Biomimetic Calcium Phosphate Mineralization with Multifunctional Elastin-Like Recombinamers

Quantitative Real-Time Analysis of Living Materials by Stimulated Raman Scattering Microscopy

Composite materials built in part from living organisms have the potential to exhibit useful autonomous, adaptive, and self-healing behavior. The physicochemical, biological, and mechanical properties of such materials can be engineered through the genetic manipulation of their living components. Successful development of living materials will require not only new methods for design and preparation but also new analytical tools that are capable of real-time noninvasive mapping of chemical compositions. Here, we establish a strategy based on stimulated Raman scattering microscopy to monitor phosphatase-catalyzed mineralization of engineered bacterial films in situ. Real-time label-free imaging elucidates the mineralization process, quantifies both the organic and inorganic components of the material as functions of time, and reveals spatial heterogeneity at multiple scales. In addition, we correlate the mechanical performance of films with the extent of mineralization. This work introduces a promising strategy for quantitatively analyzing living materials, which should contribute to the accelerated development of such materials in the future.

An Elastin-Derived Composite Matrix for Enhanced Vascularized and Innervated Bone Tissue Reconstruction: From Material Development to Preclinical Evaluation

Despite progress in bone tissue engineering, reconstruction of large bone defects remains an important clinical challenge. Here, a biomaterial designed to recruit bone cells, endothelial cells, and neuronal fibers within the same matrix is developed, enabling bone tissue regeneration. The bioactive matrix is based on modified elastin-like polypeptides (ELPs) grafted with laminin-derived adhesion peptides IKVAV and YIGSR, and the SNA15 peptide for retention of hydroxyapatite (HA) particles. The composite matrix shows suitable porosity, interconnectivity, biocompatibility for endothelial cells, and the ability to support neurites outgrowth by sensory neurons. Subcutaneous implantation leads to the formation of osteoid tissue, characterized by the presence of bone cells, vascular networks, and neuronal structures, while minimizing inflammation. Using a rat femoral condyle defect model, longitudinal micro-CT analysis is performed, which demonstrates a significant increase in the volume of mineralized tissue when using the ELP-based matrix compared to empty defects and a commercially available control (Collapat). Furthermore, visible blood vessel networks and nerve fibers are observed within the lesions after a period of two weeks. By incorporating multiple key components that support cell growth, mineralization, and tissue integration, this ELP-based composite matrix provides a holistic and versatile solution to enhance bone tissue regeneration.

Contribution of the ELRs to the development of advanced in vitro models

Developing in vitro models that accurately mimic the microenvironment of biological structures or processes holds substantial promise for gaining insights into specific biological functions. In the field of tissue engineering and regenerative medicine, in vitro models able to capture the precise structural, topographical, and functional complexity of living tissues, prove to be valuable tools for comprehending disease mechanisms, assessing drug responses, and serving as alternatives or complements to animal testing. The choice of the right biomaterial and fabrication technique for the development of these in vitro models plays an important role in their functionality. In this sense, elastin-like recombinamers (ELRs) have emerged as an important tool for the fabrication of in vitro models overcoming the challenges encountered in natural and synthetic materials due to their intrinsic properties, such as phase transition behavior, tunable biological properties, viscoelasticity, and easy processability. In this review article, we will delve into the use of ELRs for molecular models of intrinsically disordered proteins (IDPs), as well as for the development of in vitro 3D models for regenerative medicine. The easy processability of the ELRs and their rational design has allowed their use for the development of spheroids and organoids, or bioinks for 3D bioprinting. Thus, incorporating ELRs into the toolkit of biomaterials used for the fabrication of in vitro models, represents a transformative step forward in improving the accuracy, efficiency, and functionality of these models, and opening up a wide range of possibilities in combination with advanced biofabrication techniques that remains to be explored.

Transcription-Factor-Induced Aggregation of Biomimetic Oligonucleotide-b-Protein Micelles

Polymeric micelles and especially those based on natural diblocks are of particular interest due to their advantageous properties in terms of molecular recognition, biocompatibility, and biodegradability. We herein report a facile and straightforward synthesis of thermoresponsive elastin-like polypeptide (ELP) and oligonucleotide (ON) diblock bioconjugates, ON-b-ELP, through copper-catalyzed azide-alkyne cycloaddition. The resulting thermosensitive diblock copolymer self-assembles above its critical micelle temperature (CMT ∼30 °C) to form colloidally stable micelles of ∼50 nm diameter. The ON-b-ELP micelles hybridize with an ON complementary strand and maintain their size and stability. Next, we describe the capacity of these micelles to bind proteins, creating more complex structures using the classic biotin-streptavidin pairing and the specific recognition between a transcription factor protein and the ON strand. In both instances, the micelles are intact, form larger structures, and retain their sensitivity to temperature.

