Polydimethylsiloxane (PDMS Sylgard 184, Dow Corning Corporation) pre-polymer was combined with increasing amounts of cross-linker (5.7, 10.0, 14.3, 21.4, and 42.9 wt.%) and designated PDMS1, PDMS2, PDMS3, PDMS4, and PDMS5, respectively. These materials were processed by spin coating and subjected to common micro-fabrication, micro-machining, and biomedical processes: chemical immersion, oxygen plasma treatment, sterilization, and exposure to tissue culture media. The PDMS formulations were analyzed by gravimetry, goniometry, tensile testing, nano-indentation, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Spin coating of PDMS was formulation dependent with film thickness ranging from 308 microm on PDMS1 to 171 microm on PDMS5 at 200 revolutions per minute (rpm). Ultimate tensile stress (UTS) increased from 3.9 MPa (PDMS1) to 10.8 MPa (PDMS3), and then decreased down to 4.0 MPa (PDMS5). Autoclave sterilization (AS) increased the storage modulus (sigma) and UTS in all formulations, with the highest increase in UTS exhibited by PDMS5 (218%). PDMS surface hydrophilicity and micro-textures were generally unaffected when exposed to the different chemicals, except for micro-texture changes after immersion in potassium hydroxide and buffered hydrofluoric, nitric, sulfuric, and hydrofluoric acids; and minimal changes in contact angle after immersion in hexane, hydrochloric acid, photoresist developer, and toluene. Oxygen plasma treatment decreased the contact angle of PDMS2 from 109 degrees to 60 degrees. Exposure to tissue culture media resulted in increased PDMS surface element concentrations of nitrogen and oxygen.

Aggregates of charged amphiphilic molecules have been found to access a structure at elevated temperature that templates alignment of supramolecular fibrils over macroscopic scales. The thermal pathway leads to a lamellar plaque structure with fibrous texture that breaks upon cooling into large arrays of aligned nanoscale fibres and forms a strongly birefringent liquid. By manually dragging this liquid crystal from a pipette onto salty media, it is possible to extend this alignment over centimetres in noodle-shaped viscoelastic strings. Using this approach, the solution of supramolecular filaments can be mixed with cells at physiological temperatures to form monodomain gels of aligned cells and filaments. The nature of the self-assembly process and its biocompatibility would allow formation of cellular wires in situ that have any length and customized peptide compositions for use in biological applications.

We report here the self-assembly of macroscopic sacs and membranes at the interface between two aqueous solutions, one containing a megadalton polymer and the other, small self-assembling molecules bearing opposite charge. The resulting structures have a highly ordered architecture in which nanofiber bundles align and reorient by nearly 90 degrees as the membrane grows. The formation of a diffusion barrier upon contact between the two liquids prevents their chaotic mixing. We hypothesize that growth of the membrane is then driven by a dynamic synergy between osmotic pressure of ions and static self-assembly. These robust, self-sealing macroscopic structures offer opportunities in many areas, including the formation of privileged environments for cells, immune barriers, new biological assays, and self-assembly of ordered thick membranes for diverse applications.

Adult articular cartilage exhibits little capacity for intrinsic repair, and thus even minor injuries or lesions may lead to progressive damage and osteoarthritic joint degeneration, resulting in significant pain and disability. While there have been numerous attempts to develop tissue-engineered grafts or patches to repair focal chondral and osteochondral defects, there remain significant challenges in the clinical application of cell-based therapies for cartilage repair. This paper reviews the current state of cartilage tissue engineering with respect to different cell sources and their potential genetic modification, biomaterial scaffolds and growth factors, as well as preclinical testing in various animal models. This is not intended as a systematic review, rather an opinion of where the field is moving in light of current literature. While significant advances have been made in recent years, the complexity of this problem suggests that a multidisciplinary approach -combining a clinical perspective with expertise in cell biology, biomechanics, biomaterials science and high-throughput analysis will likely be necessary to address the challenge of developing functional cartilage replacements. With this approach we are more likely to realise the clinical goal of treating both focal defects and even large-scale osteoarthritic degenerative changes in the joint.

