Self-assembling peptide hydrogel fosters chondrocyte extracellular matrix production and cell division: implications for cartilage tissue repair

Evaluation of adult equine bone marrow- and adipose-derived progenitor cell chondrogenesis in hydrogel cultures

Bone marrow mesenchymal stem cells (BM-MSCs) and adipose-derived progenitor cells (ADPCs) are potential alternatives to autologous chondrocytes for cartilage resurfacing strategies. In this study, the chondrogenic potentials of these cell types were compared by quantifying neo-tissue synthesis and assaying gene expression and accumulation of extracellular matrix (ECM) components of cartilage. Adult equine progenitor cells encapsulated in agarose or self-assembling peptide hydrogels were cultured in the presence or absence of TGFbeta1 for 3 weeks. In BM-MSCs-seeded hydrogels, TGFbeta1 stimulated ECM synthesis and accumulation 3-41-fold relative to TGFbeta1-free culture. In ADPC cultures, TGFbeta1 stimulated a significant increase in ECM synthesis and accumulation in peptide (18-29-fold) but not agarose hydrogels. Chromatographic analysis of BM-MSC-seeded agarose and peptide hydrogels cultured in TGFbeta1 medium showed extensive synthesis of aggrecan-like proteoglycan monomers. ADPCs seeded in peptide hydrogel also synthesized aggrecan-like proteoglycans, although to a lesser extent than seen in BM-MSC hydrogels, whereas aggrecan-like proteoglycan synthesis in ADPC-seeded agarose was minimal. RT-PCR analysis of TGFbeta1 cultures showed detectable levels of type II collagen gene expression in BM-MSC but not ADPC cultures. Histological analysis of TGFbeta1-cultured peptide hydrogels showed the deposition of a continuous proteoglycan- and type II collagen rich ECM for BM-MSCs but not ADPCs. Therefore, this study showed both protein and gene expression evidence of superior chondrogenesis of BM-MSCs relative to ADPCs.

Low-serum media and dynamic deformational loading in tissue engineering of articular cartilage

High-serum media have been shown to produce significant improvement in the properties of tissue-engineered articular cartilage when applied in combination with dynamic deformational loading. To mitigate concerns regarding the culture variability introduced by serum, we examined the interplay between low-serum/ITS-supplemented media and dynamic deformational loading. Our results show that low serum/ITS supplementation does not support the same level of tissue formation as compared to high serum controls. In free-swelling culture, using a combination of ITS with concentrations of FBS above 2% negated the beneficial effects of ITS. Although there were beneficial effects with loading and 0.2%FBS + ITS, these constructs significantly underperformed relative to 20%FBS constructs. At 2%FBS + ITS, the free-swelling construct stiffness and composition approached or exceeded that of 20%FBS constructs. With dynamic loading, the properties of 2%FBS + ITS constructs were significantly lower than free-swelling controls and 20%FBS constructs by day 42. By priming the chondrocytes in 20%FBS prior to exposure to low-serum/ITS media, we observed that low-serum/ITS media produced significant enhancement in tissue properties compared to constructs grown continuously in 20%FBS.

Designer self-assembling Peptide nanofiber scaffolds for study of 3-d cell biology and beyond

Biomedical researchers have become increasingly aware of the limitations of the conventional 2-D tissue cell cultures where most tissue cell studies including cancer and tumor cells have been carried out. They are now searching and testing 3-D cell culture systems, something between a petri dish and a mouse. The important implications of 3-D tissue cell cultures for basic cell biology, tumor biology, high-content drug screening, and regenerative medicine and beyond are far-reaching. How can nanobiotechnology truly advance the traditional cell, tumor, and cancer biology? Why nano is important in biomedical research and medical science? A nanometer is 1000 times smaller than a micrometer, but why it matters in biology? This chapter addresses these questions. It has become more and more apparent that 3-D cell culture offers a more realistic local environment through the nanofiber scaffolds where the functional properties of cells can be observed and manipulated. A new class of designer self-assembling peptide nanofiber scaffolds now provides an ideal alternative system. Time has come to address the 3-D questions because quantitative biology requires in vitro culture systems that more authentically represent the cellular microenvironment in a living organism. In doing so, in vitro experimentation can become truly more predictive of in vivo systems.

Self-assembly of nanofiber with uniform width from wheel-type trigonal-beta-sheet-forming peptide

A novel C3 symmetric peptide conjugate "Wheel-FKFE" consisting of three beta-sheet-forming peptides with wheel-like arrangement is developed, and the morphology of self-assembled peptide conjugates in aqueous solutions is observed at various pH. The CD spectra of Wheel-FKFE show the formation of beta-sheet structures in pH 6.9 phosphate buffer, whereas random structures are formed in aqueous HCl (pH 3.3) and NaOH (pH 11) solutions. In transmission electron microscopy, nanofibers with a uniform width of 3-4 nm and lengths of several micrometers are observed in pH 6.9 phosphate buffer, whereas nanorods with the width of several nanometers and the length of several tens of nanometers are observed for that of aqueous HCl (pH 3.3) and NaOH (pH 11) solutions. The uniform width (3-4 nm) of the fibers observed in neutral solution indicates formation of columnar self-assembly of Wheel-FKFEs. The fluorescence spectrum of polarity sensitive dye, sodium 8-anilino-1-naphthalenesulfonate (ANS), in the presence of Wheel-FKFE fibers revealed that the polarity inside the fibers corresponds to that of acetone, indicating that the internal space of the fibers possesses medium hydrophobic environment.

Spatial distribution of mineralized bone matrix produced by marrow mesenchymal stem cells in self-assembling peptide hydrogel scaffold

We evaluated the osteogenic differentiation of mesenchymal stem cells (MSCs) using a new class of synthetic self-assembling peptide hydrogels, RADA 16, as a scaffold for three-dimensional culture. MSCs derived from rat bone marrow were culture-expanded and seeded into the hydrogel and further cultured in osteogenic medium containing beta-glycerophosphate, ascorbic acid, and dexamethasone for 2-4 weeks. High alkaline phosphatase activity and osteocalcin (OC) contents were detected at both the protein and gene expression levels during the culture periods. Both calcium and the OC contents increased over time, indicating the growth of a mineralized extracellular matrix within the hydrogel. Moreover, the process of the growth of the mineralized matrix determined by three-dimensional microarchitecture images was obtained by confocal laser scanning microscopy. The findings show that MSCs can differentiate into mature osteoblasts to form mineralized matrices within the hydrogel scaffold. Importantly, the differentiation can occur three dimensionally within the hydrogel, indicating that RADA 16 can be considered attractive synthetic biomaterial for use in bone tissue engineering.