Identification of the First Sulfobetaine Hydrogel-Binding Peptides via Phage Display Assay

Using the M13 phage display, a series of 7- and 12-mer peptides which interact with new sulfobetaine hydrogels are identified. Two peptides each from the 7- and 12-mer peptide libraries bind to the new sulfobetaine hydrogels with high affinity compared to the wild-type phage lacking a dedicated hydrogel binding peptide. This is the first report of peptides binding to zwitterionic sulfobetaine hydrogels and the study therefore opens up the pathway toward new phage or peptide/hydrogel hybrids with high application potential.

Emerging opportunities in bioconjugates of Elastin-like polypeptides with synthetic or natural polymers

Nature is an everlasting source of inspiration for chemical and polymer scientists seeking to develop ever more innovative materials with greater performances. Natural structural proteins are particularly scrutinized to design biomimetic materials. Often characterized by repeat peptide sequences, that together interact by inter- and intramolecular interactions and form a 3D skeleton, they contribute to the mechanical properties of individual cells, tissues, organs, and whole organisms. (Numata, K. Polymer Journal 2020, 52, 1043-1056) Among them elastin, and its main repeat sequences, have been a source of intense studies for more than 50 years resulting in the specific research field dedicated to elastin-like polypeptides (ELPs). These are currently widely investigated in different applications, namely protein purification, tissue engineering, and drug delivery, and some technologies based on ELPs are currently explored by several start-up companies. In the present review, we have summarized pioneering contributions on ELPs, progress made in their genetic engineering, and understanding of their thermal behavior and self-assembly properties. Considered as intrinsically disordered protein polymers, we have finally focused on the works where ELPs have been conjugated to other synthetic macromolecules as covalent hybrid, statistical, graft, or block copolymers, highlighting the huge opportunities that have still not been explored so far.

Design of 3D Scaffolds for Hard Tissue Engineering: From Apatites to Silicon Mesoporous Materials

Advanced bioceramics for bone regeneration constitutes one of the pivotal interests in the multidisciplinary and far-sighted scientific trajectory of Prof. Vallet Regí. The different pathologies that affect osseous tissue substitution are considered to be one of the most important challenges from the health, social and economic point of view. 3D scaffolds based on bioceramics that mimic the composition, environment, microstructure and pore architecture of hard tissues is a consolidated response to such concerns. This review describes not only the different types of materials utilized: from apatite-type to silicon mesoporous materials, but also the fabrication techniques employed to design and adequate microstructure, a hierarchical porosity (from nano to macro scale), a cell-friendly surface; the inclusion of different type of biomolecules, drugs or cells within these scaffolds and the influence on their successful performance is thoughtfully reviewed.

Advanced non-fluoride approaches to dental enamel remineralization: The next level in enamel repair management

In modern dentistry, a minimally invasive management of early caries lesions or early-stage erosive tooth wear (ETW) with synthetic remineralization systems has become indispensable. In addition to fluoride, which is still the non-plus-ultra in these early caries/ETW treatments, a number of new developments are in the test phase or have already been commercialized. Some of these systems claim that they are comparable or even superior to fluoride in terms of their ability to remineralize enamel. Besides, their use can help avoid some of the risks associated with fluoride and support treatments of patients with a high risk of caries. Two individual non-fluoride systems can be distinguished; intrinsic and extrinsic remineralization approaches. Intrinsic (protein/peptide) systems adsorb to hydroxyapatite crystals/organics located within enamel prisms and accumulate endogenous calcium and phosphate ions from saliva, which ultimately leads to the re-growth of enamel crystals. Extrinsic remineralization systems function on the basis of the external (non-saliva) supply of calcium and phosphate to the crystals to be re-grown. This article, following an introduction into enamel (re)mineralization and fluoride-assisted remineralization, discusses the requirements for non-fluoride remineralization systems, particularly their mechanisms and challenges, and summarizes the findings that underpin the most promising advances in enamel remineralization therapy.

