considering the reversible thermo-responsive behavior, we postulate that PIC gels are easy to apply and remove, and facilitate healing without eliciting foreign body responses or excessive inflammation. Biocompatibility may be higher in RGD-peptide-functionalized PIC gels due to enhanced cell binding capabilities. Full-thickness dorsal skin wounds in mice were compared to wounds treated with PIC gel and PIC-RGD gel for 3 and 7 days. No foreign body reactions and similar wound closure rates were found in all groups. The level of macrophages, myofibroblasts, epithelial migration, collagen expression, and blood vessels did not significantly differ from controls. Surprisingly, granulocyte populations in the wound decreased significantly in the PIC gel-treated groups, likely because foreign bacteria could not penetrate the gel. RGD-peptides did not further improve any effect observed for PIC. The absence of adverse effects, ease of application, and the possibilities for bio-functionalization make the biomimetic PIC hydrogels suitable for development into wound dressings.

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.

Strategies To Increase the Thermal Stability of Truly Biomimetic Hydrogels: Combining Hydrophobicity and Directed Hydrogen Bonding

Enhancing the thermal stability of proteins is an important task for protein engineering. There are several ways to increase the thermal stability of proteins in biology, such as greater hydrophobic interactions, increased helical content, decreased occurrence of thermolabile residues, or stable hydrogen bonds. Here, we describe a well-defined polymer based on β-helical polyisocyanotripeptides (TriPIC) that uses biological approaches, including hydrogen bonding and hydrophobic interactions for its exceptional thermal stability in aqueous solutions. The multiple hydrogen bonding arrays along the polymer backbone shield the hydrophobic core from water. Variable temperature CD and FTIR studies indicate that, on heating, a better packed polymer conformation further stiffens the backbone. Driven by hydrophobic interactions, TriPIC solutions give fully reversible hydrogels that can withstand high temperatures (80 °C) for extended times. Cryo-scanning electron microscopy (cryo-SEM), small-angle X-ray scattering (SAXS), and thorough rheological analysis show that the hydrogel has a bundled architecture, which gives rise to strain stiffening effects on deformation of the gel, analogous to many biological hydrogels.

Self-healing hydrogels formed by complexation between calcium ions and bisphosphonate-functionalized star-shaped polymers

Star-shaped poly(ethylene glycol) (PEG) chain termini were functionalized with alendronate to create transient networks with reversible crosslinks upon addition of calcium ions. The gelation ability of alendronate-functionalized PEG was greatly dependent on the number of arms and arm molecular weight. After mixing polymer and calcium solutions, the formed hydrogels could be cut and then brought back together without any visible interface. After 2 minutes of contact, their connection was strong enough to allow for stretching without tearing through the previous fracture surface. Oscillatory rheology showed that the hydrogels recovered between 70 and 100% of the original storage and loss modulus after rupture. Frequency sweep measurements revealed a liquid-like behavior at lower frequencies and solid-like at high frequencies. Shifting frequency curves obtained at different calcium and polymer concentrations, all data collapsed in a single common master curve. This time-concentration superposition reveals a common relaxation mechanism intrinsically connected to the calcium-bisphosphonate complexation equilibrium.

Nonlinear mechanics of hybrid polymer networks that mimic the complex mechanical environment of cells

The mechanical properties of cells and the extracellular environment they reside in are governed by a complex interplay of biopolymers. These biopolymers, which possess a wide range of stiffnesses, self-assemble into fibrous composite networks such as the cytoskeleton and extracellular matrix. They interact with each other both physically and chemically to create a highly responsive and adaptive mechanical environment that stiffens when stressed or strained. Here we show that hybrid networks of a synthetic mimic of biological networks and either stiff, flexible and semi-flexible components, even very low concentrations of these added components, strongly affect the network stiffness and/or its strain-responsive character. The stiffness (persistence length) of the second network, its concentration and the interaction between the components are all parameters that can be used to tune the mechanics of the hybrids. The equivalence of these hybrids with biological composites is striking.

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

Liquid crystal templating: an emerging technique to organise and control soft matter at multiple length scales.

Muscovite mica as a growth template of PC61BM crystallites for organic photovoltaics

The processing conditions for and (local) performance benefits of epitaxially crystallized PC₆₁BM for organic solar cells are investigated.

Synthesis of Functional Fluorescent BODIPY-based Dyes through Electrophilic Aromatic Substitution: Straightforward Approach towards Customized Fluorescent Probes

Fluorescent materials are widely used in biological and material applications as probes for imaging or sensing; however, their customization is usually complicated without the support of an organic chemistry laboratory. Here, we present a straightforward method for the customization of BODIPY cores, which are among the most commonly used fluorescent probes. The method is based on the formation of a new C-C bond through Friedel-Crafts electrophilic aromatic substitution carried out at room temperature. The method presented can be used to obtain completely customized fluorescent materials in one or two steps from commercially available compounds. Examples of the preparation of fluorescent materials for cell staining and functionalization of silica colloids are also presented.

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.