Porous thermoresponsive-co-biodegradable hydrogels as tissue-engineering scaffolds for 3-dimensional in vitro culture of chondrocytes

A new porous, thermoresponsive, partially biodegradable, chemically crosslinked hydrogel system was developed, characterized, and tested as a cartilage tissue-engineering scaffold for in vitro chondrocyte culture over a 4-week period. The hydrogel system was composed of poly(N-isopropylacrylamide), poly(D,L-lactic acid), and dextran segments. Pores in the hydrogels were generated using a salt leaching technique. The hydrogels showed thermoresponsive properties, with a lower critical solution temperature at approximately 32 degrees C. They continuously swelled at physiological temperature in phosphate buffered saline (pH 7.4) for at least 1 month. Chondrocytes isolated from embryonic chick sterna were seeded into the hydrogel scaffolds at room temperature and cultured at 37 degrees C for 4 weeks. Real-time reverse-transcriptase polymerase chain reaction quantification was conducted every week to study messenger ribonucleic acid levels of 3 chondrocyte phenotypic markers: type II collagen, type X collagen, and Indian hedgehog. Results suggested that chondrocytes maintained their phenotype during the 4-week in vitro culture and could mimic in vivo development. Chondrocytes were non-enzymatically harvested from the hydrogel scaffold at the end of the fourth week by simply lowering the temperature from 37 degrees C to room temperature. The harvested chondrocytes kept a round morphology, confirming the maintenance of the chondrocyte phenotype in the hydrogel scaffolds.

Engineering cartilage tissue

Cartilage tissue engineering is emerging as a technique for the regeneration of cartilage tissue damaged due to disease or trauma. Since cartilage lacks regenerative capabilities, it is essential to develop approaches that deliver the appropriate cells, biomaterials, and signaling factors to the defect site. The objective of this review is to discuss the approaches that have been taken in this area, with an emphasis on various cell sources, including chondrocytes, fibroblasts, and stem cells. Additionally, biomaterials and their interaction with cells and the importance of signaling factors on cellular behavior and cartilage formation will be addressed. Ultimately, the goal of investigators working on cartilage regeneration is to develop a system that promotes the production of cartilage tissue that mimics native tissue properties, accelerates restoration of tissue function, and is clinically translatable. Although this is an ambitious goal, significant progress and important advances have been made in recent years.

Self-assembling peptide nanofiber scaffolds accelerate wound healing

Cutaneous wound repair regenerates skin integrity, but a chronic failure to heal results in compromised tissue function and increased morbidity. To address this, we have used an integrated approach, using nanobiotechnology to augment the rate of wound reepithelialization by combining self-assembling peptide (SAP) nanofiber scaffold and Epidermal Growth Factor (EGF). This SAP bioscaffold was tested in a bioengineered Human Skin Equivalent (HSE) tissue model that enabled wound reepithelialization to be monitored in a tissue that recapitulates molecular and cellular mechanisms of repair known to occur in human skin. We found that SAP underwent molecular self-assembly to form unique 3D structures that stably covered the surface of the wound, suggesting that this scaffold may serve as a viable wound dressing. We measured the rates of release of EGF from the SAP scaffold and determined that EGF was only released when the scaffold was in direct contact with the HSE. By measuring the length of the epithelial tongue during wound reepithelialization, we found that SAP scaffolds containing EGF accelerated the rate of wound coverage by 5 fold when compared to controls without scaffolds and by 3.5 fold when compared to the scaffold without EGF. In conclusion, our experiments demonstrated that biomaterials composed of a biofunctionalized peptidic scaffold have many properties that are well-suited for the treatment of cutaneous wounds including wound coverage, functionalization with bioactive molecules, localized growth factor release and activation of wound repair.

Regulation of chondrocytic gene expression by biomechanical signals

Cartilage is a mechanosensitive tissue, which means that it can perceive and respond to biomechanical signals. Despite the known importance of biomechanical signals in the etiopathogenesis of arthritic diseases and their effectiveness in joint restoration, little is understood about their actions at the cellular level. Recent molecular approaches have revealed that specific biomechanical stimuli and cell interactions generate intracellular signals that are powerful inducers or suppressors of proinflammatory and reparative genes in chondrocytes. Biomechanical signals are perceived by cartilage in magnitude-, frequency-, and time-dependent manners. Static and dynamic biomechanical forces of high magnitudes induce proinflammatory genes and inhibit matrix synthesis. Contrarily, dynamic biomechanical signals of low/physiologic magnitudes are potent antiinflammatory signals that inhibit interleukin-1beta (IL-1beta)-induced proinflammatory gene transcription and abrogate IL-1beta/tumor necrosis factor-alpha-induced inhibition of matrix synthesis. Recent studies have identified nuclear factor-kB (NF-kB) transcription factors as key regulators of biomechanical signal-mediated proinflammatory and antiinflammatory actions. These signals intercept multiple steps in the NF-kappaB signaling cascade to regulate cytokine gene expression. Taken together, these findings provide insight into how biomechanical signals regulate inflammatory and reparative gene transcription, underscoring their potential in enhancing the ability of chondrocytes to curb inflammation in diseased joints.

The Amphiphilic Self-Assembling Peptide EAK16-I as a Potential Hydrophobic Drug Carrier

It is crucial for hydrophobic drugs to be dissolved and stabilized by carriers in aqueous systems and then to be delivered into target cells. An amphiphilic self-assembling peptide EAK16-I (Ac-AEAKAEAKAEAKAEAK-NH2) is reported here to be able to stabilize a model hydrophobic compound, pyrene, in aqueous solution, resulting in the formation of colloidal suspensions. Egg phosphatidylcholine (EPC) vesicles are used as plasma membranes mimic. Fluorescence data shows that the pyrene is presented in the crystalline form when stabilized by EAK16-I and molecularly migrates from its peptide encapsulations into the membrane bilayers of EPC vesicles when the suspension is mixed with EPC vesicles. Furthermore, the release rate can be controlled by changing peptide-to-pyrene ratio, and the higher ratios lead to the slower release rates due to a thicker encapsulation on the pyrene microcrystals. This demonstrates that EAK16-I, as a promising nanobiomaterial, has the potential to be a hydrophobic compounds carrier.