Functional biomedical materials derived from proteins in the acquired salivary pellicle

In the oral environment, the acquired salivary pellicle (ASP) on the tooth surface comprises proteins, glycoproteins, carbohydrates, and lipids. The ASP can specifically and rapidly adsorb on the enamel surface to provide effective lubrication, protection, hydration, and remineralisation, as well as be recognised by various bacteria to form a microbial biofilm (plaque). The involved proteins, particularly various phosphoproteins such as statherins, histatins, and proline-rich proteins, are vital to their specific functions. This review first describes the relationship between the biological functions of these proteins and their structures. Subsequently, recent advances in functional biomedical materials derived from these proteins are reviewed in terms of dental/bone therapeutic materials, antibacterial materials, tissue engineering materials, and coatings for medical devices. Finally, perspectives and challenges regarding the rational design and biomedical applications of ASP-derived materials are discussed.

Structure-Property Relationships of Elastin-like Polypeptides: A Review of Experimental and Computational Studies

Elastin is a structural protein with outstanding mechanical properties (e.g., elasticity and resilience) and biologically relevant functions (e.g., triggering responses like cell adhesion or chemotaxis). It is formed from its precursor tropoelastin, a 60-72 kDa water-soluble and temperature-responsive protein that coacervates at physiological temperature, undergoing a phenomenon termed lower critical solution temperature (LCST). Inspired by this behavior, many scientists and engineers are developing recombinantly produced elastin-inspired biopolymers, usually termed elastin-like polypeptides (ELPs). These ELPs are generally comprised of repetitive motifs with the sequence VPGXG, which corresponds to repeats of a small part of the tropoelastin sequence, X being any amino acid except proline. ELPs display LCST and mechanical properties similar to tropoelastin, which renders them promising candidates for the development of elastic and stimuli-responsive protein-based materials. Unveiling the structure-property relationships of ELPs can aid in the development of these materials by establishing the connections between the ELP amino acid sequence and the macroscopic properties of the materials. Here we present a review of the structure-property relationships of ELPs and ELP-based materials, with a focus on LCST and mechanical properties and how experimental and computational studies have aided in their understanding.

The Significance and Utilisation of Biomimetic and Bioinspired Strategies in the Field of Biomedical Material Engineering: The Case of Calcium Phosphat-Protein Template Constructs

This review provides a summary of recent research on biomimetic and bioinspired strategies applied in the field of biomedical material engineering and focusing particularly on calcium phosphate-protein template constructs inspired by biomineralisation. A description of and discussion on the biomineralisation process is followed by a general summary of the application of the biomimetic and bioinspired strategies in the fields of biomedical material engineering and regenerative medicine. Particular attention is devoted to the description of individual peptides and proteins that serve as templates for the biomimetic mineralisation of calcium phosphate. Moreover, the review also presents a description of smart devices including delivery systems and constructs with specific functions. The paper concludes with a summary of and discussion on potential future developments in this field.

Biopolymer-based strategies in the design of smart medical devices and artificial organs

Advances in regenerative medicine and in modern biomedical therapies are fast evolving and set goals causing an upheaval in the field of materials science. This review discusses recent developments involving the use of biopolymers as smart materials, in terms of material properties and stimulus-responsive behavior, in the presence of environmental physico-chemical changes. An overview on the transformations that can be triggered in natural-based polymeric systems (sol-gel transition, polymer relaxation, cross-linking, and swelling) is presented, with specific focus on the benefits these materials can provide in biomedical applications.

Human adipose derived stem cells are superior to human osteoblasts (HOB) in bone tissue engineering on a collagen-fibroin-ELR blend

The ultrastructure of the bone provides a unique mechanical strength against compressive, torsional and tensional stresses. An elastin-like recombinamer (ELR) with a nucleation sequence for hydroxyapatite was incorporated into films prepared from a collagen - silk fibroin blend carrying microchannel patterns to stimulate anisotropic osteogenesis. SEM and fluorescence microscopy showed the alignment of adipose-derived stem cells (ADSCs) and the human osteoblasts (HOBs) on the ridges and in the grooves of microchannel patterned collagen-fibroin-ELR blend films. The Young's modulus and the ultimate tensile strength (UTS) of untreated films were 0.58 ± 0.13 MPa and 0.18 ± 0.05 MPa, respectively. After 28 days of cell culture, ADSC seeded film had a Young's modulus of 1.21 ± 0.42 MPa and UTS of 0.32 ± 0.15 MPa which were about 3 fold higher than HOB seeded films. The difference in Young's modulus was statistically significant (p: 0.02). ADSCs attached, proliferated and mineralized better than the HOBs. In the light of these results, ADSCs served as a better cell source than HOBs for bone tissue engineering of collagen-fibroin-ELR based constructs used in this study. We have thus shown the enhancement in the tensile mechanical properties of the bone tissue engineered scaffolds by using ADSCs.