Critical behaviour in the nonlinear elastic response of hydrogels

In this paper we study the elastic response of synthetic hydrogels to an applied shear stress. The hydrogels studied here have previously been shown to mimic the behaviour of biopolymer networks when they are sufficiently far above the gel point. We show that near the gel point they exhibit an elastic response that is consistent with the predicted critical behaviour of networks near or below the isostatic point of marginal stability. This point separates rigid and floppy states, distinguished by the presence or absence of finite linear elastic moduli. Recent theoretical work has also focused on the response of such networks to finite or large deformations, both near and below the isostatic point. Despite this interest, experimental evidence for the existence of criticality in such networks has been lacking. Using computer simulations, we identify critical signatures in the mechanical response of sub-isostatic networks as a function of applied shear stress. We also present experimental evidence consistent with these predictions. Furthermore, our results show the existence of two distinct critical regimes, one of which arises from the nonlinear stretch response of semi-flexible polymers.

Directing Soft Matter in Water Using Electric Fields

Directing the spatial organization of functional supramolecular and polymeric materials at larger length scales is essential for many biological and molecular optoelectronic applications. Although the application of electrical fields is one of the most powerful approaches to induce spatial control, it is rarely applied experimentally in aqueous solutions, since the low susceptibility of soft and biological materials requires the use of high fields, which leads to parasitic heating and electrochemical degradation. In this work, we demonstrate that we can apply electric fields when we use a mineral liquid crystal as a responsive template. Besides aligning and positioning functional soft matter, we show that the concentration of the liquid crystal template controls the morphology of the assembly. As our setup is very easy to operate and our approach lacks specific molecular interactions, we believe it will be applicable for a wide range of (aqueous) materials.

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?

Solid-state NMR characterization of tri-ethyleneglycol grafted polyisocyanopeptides

In aqueous media, ethylene glycol substituted polyisocyanopeptides (PICPs) change their state (undergo a sol-to-gel transition) as a response to temperature. This makes them promising materials for various biomedical applications, for instance, for controlled drug release and non-damaging wound dressing. To utilize PICP in biomedical applications, understanding of the origin of the gelation process is needed, but this is experimentally difficult because of the notoriously low gelator concentration in combination with the slow polymer dynamics in the sample. This paper describes a detailed characterization of the dried state of PICPs by solid-state NMR measurements. Both the (13) C and the (1) H NMR resonances were assigned using a combination of 1D cross-polarization magic angle spinning, 2D (13) C-(1) H heteronuclear correlation spectra and (1) H-(1) H single quantum-double quantum experiments. In addition, the chemical groups involved in dipolar interaction with each other were used to discuss the dynamics and spatial conformation of the polymer. In contrast to other PICP polymers, two resonances for the backbone carbon are observed, which are present in equal amounts. The possible origin of these resonances is discussed in the last section of this work. The data obtained during the current studies will be further used in elucidating mechanisms of the bundling and gelation. A comprehensive picture will make it possible to tailor polymer properties to meet specific needs in different applications. Copyright © 2015 John Wiley & Sons, Ltd.

Unusual temperature dependence of elastic constants of an ambient-temperature discotic nematic liquid crystal

We report the first experimental studies on the temperature dependence of viscoelastic properties of a room temperature discotic nematic liquid crystal. The splay elastic constant is greater than the bend elastic constant and both show unusual temperature and order parameter dependence. The rotational viscosity is remarkably larger than conventional calamitic liquid crystals. We provide a simple physical explanation based on the columnar short-range order to account for the the unusual temperature dependence of the elastic constants.

Fibrin-fiber architecture influences cell spreading and differentiation

The mechanical and structural properties of the extracellular matrix (ECM) play an important role in regulating cell fate. The natural ECM has a complex fibrillar structure and shows nonlinear mechanical properties, which are both difficult to mimic synthetically. Therefore, systematically testing the influence of ECM properties on cellular behavior is very challenging. In this work we show two different approaches to tune the fibrillar structure and mechanical properties of fibrin hydrogels. Addition of extra thrombin before gelation increases the protein density within the fibrin fibers without significantly altering the mechanical properties of the resulting hydrogel. On the other hand, by forming a composite hydrogel with a synthetic biomimetic polyisocyanide network the protein density within the fibrin fibers decreases, and the mechanics of the composite material can be tuned by the PIC/fibrin mass ratio. The effect of the changes in gel structure and mechanics on cellular behavior are investigated, by studying human mesenchymal stem cell (hMSC) spreading and differentiation on these gels. We find that the trends observed in cell spreading and differentiation cannot be explained by the bulk mechanics of the gels, but correlate to the density of the fibrin fibers the gels are composed of. These findings strongly suggest that the microscopic properties of individual fibers in fibrous networks play an essential role in determining cell behavior.

Ultra-responsive soft matter from strain-stiffening hydrogels

The stiffness of hydrogels is crucial for their application. Nature's hydrogels become stiffer as they are strained. This stiffness is not constant but increases when the gel is strained. This stiffening is used, for instance, by cells that actively strain their environment to modulate their function. When optimized, such strain-stiffening materials become extremely sensitive and very responsive to stress. Strain stiffening, however, is unexplored in synthetic gels since the structural design parameters are unknown. Here we uncover how readily tuneable parameters such as concentration, temperature and polymer length impact the stiffening behaviour. Our work also reveals the marginal point, a well-described but never observed, critical point in the gelation process. Around this point, we observe a transition from a low-viscous liquid to an elastic gel upon applying minute stresses. Our experimental work in combination with network theory yields universal design principles for future strain-stiffening materials.

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.