Novel opportunities and challenges offered by nanobiomaterials in tissue engineering

Over the last decades, tissue engineering has demonstrated an unquestionable potential to regenerate damaged tissues and organs. Some tissue-engineered solutions recently entered the clinics (eg, artificial bladder, corneal epithelium, engineered skin), but most of the pathologies of interest are still far from being solved. The advent of stem cells opened the door to large-scale production of "raw living matter" for cell replacement and boosted the overall sector in the last decade. Still reliable synthetic scaffolds fairly resembling the nanostructure of extracellular matrices, showing mechanical properties comparable to those of the tissues to be regenerated and capable of being modularly functionalized with biological active motifs, became feasible only in the last years thanks to newly introduced nanotechnology techniques of material design, synthesis, and characterization. Nanostructured synthetic matrices look to be the next generation scaffolds, opening new powerful pathways for tissue regeneration and introducing new challenges at the same time. We here present a detailed overview of the advantages, applications, and limitations of nanostructured matrices with a focus on both electrospun and self-assembling scaffolds.

In situ cross-linking of elastin-like polypeptide block copolymers for tissue repair

Rapid cross-linking of elastin-like polypeptides (ELPs) with hydroxymethylphosphines (HMPs) in aqueous solution is attractive for minimally invasive in vivo implantation of biomaterials and tissue engineering scaffolds. In order to examine the independent effect of the location and number of reactive sites on the chemical cross-linking kinetics of ELPs and the mechanical properties of the resulting hydrogels, we have designed ELP block copolymers comprised of cross-linkable, hydrophobic ELP blocks with periodic Lys residues (A block) and aliphatic, hydrophilic ELP blocks with no cross-linking sites (B block); three different block architectures, A, ABA, and BABA were synthesized in this study. All ELP block copolymers were rapidly cross-linked with HMPs within several minutes under physiological conditions. The inclusion of the un-cross-linked hydrophilic block, its length relative to the cross-linkable hydrophobic block, and the block copolymer architecture all had a significant effect on swelling ratios of the cross-linked hydrogels, their microstructure, and mechanical properties. Fibroblasts embedded in the ELP hydrogels survived the cross-linking process and remained viable for at least 3 days in vitro when the gels were formed from an equimolar ratio of HMPs and Lys residues of ELPs. DNA quantification of the embedded cells indicated that the cell viability within triblock ELP hydrogels was statistically greater than that in the monoblock gels at day 3. These results suggest that the mechanical properties of ELP hydrogels and the microenvironment that they present to cells can be tuned by the design of the block copolymer architecture.

Modular self-assembling biomaterials for directing cellular responses

Self-assembling biomaterials are promising as cell-interactive matrices because they can be constructed in a modular fashion, which enables the independent and simultaneous tuning of several of their physicochemical and biological properties. Such modularity facilitates the optimization of multi-component matrices for use in complex biological environments such as 3-D cell culture or scaffolds for regenerative medicine. This Highlight will discuss recent strategies for producing modular self-assembling biomaterials, with a particular focus on how ligand presentation and matrix mechanics can be controlled in modular ways. In addition, it will discuss key hurdles that remain for employing these materials as cell-interactive scaffolds in biomedical applications, particularly those that relate to how they may interface with the immune system.

Peptide-based Biopolymers in Biomedicine and Biotechnology

Peptides are emerging as a new class of biomaterials due to their unique chemical, physical, and biological properties. The development of peptide-based biomaterials is driven by the convergence of protein engineering and macromolecular self-assembly. This review covers the basic principles, applications, and prospects of peptide-based biomaterials. We focus on both chemically synthesized and genetically encoded peptides, including poly-amino acids, elastin-like polypeptides, silk-like polymers and other biopolymers based on repetitive peptide motifs. Applications of these engineered biomolecules in protein purification, controlled drug delivery, tissue engineering, and biosurface engineering are discussed.

Modification of hydrophilic and hydrophobic surfaces using an ionic-complementary peptide

Ionic-complementary peptides are novel nano-biomaterials with a variety of biomedical applications including potential biosurface engineering. This study presents evidence that a model ionic-complementary peptide EAK16-II is capable of assembling/coating on hydrophilic mica as well as hydrophobic highly ordered pyrolytic graphite (HOPG) surfaces with different nano-patterns. EAK16-II forms randomly oriented nanofibers or nanofiber networks on mica, while ordered nanofibers parallel or oriented 60° or 120° to each other on HOPG, reflecting the crystallographic symmetry of graphite (0001). The density of coated nanofibers on both surfaces can be controlled by adjusting the peptide concentration and the contact time of the peptide solution with the surface. The coated EAK16-II nanofibers alter the wettability of the two surfaces differently: the water contact angle of bare mica surface is measured to be <10°, while it increases to 20.3±2.9° upon 2 h modification of the surface using a 29 µM EAK16-II solution. In contrast, the water contact angle decreases significantly from 71.2±11.1° to 39.4±4.3° after the HOPG surface is coated with a 29 µM peptide solution for 2 h. The stability of the EAK16-II nanofibers on both surfaces is further evaluated by immersing the surface into acidic and basic solutions and analyzing the changes in the nanofiber surface coverage. The EAK16-II nanofibers on mica remain stable in acidic solution but not in alkaline solution, while they are stable on the HOPG surface regardless of the solution pH. This work demonstrates the possibility of using self-assembling peptides for surface modification applications.

Nanotechnology in regenerative medicine: the materials side

Regenerative medicine is an emerging multidisciplinary field that aims to restore, maintain or enhance tissues and hence organ functions. Regeneration of tissues can be achieved by the combination of living cells, which will provide biological functionality, and materials, which act as scaffolds to support cell proliferation. Mammalian cells behave in vivo in response to the biological signals they receive from the surrounding environment, which is structured by nanometre-scaled components. Therefore, materials used in repairing the human body have to reproduce the correct signals that guide the cells towards a desirable behaviour. Nanotechnology is not only an excellent tool to produce material structures that mimic the biological ones but also holds the promise of providing efficient delivery systems. The application of nanotechnology to regenerative medicine is a wide issue and this short review will only focus on aspects of nanotechnology relevant to biomaterials science. Specifically, the fabrication of materials, such as nanoparticles and scaffolds for tissue engineering, and the nanopatterning of surfaces aimed at eliciting specific biological responses from the host tissue will be addressed.