Recombinant DNA technology and click chemistry: a powerful combination for generating a hybrid elastin-like-statherin hydrogel to control calcium phosphate mineralization

Understanding the mechanisms responsible for generating different phases and morphologies of calcium phosphate by elastin-like recombinamers is supreme for bioengineering of advanced multifunctional materials. The generation of such multifunctional hybrid materials depends on the properties of their counterparts and the way in which they are assembled. The success of this assembly depends on the different approaches used, such as recombinant DNA technology and click chemistry. In the present work, an elastin-like recombinamer bearing lysine amino acids distributed along the recombinamer chain has been cross-linked via Huisgen [2 + 3] cycloaddition. The recombinamer contains the SNA15 peptide domains inspired by salivary statherin, a peptide epitope known to specifically bind to and nucleate calcium phosphate. The benefit of using click chemistry is that the hybrid elastin-like-statherin recombinamers cross-link without losing their fibrillar structure. Mineralization of the resulting hybrid elastin-like-statherin recombinamer hydrogels with calcium phosphate is described. Thus, two different hydroxyapatite morphologies (cauliflower- and plate-like) have been formed. Overall, this study shows that crosslinking elastin-like recombinamers leads to interesting matrix materials for the generation of calcium phosphate composites with potential applications as biomaterials.

Intrafibrillar Mineralization of Self-Assembled Elastin-Like Recombinamer Fibrils

Biomineralization of bone, a controlled process where hydroxyapatite nanocrystals preferentially deposit in collagen fibrils, is achieved by the interplay of the collagen matrix and noncollagenous proteins. Mimicking intrafibrillar mineralization in synthetic systems is highly attractive for the development of advanced hybrid materials with elaborated morphologies and outstanding mechanical properties, as well as understanding the mechanisms of biomineralization. Inspired by nature, intrafibrillar mineralization of collagen fibrils has been successfully replicated in vitro via biomimetic systems, where acidic polymeric additives are used as analogue of noncollagenous proteins in mediating mineralization. The development of synthetic templates that mimic the structure and functions of collagenous matrix in mineralization has yet to be explored. In this study, we demonstrated that self-assembled fibrils of elastin-like recombinamers (ELRs) can induce intrafibrillar mineralization. The ELRs displayed a disordered structure at low temperature but self-assembled into nanofibrils above its inverse transition temperature. In the presence of the self-assembled ELR fibrils, polyaspartate-stabilized amorphous calcium phosphates preferentially infiltrated into the fibrils and then crystallized into hydroxyapatite nanocrystals with their [001] axes aligned parallel to the long axis of the ELR fibril. As the recombinant technology enables designing and producing well-defined ELRs, their molecular and structural properties can be fine-tuned. By examining the ultrastructure of the self-assembled ELRs fibrils as well as their mineralization, we concluded that the spatial confinement formed by a continuum β-spiral structure in an unperturbed fibrillar structure rather than electrostatic interactions or bioactive sequences in the recombinamer composition played the crucial role in inducing intrafibrillar mineralization.

3D silicon doped hydroxyapatite scaffolds decorated with Elastin-like Recombinamers for bone regenerative medicine

The current study reports on the manufacturing by rapid prototyping technique of three-dimensional (3D) scaffolds based on silicon substituted hydroxyapatite with Elastin-like Recombinamers (ELRs) functionalized surfaces. Silicon doped hydroxyapatite (Si-HA), with Ca₁₀(PO₄)_5.7(SiO₄)_0.3(OH)_1.7h_0.3 nominal formula, was surface functionalized with two different types of polymers designed by genetic engineering: ELR-RGD that contain cell attachment specific sequences and ELR-SN_A15/RGD with both hydroxyapatite and cells domains that interact with the inorganic phase and with the cells, respectively. These hybrid materials were subjected to in vitro assays in order to clarify if the ELRs coating improved the well-known biocompatible and bone regeneration properties of calcium phosphates materials. The in vitro tests showed that there was a total and homogeneous colonization of the 3D scaffolds by Bone marrow Mesenchymal Stromal Cells (BMSCs). In addition, the BMSCs were viable and able to proliferate and differentiate into osteoblasts.