ature uses self-assembly to create a widespread variety of complex structures with elaborate geometries and outstanding properties 1 such as hierarchical order, adaptability, selfhealing and bioactivity. Developing new bioinspired processes based on dynamic self-assembly could facilitate the fabrication of synthetic three-dimensional (3D) materials with enhanced complexity, dynamic properties and functionality 2 . Proteins are particularly attractive building blocks because of their versatility and biofunctionality 3 . Elastin-like polypeptides (ELPs) 4 are recombinant proteins that have generated great interest 5 as a result of their modular structure, bioactivity, ease of design and production, and the possibility to create robust and elastic materials 5,6 . ELPs allow for a tunable molecular design 7 and are based on the tropoelastin recurrent motif Val-Pro-Gly-X-Gly (VPGXG), in which X is any amino acid other than proline 7 . This repeating pentapeptide provides ELPs with a thermoresponsive behaviour. Below a critical transition temperature (T t ), the ELP molecule undergoes a reversible-phase transition wherein the protein is soluble in aqueous solution and becomes highly solvated, surrounded by clatharate-like water structures. Above the T t , the hydrophobic domains dehydrate and the protein chain hydrophobically collapses and aggregates to form a phaseseparated state 8 .The use of natural and synthetic proteins to create functional materials has been hindered by the difficulty in controlling their conformation and nanoscale assembly with the precision required to form macroscopic materials. This limitation has driven the development of simpler and more-predictable peptide-based materials 9,10 . Peptide amphiphiles (PAs), for example, are synthetic molecules that can self-assemble into nanofibres and create functional 3D hydrogels that emulate the fibrous architecture of the extracellular matrix (ECM) 11,12 . Nonetheless, most peptide and/or protein materials are formed through equilibrium-based self-assembly approaches that are capable of generating stable supramolecular structures, but with limited hierarchy and spatiotemporal control, which has hindered their functionality 2 .Novel approaches based on the dynamic self-assembly of inorganic building blocks [13][14][15] , actin self-organization 16 and the combination of top-down processes with peptide self-assembly have been reported recently 17 . In particular, Stupp and co-workers have described a self-assembling membrane system obtained through strong electrostatic interactions between PAs and oppositely charged polysaccharides 18 . However, the possibility to exploit the unique structural and functional properties of proteins to create dynamic hierarchical materials remains an elusive target. In this study, we attempt to overcome this hurdle by using self-assembling peptides to promote protein conformational changes and guide their assembly into complex, yet functional, materials. We report the discovery and development of a protein/peptide system t...

Rapid bone regeneration within a three-dimensional defect without the use of bone grafts, exogenous growth factors, or cells remains a major challenge. We report here on the use of selfassembling peptide nanostructured gels to promote bone regeneration that have the capacity to mineralize in biomimetic fashion. The main molecular design was the use of phosphoserine residues in the sequence of a peptide amphiphile known to nucleate hydroxyapatite crystals on the surfaces of nanofibers. We tested the system in a rat femoral critical size defect by placing preassembled nanofiber gels in a 5 mm gap and analyzed bone formation with micro-computed tomography and histology. We found within 4 weeks significantly higher bone formation relative to controls lacking phosphorylated residues and comparable bone formation to that observed in animals treated with a clinically used allogenic bone matrix.

A major goal in materials science is to develop bioinspired functional materials based on the precise control of molecular building blocks across length scales. Here we report a protein-mediated mineralization process that takes advantage of disorder–order interplay using elastin-like recombinamers to program organic–inorganic interactions into hierarchically ordered mineralized structures. The materials comprise elongated apatite nanocrystals that are aligned and organized into microscopic prisms, which grow together into spherulite-like structures hundreds of micrometers in diameter that come together to fill macroscopic areas. The structures can be grown over large uneven surfaces and native tissues as acid-resistant membranes or coatings with tuneable hierarchy, stiffness, and hardness. Our study represents a potential strategy for complex materials design that may open opportunities for hard tissue repair and provide insights into the role of molecular disorder in human physiology and pathology.

The fabrication of multi-level SU-8 microstructures using multiple coating and exposure steps and a single developing step has been achieved for up to six layers of SU-8. Alternating layers of SU-8 2010 (thin) and SU-8 2100 (thick) photoresist films were spin coated, followed by soft-bake, ultraviolet (UV) exposure and post-exposure bake steps. The multiple SU-8 layers were simultaneously developed to create patterned microstructures with overall thicknesses of up to 500 µm and minimum lateral feature size of 10 µm. The use of a single developing step facilitated fabrication of complex multi-level SU-8 microstructures that might be difficult, or even impossible, to achieve by sequential processing of multiple SU-8 layers that are individually coated, baked, exposed and developed.