Designer self-assembling peptide scaffolds for 3-d tissue cell cultures and regenerative medicine

Biomaterial science has made enormous progress in the last few decades. Nonetheless, innovative biomaterials are still urgently needed to provide in vitro cell-culture models that more closely resemble three-dimensional (3-D) cell interactions and cyto-architectures in bodies and tissues. In this review, the recent advances toward this goal through molecular engineering of various designer self-assembling peptide scaffolds are discussed. These peptide scaffolds can be commercially and custom-tailor synthesized materials with high purity and may be not only useful for specific 3-D tissue cell cultures but also for tissue repair and regenerative therapies. Furthermore, these designer self-assembling peptide scaffolds have recently become powerful tools for regenerative medicine to repair nervous tissue, to stop bleeding in seconds, to repair infarctuated myocardia, as well as being useful medical devices for slow drug release.

Protease-sensitive fluorescent nanofibers

We report the design and synthesis of enzyme-responsive nanofibers. The fibers are composed of self-assembled hydrophobic beta-sheet peptides incorporating protease-sensitive domains, fluorescent reporters, and hydrophilic poly(ethylene glycol) (PEG) units. Using urokinase plasminogen activator (uPA) as a model system, nanofibers were developed to release fluorescent fragments upon uPA incubation. These protease-sensitive nanofibers may have considerable biomedical applications as diagnostic sensors or for protease-assisted drug deliveries.

Nanofibers and their applications in tissue engineering

Developing scaffolds that mimic the architecture of tissue at the nanoscale is one of the major challenges in the field of tissue engineering. The development of nanofibers has greatly enhanced the scope for fabricating scaffolds that can potentially meet this challenge. Currently, there are three techniques available for the synthesis of nanofibers: electrospinning, self-assembly, and phase separation. Of these techniques, electrospinning is the most widely studied technique and has also demonstrated the most promising results in terms of tissue engineering applications. The availability of a wide range of natural and synthetic biomaterials has broadened the scope for development of nanofibrous scaffolds, especially using the electrospinning technique. The three dimensional synthetic biodegradable scaffolds designed using nanofibers serve as an excellent framework for cell adhesion, proliferation, and differentiation. Therefore, nanofibers, irrespective of their method of synthesis, have been used as scaffolds for musculoskeletal tissue engineering (including bone, cartilage, ligament, and skeletal muscle), skin tissue engineering, vascular tissue engineering, neural tissue engineering, and as carriers for the controlled delivery of drugs, proteins, and DNA. This review summarizes the currently available techniques for nanofiber synthesis and discusses the use of nanofibers in tissue engineering and drug delivery applications.

Electrostatically tuned rate of peptide self-assembly resolved by multiple particle tracking

Hydrogels formed from the self-assembly of oligopeptides are being extensively studied for biomedical applications. The kinetics of their gelation, as well as a quantitative description of the forces controlling the rate of assembly has not yet been addressed. We report here the use of multiple particle tracking to measure the self-assembly kinetics of the model peptide FKFEFKFE (KFE8). KFE8 forms well-defined β-sheet intermediates and is often used as a model peptide system that forms a fibrous network in aqueous solvent. We find that increasing the pH of this system from 3.5 to 4.0 decreases the time of KFE8 gelation by almost hundredfold, from hours to minutes. A remarkable self-similarity between measurements performed at different pH suggests that, although accelerated by the pH increase, gelation follows an invariable mechanism. We propose a semi-quantitative interpretation for the order of magnitudes of gelation time using a simple model for the interaction driving the self-assembly in terms of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Such understanding is important for the development of current and future therapeutic applications ( drug delivery).

Hydrogels as Extracellular Matrices for Skeletal Tissue Engineering: State-of-the-Art and Novel Application in Organ Printing

Organ printing, a novel approach in tissue engineering, applies layered computer-driven deposition of cells and gels to create complex 3-dimensional cell-laden structures. It shows great promise in regenerative medicine, because it may help to solve the problem of limited donor grafts for tissue and organ repair. The technique enables anatomical cell arrangement using incorporation of cells and growth factors at predefined locations in the printed hydrogel scaffolds. This way, 3-dimensional biological structures, such as blood vessels, are already constructed. Organ printing is developing fast, and there are exciting new possibilities in this area. Hydrogels are highly hydrated polymer networks used as scaffolding materials in organ printing. These hydrogel matrices are natural or synthetic polymers that provide a supportive environment for cells to attach to and proliferate and differentiate in. Successful cell embedding requires hydrogels that are complemented with biomimetic and extracellular matrix components, to provide biological cues to elicit specific cellular responses and direct new tissue formation. This review surveys the use of hydrogels in organ printing and provides an evaluation of the recent advances in the development of hydrogels that are promising for use in skeletal regenerative medicine. Special emphasis is put on survival, proliferation and differentiation of skeletal connective tissue cells inside various hydrogel matrices.

Light and low-frequency pulsatile hydrostatic pressure enhances extracellular matrix formation by bone marrow mesenchymal cells: An in-vitro study with special reference to cartilage repair

Ovine bone marrow mesenchymal cells (BMMCs) were seeded on to non-woven filamentous plasma-treated polyester scaffolds and cultured in a chondrogenic medium for 4 weeks. Thereafter a pulsatile hydrostatic pressure (PHP) was applied to these cell-scaffolds constructs at an amplitude of 0.1 MPa and frequency of 0.25 Hz, for 30 min a day, over a period of 10 days. Samples ( n=6) were removed 24 h after PHP stimulation at days 1, 4, 7, and 10 for biochemical analysis. Similar analyses were conducted, at the same time points, on control samples that were not subjected to a PHP. The results showed that the glycosaminoglycan (GAG) content did not significantly increase until after the application of a PHP for 7 days. The GAG content was 1.5 and 2.7 times higher in the PHP group than in the control group at days 7 and 10 respectively ( p < 0.01). The deoxyribonucleic acid (DNA) content was 1.5 times greater in the PHP group than in the control group at day 10 ( p < 0.01). GAG synthesis amounts, expressed as the total GAG contents per microgram of DNA, were 1.6 and 1.8 times higher in the PHP group than in the control group at days 7 and 10 respectively ( p < 0.01). The total collagen content in the medium did not change until after PHP application for 10 days, when it was 1.9 times higher than the control ( p < 0.05). The results suggest that a light PHP applied at a low frequency has a cumulative stimulatory effect on the BMMCs' metabolic activities including cell proliferation and synthesis of the extracellular matrix.