STATEMENT OF SIGNIFICANCE

Bone tissue engineering is an area of increasing interest because its main applications are directly related to the rising life expectancy of the population, which promotes higher rates of several bone pathologies, so innovative strategies are needed for bone tissue regeneration therapies. Here we use the rapid prototyping technology to allow moulding ceramic 3D scaffolds and we use different bio-polymers for the functionalization of their surfaces in order to enhance the biological response. Combining the ceramic material (silicon doped hydroxyapatite, Si-HA) and the Elastin like Recombinamers (ELRs) polymers with the presence of the integrin-mediate adhesion domain alone or in combination with SNA15 peptide that possess high affinity for hydroxyapatite, provided an improved Bone marrow Mesenchymal Stromal Cells (BMSCs) differentiation into osteoblastic linkage.

Engineered protein coatings to improve the osseointegration of dental and orthopaedic implants

Here we present the design of an engineered, elastin-like protein (ELP) that is chemically modified to enable stable coatings on the surfaces of titanium-based dental and orthopaedic implants by novel photocrosslinking and solution processing steps. The ELP includes an extended RGD sequence to confer bio-signaling and an elastin-like sequence for mechanical stability. ELP thin films were fabricated on cp-Ti and Ti6Al4V surfaces using scalable spin and dip coating processes with photoactive covalent crosslinking through a carbene insertion mechanism. The coatings withstood procedures mimicking dental screw and hip replacement stem implantations, a key metric for clinical translation. They promoted rapid adhesion of MG63 osteoblast-like cells, with over 80% adhesion after 24 h, compared to 38% adhesion on uncoated Ti6Al4V. MG63 cells produced significantly more mineralization on ELP coatings compared to uncoated Ti6Al4V. Human bone marrow mesenchymal stem cells (hMSCs) had an earlier increase in alkaline phosphatase activity, indicating more rapid osteogenic differentiation and mineral deposition on adhesive ELP coatings. Rat tibia and femur in vivo studies demonstrated that cell-adhesive ELP-coated implants increased bone-implant contact area and interfacial strength after one week. These results suggest that ELP coatings withstand surgical implantation and promote rapid osseointegration, enabling earlier implant loading and potentially preventing micromotion that leads to aseptic loosening and premature implant failure.

Formation of calcium phosphate nanostructures under the influence of self-assembling hybrid elastin-like-statherin recombinamers

The self-assembly properties of elastin-like-statherin recombinamers have great influence on calcium phosphate mineralization.

Biomimetic Mineralization of Recombinamer-Based Hydrogels toward Controlled Morphologies and High Mineral Density

The use of insoluble organic matrices as a structural template for the bottom-up fabrication of organic-inorganic nanocomposites is a powerful way to build a variety of advanced materials with defined and controlled morphologies and superior mechanical properties. Calcium phosphate mineralization in polymeric hydrogels is receiving significant attention in terms of obtaining biomimetic hierarchical structures with unique mechanical properties and understanding the mechanisms of the biomineralization process. However, integration of organic matrices with hydroxyapatite nanocrystals, different in morphology and composition, has not been well-achieved yet at nanoscale. In this study, we synthesized thermoresponsive hydrogels, composed of elastin-like recombinamers (ELRs), to template mineralization of hydroxyapatite nanocrystals using a biomimetic polymer-induced liquid-precursor (PILP) mineralization process. Different from conventional mineralization where minerals were deposited on the surface of organic matrices, they were infiltrated into the frameworks of ELR matrices, preserving their microporous structure. After 14 days of mineralization, an average of 78 μm mineralization depth was achieved. Mineral density up to 1.9 g/cm(3) was found after 28 days of mineralization, which is comparable to natural bone and dentin. In the dry state, the elastic modulus and hardness of the mineralized hydrogels were 20.3 ± 1.7 and 0.93 ± 0.07 GPa, respectively. After hydration, they were reduced to 4.50 ± 0.55 and 0.10 ± 0.03 GPa, respectively. These values were lower but still on the same order of magnitude as those of natural hard tissues. The results indicated that inorganic-organic hybrid biomaterials with controlled morphologies can be achieved using organic templates of ELRs. Notably, the chemical and physical properties of ELRs can be tuned, which might help elucidate the mechanisms by which living organisms regulate the mineralization process.