Osteochondral defect repair with autologous bone marrow-derived mesenchymal stem cells in an injectable, in situ, cross-linked synthetic extracellular matrix

A co-cross-linked synthetic extracellular matrix (sECM) composed of chemically modified hyaluronic acid and gelatin was used as a cell delivery vehicle for osteochondral defect repair in a rabbit model. A full-thickness defect was created in the patellar groove of the femoral articular cartilage in each of 2 rabbit joints, and 4 experimental groups were assigned (12 rabbits/group): untreated control, autologous mesenchymal stem cells (MSCs) only, sECM only, and MSCs + sECM. The sECM hydrogels were allowed to cross-link in the defect in situ. Rabbits were sacrificed at 4, 8, and 12 weeks post-surgery, and cartilage repair was evaluated and scored. In the controls, defects were filled with fibrous tissue. In the MSC-only group, hyaline-like cartilage filled the peripheral area of the defect, but the center was filled with fibrous tissue. In the sECM-only group, hyaline cartilage with zonal architecture filled the defect at 12 weeks, but an interface between repaired and adjacent host cartilage was evident. In the MSCs + sECM group, defects were completely filled with elastic, firm, translucent cartilage at 12 weeks and showed superior integration of the repair tissue with the normal cartilage. The sECM delivers and retains MSCs, and the injectable cell-seeded sECM could be delivered arthroscopically in the clinic.

Injectable biodegradable hydrogel composites for rabbit marrow mesenchymal stem cell and growth factor delivery for cartilage tissue engineering

We investigated the development of an injectable, biodegradable hydrogel composite of oligo(poly(ethylene glycol) fumarate) (OPF) with encapsulated rabbit marrow mesenchymal stem cells (MSCs) and gelatin microparticles (MPs) loaded with transforming growth factor-beta1 (TGF-beta1) for cartilage tissue engineering applications. Rabbit MSCs and TGF-beta1-loaded MPs were mixed with OPF, a poly(ethylene glycol)-diacrylate crosslinker and the radical initiators ammonium persulfate and N,N,N',N'-tetramethylethylenediamine, and then crosslinked at 37 degrees C for 8 min to form hydrogel composites. Three studies were conducted over 14 days in order to examine the effects of: (1) the composite formulation, (2) the MSC seeding density, and (3) the TGF-beta1 concentration on the chondrogenic differentiation of encapsulated rabbit MSCs. Bioassay results showed no significant difference in DNA amount between groups, however, groups with MPs had a significant increase in glycosaminoglycan content per DNA starting at day 7 as compared to controls at day 0. Chondrocyte-specific gene expression of type II collagen and aggrecan were only evident in groups containing TGF-beta1-loaded MPs and varied with TGF-beta1 concentration in a dose-dependent manner. Specifically, type II collagen gene expression exhibited a 161+/-49-fold increase and aggrecan gene expression a 221+/-151-fold increase after 14 days with the highest dose of TGF-beta1 (16 ng/ml). These results indicate that encapsulated rabbit MSCs remained viable over the culture period and differentiated into chondrocyte-like cells, thus suggesting the potential of OPF composite hydrogels as part of a novel strategy for localized delivery of stem cells and bioactive molecules.

Immobilized sonic hedgehog N‐terminal signaling domain enhances differentiation of bone marrow‐derived mesenchymal stem cells

The signaling domain of Sonic hedgehog (Shh), a potent upstream regulator of cell fate that has been implicated in osteoblast differentiation from undifferentiated mesenchymal cells in its endogenous form, was investigated in an immobilized form as a means for accelerating differentiation of uncommitted cells to the osteoblast phenotype. A recombinant cysteine-modified N-terminal Shh (mShh) was synthesized, purified, and immobilized onto interpenetrating polymer network (IPN) surfaces also grafted with a bone sialoprotein-derived peptide containing the Arg-Gly-Asp (RGD) sequence (bsp-RGD (15)), at calculated densities of 2.42 and 10 pmol/cm2, respectively. The mitogenic effect of mShh was dependent on the mode of presentation, as surfaces with immobilized mShh and bsp-RGD (15) had no effect on the growth rate of rat bone marrow-derived mesenchymal stem cells (BMSCs), while soluble mShh enhanced cell growth compared to similar surface without mShh supplementation. In conjunction with media supplemented with bone morphogenetic protein-2 and -4, mShh and bsp-RGD (15)-grafted IPN surfaces enhanced the alkaline phosphatase activity of BMSCs compared with tissue culture polystyrene and bsp-RGD (15)-grafted IPN surfaces supplemented with soluble mShh, indicating enhanced osteoblast differentiation. The adhesive peptide bsp-RGD (15) was necessary for cell attachment and proliferation, as well as differentiation in response to immobilized mShh. The addition of immobilized Shh substantially improved the differentiation of uncommitted BMSCs to the osteoblast lineage, and therefore warrants further testing in vivo to examine the effect of the stated biomimetic system on peri-implant bone formation and implant fixation.

Fabrication of microfluidic hydrogels using molded gelatin as a sacrificial element

This paper describes a general procedure for the formation of hydrogels that contain microfluidic networks. In this procedure, micromolded meshes of gelatin served as sacrificial materials. Encapsulation of gelatin meshes in a hydrogel and subsequent melting and flushing of the gelatin left behind interconnected channels in the hydrogel. The channels were as narrow as approximately 6 microm, and faithfully replicated the features in the original gelatin mesh. Fifty micrometre wide microfluidic networks in collagen and fibrin readily enabled delivery of macromolecules and particles into the channels and transport of macromolecules from channels into the bulk of the gels. Microfluidic gels were also suitable as scaffolds for cell culture, and could be seeded by human microvascular endothelial cells to form rudimentary endothelial networks for potential use in tissue engineering.

Surface engineered and drug releasing pre-fabricated scaffolds for tissue engineering

A wide range of polymeric scaffolds have been intensively studied for use as implantable and temporal devices in tissue engineering. Biodegradable and biocompatible scaffolds having a highly open porous structure and good mechanical strength are needed to provide an optimal microenvironment for cell proliferation, migration, and differentiation, and guidance for cellular in-growth from host tissue. A variety of natural and synthetic polymeric scaffolds can be fabricated in the form of a solid foam, nanofibrous matrix, microsphere, or hydrogel. Biodegradable porous scaffolds can be surface engineered to provide an extracellular matrix mimicking environment for better cell adhesion and tissue in-growth. Furthermore, scaffolds can be designed to release bioactive molecules, such as growth factors, DNA, or drugs, in a sustained manner to facilitate tissue regeneration. This paper reviews the current status of surface engineered and drug releasing scaffolds for tissue engineering.

Enzymatic formation of modular cell-instructive fibrin analogs for tissue engineering

The molecular engineering of cell-instructive artificial extracellular matrices is a powerful means to control cell behavior and enable complex processes of tissue formation and regeneration. This work reports on a novel method to produce such smart biomaterials by recapitulating the crosslinking chemistry and the biomolecular characteristics of the biopolymer fibrin in a synthetic analog. We use activated coagulation transglutaminase factor XIIIa for site-specific coupling of cell adhesion ligands and engineered growth factor proteins to multiarm poly(ethylene glycol) macromers that simultaneously form proteolytically sensitive hydrogel networks in the same enzyme-catalyzed reaction. Growth factor proteins are quantitatively incorporated and released upon cell-derived proteolytic degradation of the gels. Primary stromal cells can invade and proteolytically remodel these networks both in an in vitro and in vivo setting. The synthetic ease and potential to engineer their physicochemical and bioactive characteristics makes these hybrid networks true alternatives for fibrin as provisional drug delivery platforms in tissue engineering.

Designer self-assembling peptide materials

Understanding of macromolecular materials at the molecular level is becoming increasingly important for a new generation of nanomaterials for nanobiotechnology and other disciplines, namely, the design, synthesis, and fabrication of nanodevices at the molecular scale from bottom up. Basic engineering principles for microfabrication can be learned through fully grasping the molecular self-assembly and programmed assembly phenomena. Self- and programmed-assembly phenomena are ubiquitous in nature. Two key elements in molecular macrobiological material productions are chemical complementarity and structural compatibility, both of which require weak and non-covalent interactions that bring building blocks together during self-assembly. Significant advances have been made during the 1990s at the interface of materials chemistry and biology. They include the design of helical ribbons, peptide nanofiber scaffolds for three-dimensional cell cultures and tissue engineering, peptide surfactants for solubilizing and stabilizing diverse types of membrane proteins and their complexes, and molecular ink peptides for arbitrary printing and coating surfaces as well as coiled-coil helical peptides for multi-length scale fractal structures. These designer self-assembling peptides have far reaching implications in a broad spectrum of applications in biology, medicine, nanobiotechnology, and nanobiomedical technology, some of which are beyond our current imaginations. [image: see text]

Design and application of stimulus-responsive peptide systems

The ability of peptides and proteins to change conformations in response to external stimuli such as temperature, pH and the presence of specific small molecules is ubiquitous in nature. Exploiting this phenomenon, numerous natural and designed peptides have been used to engineer stimulus-responsive systems with potential applications in important research areas such as biomaterials, nanodevices, biosensors, bioseparations, tissue engineering and drug delivery. This review describes prominent examples of both natural and designed synthetic stimulus-responsive peptide systems. While the future looks bright for stimulus-responsive systems based on natural and rationally engineered peptides, it is expected that the range of stimulants used to manipulate such systems will be significantly broadened through the use of combinatorial protein engineering approaches such as directed evolution. These new proteins and peptides will continue to be employed in exciting and high-impact research areas including bionanotechnology and synthetic biology.

Ile-phe dipeptide self-assembly: clues to amyloid formation

Peptidic self-assembled nanostructures are said to have a wide range of applications in nanotechnology, yet the mechanistic details of hierarchical self-assembly are still poorly understood. The Phe-Phe recognition motif of the Alzheimer's Abeta peptide is the smallest peptide able to assemble into higher-order structures. Here, we show that the Ile-Phe dipeptide analog is also able to self-associate in aqueous solution as a transparent, thermoreversible gel formed by a network of fibrillar nanostructures that exhibit strong birefringence upon Congo red binding. Besides, a second dipeptide Val-Phe, differing only in a methyl group from the former, is unable to self-assemble. The detailed analysis of the differential polymeric behavior of these closely related molecules provides insight into the forces triggering the first steps in self-assembly processes such as amyloid formation.

Biological designer self-assembling peptide nanofiber scaffolds significantly enhance osteoblast proliferation, differentiation and 3-D migration

A class of self-assembling peptide nanofiber scaffolds has been shown to be an excellent biological material for 3-dimension cell culture and stimulating cell migration into the scaffold, as well as for repairing tissue defects in animals. We report here the development of several peptide nanofiber scaffolds designed specifically for osteoblasts. We designed one of the pure self-assembling peptide scaffolds RADA16-I through direct coupling to short biologically active motifs. The motifs included osteogenic growth peptide ALK (ALKRQGRTLYGF) bone-cell secreted-signal peptide, osteopontin cell adhesion motif DGR (DGRGDSVAYG) and 2-unit RGD binding sequence PGR (PRGDSGYRGDS). We made the new peptide scaffolds by mixing the pure RAD16 and designer-peptide solutions, and we examined the molecular integration of the mixed nanofiber scaffolds using AFM. Compared to pure RAD16 scaffold, we found that these designer peptide scaffolds significantly promoted mouse pre-osteoblast MC3T3-E1 cell proliferation. Moreover, alkaline phosphatase (ALP) activity and osteocalcin secretion, which are early and late markers for osteoblastic differentiation, were also significantly increased. We demonstrated that the designer, self-assembling peptide scaffolds promoted the proliferation and osteogenic differentiation of MC3T3-E1. Under the identical culture medium condition, confocal images unequivocally demonstrated that the designer PRG peptide scaffold stimulated cell migration into the 3-D scaffold. Our results suggest that these designer peptide scaffolds may be very useful for promoting bone tissue regeneration.