Fabricated Elastin

The mechanical stability, elasticity, inherent bioactivity, and self-assembly properties of elastin make it a highly attractive candidate for the fabrication of versatile biomaterials. The ability to engineer specific peptide sequences derived from elastin allows the precise control of these physicochemical and organizational characteristics, and further broadens the diversity of elastin-based applications. Elastin and elastin-like peptides can also be modified or blended with other natural or synthetic moieties, including peptides, proteins, polysaccharides, and polymers, to augment existing capabilities or confer additional architectural and biofunctional features to compositionally pure materials. Elastin and elastin-based composites have been subjected to diverse fabrication processes, including heating, electrospinning, wet spinning, solvent casting, freeze-drying, and cross-linking, for the manufacture of particles, fibers, gels, tubes, sheets and films. The resulting materials can be tailored to possess specific strength, elasticity, morphology, topography, porosity, wettability, surface charge, and bioactivity. This extraordinary tunability of elastin-based constructs enables their use in a range of biomedical and tissue engineering applications such as targeted drug delivery, cell encapsulation, vascular repair, nerve regeneration, wound healing, and dermal, cartilage, bone, and dental replacement.

Elastin-like polypeptide based nanoparticles: design rationale toward nanomedicine

Elastin-like polypeptides (ELPs) are characterized by a high sequence control, temperature responsiveness and biocompatibility, which make them highly interesting as smart materials for application in nanomedicine. In particular the construction of ELP-based nanoparticles has recently become a focal point of attention in materials research. This review will give an overview of the ELP-based nanoparticles that have been developed until now and their underlying design principles. First a short introduction on ELPs and their stimulus-responsive behavior will be given. This characteristic has been applied for the development of ELP-based block copolymers that can self-assemble into nanoparticles. Both the fully ELP-based as well as several ELP hybrid materials that have been reported to form nanoparticles will be discussed, which is followed by a concise description of the promising biomedical applications reported for this class of materials.

Hybrid nanotopographical surfaces obtained by biomimetic mineralization of statherin-inspired elastin-like recombinamers

Modification of surfaces mimicking unique chemical and physical features of mineralized tissues is of major interest for obtaining biomaterials for replacing and regenerating biological tissues. Here, human salivary statherin-inspired genetically engineered recombinamers (ELRs, HSS) on biomedical surfaces regulates mineralization to form an amorphous-calcium-phosphate (ACP) layer that reproduces the original substrate nanotopography. The HSS-ELRs carry a statherin-derived peptide with high affinity to tooth enamel. They are tethered to nanorough surfaces and mineralized using an enzyme-directed process. A homogeneous layer of ACP-minerals forms on HSS-coated surfaces retaining the original nanotopography of the substrate. In contrast, biomineralization of control surfaces results in uncontrolled growth of minerals. This suggest the statherin-inspired ELRs have ability to induce and control growth of the minerals on the biofunctional surfaces. Likely, the HSS-ELR coating have similar bioactivity to that of statherin in human saliva. The hybrid nanorough surfaces improve adhesion and differentiation of preosteoblasts and show potential for dental and orthopedic implants integration. This method enables the combination and tailoring of nanotopographical and biochemical cues to design functionalized surfaces to investigate and potentially direct the stem cell fate.

Bioactive membranes for bone regeneration applications: effect of physical and biomolecular signals on mesenchymal stem cell behavior

This study focuses on the in vitro characterization of bioactive elastin-like recombinamer (ELR) membranes for bone regeneration applications. Four bioactive ELRs exhibiting epitopes designed to promote mesenchymal stem cell adhesion (RGDS), endothelial cell adhesion (REDV), mineralization (HAP), and both cell adhesion and mineralization (HAP-RGDS) were synthesized using standard recombinant protein techniques. The materials were then used to fabricate ELR membranes incorporating a variety of topographical micropatterns including channels, holes and posts. Primary rat mesenchymal stem cells (rMSCs) were cultured on the different membranes and the effects of biomolecular and physical signals on cell adhesion, morphology, proliferation, and differentiation were evaluated. All results were analyzed using a custom-made MATLAB program for high throughput image analysis. Effects on cell morphology were mostly dependent on surface topography, while cell proliferation and cell differentiation were largely dependent on the biomolecular signaling from the ELR membranes. In particular, osteogenic differentiation (evaluated by staining for the osteoblastic marker osterix) was significantly enhanced on cells cultured on HAP membranes. Remarkably, cells growing on membranes containing the HAP sequence in non-osteogenic differentiation media exhibited significant up-regulation of the osteogenic marker as early as day 5, while those growing on fibronectin-coated glass in osteogenic differentiation media did not. These results are part of our ongoing effort to develop an optimized molecularly designed periosteal graft.

Rapid biomimetic mineralization of hydroxyapatite-g-PDLLA hybrid microspheres

Hydroxyapatite-graft-poly(D,L-lactide) (HA-g-PDLLA) nanoparticles were synthesized here to fabricate hybrid microspheres with diameter in the range of 150-200 μm by emulsion solvent evaporation techniques. The as-obtained microspheres were treated with alkaline solution in order to selectively degrade the PDLLA layer which covered on the surface of hybrid microspheres and instead to generate a dense coating of HA nanoparticles. The hybrid microspheres with enriched HA nanoparticles on the surface were further immersed in simulated body fluid (SBF) solution to evaluate the bone-forming ability of the bioactive hybrid microspheres via the in vitro biomimetic mineralization process. The resultant microspheres were analyzed by using X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) to understand the nucleation and growth of bioactive calcium phosphate (Ca-P) crystals as a function of surface treatment. Results in this work clearly demonstrated that the existing HA nanoparticles on the surface of hybrid microspheres after alkaline treatment greatly affect the growth of the bone-like Ca-P crystals in SBF solutions. The biomimetic hybrid microspheres were found to be excellent candidates for use as injectable scaffolds for bone tissue engineering.

Functionalization of biomaterials with small osteoinductive moieties

Human mesenchymal stem cells (MSCs) are currently recognized as a powerful cell source for regenerative medicine, notably for their capacity to differentiate into multiple cell types. The combination of MSCs with biomaterials functionalized with instructive cues can be used as a strategy to direct specific lineage commitment, and can thus improve the therapeutic efficacy of these cells. In terms of biomaterial design, one common approach is the functionalization of materials with ligands capable of directly binding to cell receptors and trigger specific differentiation signaling pathways. Other strategies focus on the use of moieties that have an indirect effect, acting, for example, as sequesters of bioactive ligands present in the extracellular milieu that, in turn, will interact with cells. Compared with complex biomolecules, the use of simple compounds, such as chemical moieties and peptides, and other small molecules can be advantageous by leading to less expensive and easily tunable biomaterial formulations. This review describes different strategies that have been used to promote substrate-mediated guidance of osteogenic differentiation of immature osteoblasts, osteoprogenitors and MSCs, through chemically conjugated small moieties, both in two- and three-dimensional set-ups. In each case, the selected moiety, the coupling strategy and the main findings of the study were highlighted. The latest advances and future perspectives in the field are also discussed.

Mimicking bone nanostructure by combining block copolymer self-assembly and 1D crystal nucleation

The orientation and spatial distribution of nanocrystals in the organic matrix are two distinctive structural characteristics associated with natural bone. Synthetic soft materials have been used to successfully control the orientation of mineral crystals. The spatial distribution of minerals in a synthetic scaffold, however, has yet to be reproduced in a biomimetic manner. Herein, we report using block copolymer-decorated polymer nanofibers to achieve biomineralized fibrils with precise control of both mineral crystal orientation and spatial distribution. Exquisite nanoscale structural control in biomimetic hybrid materials has been demonstrated.

Enhanced cell-material interactions through the biofunctionalization of polymeric surfaces with engineered peptides

Research on surface modification of polymeric materials to guide the cellular activity in biomaterials designed for tissue engineering applications has mostly focused on the use of natural extracellular matrix (ECM) proteins and short peptides, such as RGD. However, the use of engineered proteins can gather the advantages of these strategies and avoid the main drawbacks. In this study, recombinant engineered proteins called elastin-like recombinamers (ELRs) have been used to functionalize poly(lactic) acid (PLA) model surfaces. The structure of the ELRs has been designed to include the integrin ligand RGDS and the cross-linking module VPGKG. Surface functionalization has been characterized and optimized by means of ELISA and atomic force microscopy (AFM). The results suggest that ELR functionalization creates a nonfouling canvas able to restrict unspecific adsorption of proteins. Moreover, AFM analysis reveals the conformation and disposition of ELRs on the surface. Biological performance of PLA surfaces functionalized with ELRs has been studied and compared with the use of short peptides. Cell response has been assessed for different functionalization conditions in the presence and absence of the bovine serum albumin (BSA) protein, which could interfere with the surface-cell interaction by adsorbing on the interface. Studies have shown that ELRs are able to elicit higher rates of cell attachment, stronger cell anchorages and faster levels of proliferation than peptides. This work has demonstrated that the use of engineered proteins is a more efficient strategy to guide the cellular activity than the use of short peptides, because they not only allow for better cell attachment and proliferation, but also can provide more complex properties such as the creation of nonfouling surfaces.