Tissue engineering of small intestine--current status

Short bowel syndrome (SBS) has always posed a great threat to patients and has been one of the biggest challenges for doctors due to its high morbidity and mortality. So far, parenteral nutrition (PN) and small bowel transplantation remain the only viable therapeutic options. However, sepsis and liver failure associated with PN and limited availability of the donor organs and high graft rejection rates associated with transplantation have limited their use to a nonpermanent solution. Clearly, there is a need for an alternative therapy whereby increasing the absorptive surface area would help neonates and adults suffering from permanent intestinal failure. Techniques such as sequential intestinal lengthening are being explored in animal models with little success. Attempts to engineer small intestine since the late 1980s have achieved varying degrees of success in animal models with evolving refinements in biotechnology. The most encouraging results so far have been the generation of intestinal neomucosa in the form of cysts when intestinal epithelial organoid units isolated from neonatal rats were seeded onto biodegradable polymers before implantation in syngeneic adult rats' omentum. Although still experimental, continued attempts worldwide using cultured stem cells and improved polymer technology offer promise to provide an off-the-shelf artificial intestine as a novel therapy for patients with SBS. This article reviews the current status of progress in the field of small intestinal tissue engineering and addresses various types of cell sources and scaffold material having potential to be used in this field.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

Amyloid Fibrils: From Disease to Design. New Biomaterial Applications for Self-Assembling Cross-β Fibrils

Amyloid fibrils are self-assembling protein aggregates. They are essentially insoluble and resilient nanofibres that offer great potential as materials for nanotechnology and bionanotechnology. Fibrils are associated with several debilitating diseases, for example Alzheimer’s disease, but recent advances suggest they also have positive functions in nature and can be formed in vitro from generic proteins. This article explores how the unique nanotopography and advantageous properties of fibrils may be used to develop tools for probing cell behaviour, protein-based biomimetic materials for supporting cells, or platforms for biosensors and enzyme immobilization.

Nanostructured materials for applications in drug delivery and tissue engineering

Research in the areas of drug delivery and tissue engineering has witnessed tremendous progress in recent years due to their unlimited potential to improve human health. Meanwhile, the development of nanotechnology provides opportunities to characterize, manipulate and organize matter systematically at the nanometer scale. Biomaterials with nano-scale organizations have been used as controlled release reservoirs for drug delivery and artificial matrices for tissue engineering. Drug-delivery systems can be synthesized with controlled composition, shape, size and morphology. Their surface properties can be manipulated to increase solubility, immunocompatibility and cellular uptake. The limitations of current drug delivery systems include suboptimal bioavailability, limited effective targeting and potential cytotoxicity. Promising and versatile nano-scale drug-delivery systems include nanoparticles, nanocapsules, nanotubes, nanogels and dendrimers. They can be used to deliver both small-molecule drugs and various classes of biomacromolecules, such as peptides, proteins, plasmid DNA and synthetic oligodeoxynucleotides. Whereas traditional tissue-engineering scaffolds were based on hydrolytically degradable macroporous materials, current approaches emphasize the control over cell behaviors and tissue formation by nano-scale topography that closely mimics the natural extracellular matrix (ECM). The understanding that the natural ECM is a multifunctional nanocomposite motivated researchers to develop nanofibrous scaffolds through electrospinning or self-assembly. Nanocomposites containing nanocrystals have been shown to elicit active bone growth. Drug delivery and tissue engineering are closely related fields. In fact, tissue engineering can be viewed as a special case of drug delivery where the goal is to accomplish controlled delivery of mammalian cells. Controlled release of therapeutic factors in turn will enhance the efficacy of tissue engineering. From a materials point of view, both the drug-delivery vehicles and tissue-engineering scaffolds need to be biocompatible and biodegradable. The biological functions of encapsulated drugs and cells can be dramatically enhanced by designing biomaterials with controlled organizations at the nanometer scale. This review summarizes the most recent development in utilizing nanostructured materials for applications in drug delivery and tissue engineering.

Designer self-assembling peptide nanofiber scaffolds for adult mouse neural stem cell 3-dimensional cultures

Biomedical researchers have become increasingly aware of the limitations of conventional 2-dimensional tissue cell culture systems, including coated Petri dishes, multi-well plates and slides, to fully address many critical issues in cell biology, cancer biology and neurobiology, such as the 3-D microenvironment, 3-D gradient diffusion, 3-D cell migration and 3-D cell-cell contact interactions. In order to fully understand how cells behave in the 3-D body, it is important to develop a well-controlled 3-D cell culture system where every single ingredient is known. Here we report the development of a 3-D cell culture system using a designer peptide nanofiber scaffold with mouse adult neural stem cells. We attached several functional motifs, including cell adhesion, differentiation and bone marrow homing motifs, to a self-assembling peptide RADA16 (Ac-RADARADARADARADA-COHN2). These functionalized peptides undergo self-assembly into a nanofiber structure similar to Matrigel. During cell culture, the cells were fully embedded in the 3-D environment of the scaffold. Two of the peptide scaffolds containing bone marrow homing motifs significantly enhanced the neural cell survival without extra soluble growth and neurotrophic factors to the routine cell culture media. In these designer scaffolds, the cell populations with β-Tubulin⁺, GFAP⁺ and Nestin⁺ markers are similar to those found in cell populations cultured on Matrigel. The gene expression profiling array experiments showed selective gene expression, possibly involved in neural stem cell adhesion and differentiation. Because the synthetic peptides are intrinsically pure and a number of desired function cellular motifs are easy to incorporate, these designer peptide nanofiber scaffolds provide a promising controlled 3-D culture system for diverse tissue cells, and are useful as well for general molecular and cell biology.

Scaffold-free, engineered porcine cartilage construct for cartilage defect repair--in vitro and in vivo study

This study introduces an implantable scaffold-free (SF) cartilage tissue construct that is composed of chondrocytes and their self-produced extracellular matrix (ECM). Chondrocytes were isolated from the articular cartilages from knees of domestic pigs (2-week old) and monolayer-cultured for 3-4 days in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum and 50 microg/mL of ascorbic acid. Briefly treated with 0.25% trypsin-ethylenediaminetetraacetic acid (EDTA), an intact chondrocytes/ECM membrane, as a cell sheet was released from the plate bottom and subsequently centrifuged into a pellet-type construct. Each was grown in vitro for up to 5 weeks and subjected to various assays at different time points (1, 7, 14, 21, and 35 days). For in vivo implantation, full-thickness defects (n = 4) were manually created on the femoro-patellar groove of the left porcine knee and 1-week-cultured SF construct was implanted as an allograft for a month. One defect (#1) was an empty control and the remaining three received different recipes; construct only (#2) or 0.25% trypsin/EDTA-treated first and then construct and collagen gel (#3) or construct and collagen gel (#4). While the total cell numbers significantly increased by 2 weeks and then remained stable, cell viability stayed in the mid-70% range through the entire culture period. Biochemical assay found continuous glycosaminoglycan (GAG) accumulation. Histology exhibited that cell distribution was even in the construct and GAG intensity became stronger and uniform with time. Real-time reverse transcription polymerase chain reaction (RT-PCR) results showed that phenotypic stability peaked at 2 weeks, which was arable to that of freshly isolated chondrocytes. Upon analysis of the retrieved implants, some promising results were witnessed in the defects (#3) retaining not only their intact mass but also chondrocytic morphology with lacuna formation.