Calcium binding peptide motifs from calmodulin confer divalent ion selectivity to elastin-like polypeptides

Calcium-sensitive elastin-like polypeptides (CELPs) were synthesized by periodically interspersing a calcium-binding peptide sequence from calmodulin within an elastin-like polypeptide (ELP) with the goal of creating thermal and calcium responsive peptide polymers. The CELPs exhibit high sensitivity to calcium compared to monovalent cations but do not exhibit the exquisite selectivity for calcium over other divalent cations, such as magnesium, that is displayed by calmodulin. The CELPs were further used as a building block for the synthesis of calcium-sensitive nanoparticles by fusing a hydrophilic, noncalcium-sensitive ELP block with a CELP block that becomes more hydrophobic upon calcium binding. We show that addition of calcium at concentrations between 50 and 500 mM imparts sufficient amphiphilicity to the diblock polypeptide between 33 and 46 °C to trigger its self-assembly into monodisperse spherical micelles with a hydrodynamic radius of ∼50 nm.

Layer-by-layer film growth using polysaccharides and recombinant polypeptides: a combinatorial approach

Nanostructured films consisting of polysaccharides and elastin-like recombinamers (ELRs) are fabricated in a layer-by-layer manner. A quartz-crystal microbalance with dissipation monitoring (QCM-D) is used to follow the buildup of hybrid films containing one polysaccharide (chitosan or alginate) and one of several ELRs that differ in terms of amino acid content, length, and biofunctionality in situ at pH 4.0 and pH 5.5. The charge density of the ingredients at each pH is determined by measuring their ζ-potential, and the thickness of a total of 36 different films containing five bilayers is estimated using the Voigt-based viscoelastic model. A comparison of the values obtained reveals that thicker films can be obtained when working at a pH close to the acidity constant of the polysaccharide used (near-pKa conditions), suggesting that the construction of such films is more favorable when based on the presence of hydrophobic interactions between ELRs and partially neutralized polysaccharides. Further analysis shows that the molecular weight of the ELRs plays only a minor role in defining the growth tendency. When taken together, these results point to the most favorable conditions for constructing nanostructured films from natural and distinct recombinant polypeptides that can be tuned to exhibit specialized biofunctionality for tissue-engineering, drug-delivery, and biotechnological applications.

New developments in polymer-controlled, bioinspired calcium phosphate mineralization from aqueous solution

The polymer-controlled and bioinspired precipitation of inorganic minerals from aqueous solution at near-ambient or physiological conditions avoiding high temperatures or organic solvents is a key research area in materials science. Polymer-controlled mineralization has been studied as a model for biomineralization and for the synthesis of (bioinspired and biocompatible) hybrid materials for a virtually unlimited number of applications. Calcium phosphate mineralization is of particular interest for bone and dental repair. Numerous studies have therefore addressed the mineralization of calcium phosphate using a wide variety of low- and high-molecular-weight additives. In spite of the growing interest and increasing number of experimental and theoretical data, the mechanisms of polymer-controlled calcium phosphate mineralization are not entirely clear to date, although the field has made significant progress in the last years. A set of elegant experiments and calculations has shed light on some details of mineral formation, but it is currently not possible to preprogram a mineralization reaction to yield a desired product for a specific application. The current article therefore summarizes and discusses the influence of (macro)molecular entities such as polymers, peptides, proteins and gels on biomimetic calcium phosphate mineralization from aqueous solution. It focuses on strategies to tune the kinetics, morphologies, final dimensions and crystal phases of calcium phosphate, as well as on mechanistic considerations.

Rational design of thermostable vaccines by engineered peptide-induced virus self-biomineralization under physiological conditions

The development of vaccines against infectious diseases represents one of the most important contributions to medical science. However, vaccine-preventable diseases still cause millions of deaths each year due to the thermal instability and poor efficacy of vaccines. Using the human enterovirus type 71 vaccine strain as a model, we suggest a combined, rational design approach to improve the thermostability and immunogenicity of live vaccines by self-biomineralization. The biomimetic nucleating peptides are rationally integrated onto the capsid of enterovirus type 71 by reverse genetics so that calcium phosphate mineralization can be biologically induced onto vaccine surfaces under physiological conditions, generating a mineral exterior. This engineered self-biomineralized virus was characterized in detail for its unique structural, virological, and chemical properties. Analogous to many exteriors, the mineral coating confers some new properties on enclosed vaccines. The self-biomineralized vaccine can be stored at 26 °C for more than 9 d and at 37 °C for approximately 1 wk. Both in vitro and in vivo experiments demonstrate that this engineered vaccine can be used efficiently after heat treatment or ambient temperature storage, which reduces the dependence on a cold chain. Such a combination of genetic technology and biomineralization provides an economic solution for current vaccination programs, especially in developing countries that lack expensive refrigeration infrastructures.