Formation of colloidal suspension of hydrophobic compounds with an amphiphilic self-assembling peptide

The amphiphilic self-assembling peptide EAK16-II was found to be able to stabilize hydrophobic compounds in aqueous solution. Micro/nanocrystals of a hydrophobic compound, pyrene, and a hydrophobic anticancer agent, ellipticine, were stabilized by EAK16-II to form colloidal suspensions in water. Initial evidence of the association between EAK16-II and hydrophobic compounds was the observation of a clouding phenomenon and a difference in fluorescence spectra of the solution. A further investigation on the interaction between EAK16-II and pyrene was carried out using fluorescence spectroscopy and scanning electron microscopy (SEM). It was found that the pyrene-peptide complex formation required mechanical stirring, and the freshly prepared peptide solution (containing peptide monomers and/or peptide protofibrils) was more effective at stabilizing pyrene than the mature fibrils in aged peptide solutions. The time duration over which the complex formed was about 22 h. The data on the complexation of pyrene and EAK16-II at various concentrations suggested that the maximum amount of stabilized pyrene was concentration dependent. SEM images showed that peptide concentration did not significantly affect the size of the complexes/suspensions but altered the structures of the peptide coating on the surface of the complex. Atomic force microscopy (AFM) was conducted to study the interaction of EAK16-II with a model hydrophobic surface, which provided some detailed information of how peptide adsorbed onto the hydrophobic compounds and stabilize them. This study shows the potential of self-assembling peptides for encapsulation of hydrophobic compounds.

Patterning amyloid peptide fibrils by AFM charge writing

Surface charge patterns generated by atomic force microscopy-based charge writing were used to pattern amyloid-like peptide fibrils on a solid substrate. Fibrils of the short peptide TTR105-115 were encapsulated inside water droplets of a water-in-perfluorocarbon oil emulsion and retained their rod morphology. They were observed to deposit selectively with a lateral resolution of approximately 1 microm onto negatively charged patterns on a polymethyl-methacrylate substrate.

Enzyme-catalysed assembly of DNA hydrogel

DNA is a remarkable polymer that can be manipulated by a large number of molecular tools including enzymes. A variety of geometric objects, periodic arrays and nanoscale devices have been constructed. Previously we synthesized dendrimer-like DNA and DNA nanobarcodes from branched DNA via ligases. Here we report the construction of a hydrogel entirely from branched DNA that are three-dimensional and can be crosslinked in nature. These DNA hydrogels were biocompatible, biodegradable, inexpensive to fabricate and easily moulded into desired shapes and sizes. The distinct difference of the DNA hydrogel to other bio-inspired hydrogels (including peptide-based, alginate-based and DNA (linear)-polyacrylamide hydrogels) is that the crosslinking is realized via efficient, ligase-mediated reactions. The advantage is that the gelling processes are achieved under physiological conditions and the encapsulations are accomplished in situ-drugs including proteins and even live mammalian cells can be encapsulated in the liquid phase eliminating the drug-loading step and also avoiding denaturing conditions. Fine tuning of these hydrogels is easily accomplished by adjusting the initial concentrations and types of branched DNA monomers, thus allowing the hydrogels to be tailored for specific applications such as controlled drug delivery, tissue engineering, 3D cell culture, cell transplant therapy and other biomedical applications.

Molecular designer self-assembling peptides

Chemistry has generally been associated with inorganic and organic syntheses, metal-organic composites, coordinate metal chemistry, catalyses, block copolymer, coating, thin film, industrial surfactants and small-molecule drug development. That is about to change. Chemistry will also expand to the discovery and fabrication of biological and molecular materials with diverse structures, functionalities and utilities. The advent of biotechnology, nanotechnology and nanobiotechnology has accelerated this trend. Nature has selected and evolved numerous molecular architectural motifs at nanometer scale over billions of years for particular functions. These molecular nanomotifs can now be designed for new materials and nanodevices from the bottom up. Chemistry will again harness Nature's enormous power to benefit other disciplines and society. This tutorial review focuses on two self-assembling peptide systems.

In-Situ Formation of Biodegradable Hydrogels by Stereocomplexation of PEG−(PLLA)8 and PEG−(PDLA)8 Star Block Copolymers

Eight-arm poly(ethylene glycol)-poly(L-lactide), PEG-(PLLA)(8), and poly(ethylene glycol)-poly(D-lactide), PEG-(PDLA)(8), star block copolymers were synthesized by ring-opening polymerization of either L-lactide or D-lactide at room temperature in the presence of a single-site ethylzinc complex and 8-arm PEG (M(n) = 21.8 x 10(3) or 43.5 x 10(3)) as a catalyst and initiator, respectively. High lactide conversions (>95%) and well-defined copolymers with PLLA or PDLA blocks of the desired molecular weights were obtained. Star block copolymers were water-soluble when the number of lactyl units per poly(lactide) (PLA) block did not exceed 14 and 17 for PEG21800-(PLA)(8) and PEG43500-(PLA)(8), respectively. PEG-(PLA)(8) stereocomplexed hydrogels were prepared by mixing aqueous solutions with equimolar amounts of PEG-(PLLA)(8) and PEG-(PDLA)(8) in a polymer concentration range of 5-25 w/v % for PEG21800-(PLA)(8) star block copolymers and of 6-8 w/v % for PEG43500-(PLA)(8) star block copolymers. The gelation is driven by stereocomplexation of the PLLA and PDLA blocks, as confirmed by wide-angle X-ray scattering experiments. The stereocomplexed hydrogels were stable in a range from 10 to 70 degrees C, depending on their aqueous concentration and the PLA block length. Stereocomplexed hydrogels at 10 w/v % polymer concentration showed larger hydrophilic and hydrophobic domains as compared to 10 w/v % single enantiomer solutions, as determined by cryo-TEM. Correspondingly, dynamic light scattering showed that 1 w/v % solutions containing both PEG-(PLLA)(8) and PEG-(PDLA)(8) have larger "micelles" as compared to 1 w/v % single enantiomer solutions. With increasing polymer concentration and PLLA and PDLA block length, the storage modulus of the stereocomplexed hydrogels increases and the gelation time decreases. Stereocomplexed hydrogels with high storage moduli (up to 14 kPa) could be obtained at 37 degrees C in PBS. These stereocomplexed hydrogels are promising for use in biomedical applications, including drug delivery and tissue engineering, because they are biodegradable and the in-situ formation allows for easy immobilization of drugs and cells.