Highly extensible, tough, and elastomeric nanocomposite hydrogels from poly(ethylene glycol) and hydroxyapatite nanoparticles

Unique combinations of hard and soft components found in biological tissues have inspired researchers to design and develop synthetic nanocomposite gels and hydrogels with elastomeric properties. These elastic materials can potentially be used as synthetic mimics for diverse tissue engineering applications. Here we present a set of elastomeric nanocomposite hydrogels made from poly(ethylene glycol) (PEG) and hydroxyapatite nanoparticles (nHAp). The aqueous nanocomposite PEG-nHAp precursor solutions can be injected and then covalently cross-linked via photopolymerization. The resulting PEG-nHAp hydrogels have interconnected pore sizes ranging from 100 to 300 nm. They have higher extensibilities, fracture stresses, compressive strengths, and toughness when compared with conventional PEO hydrogels. The enhanced mechanical properties are a result of polymer nanoparticle interactions that interfere with the permanent cross-linking of PEG during photopolymerization. The effect of nHAp concentration and temperature on hydrogel swelling kinetics was evaluated under physiological conditions. An increase in nHAp concentration decreased the hydrogel saturated swelling degree. The combination of PEG and nHAp nanoparticles significantly improved the physical and chemical hydrogel properties as well as some biological characteristics such as osteoblast cell adhesion. Further development of these elastomeric materials can potentially lead to use as a matrix for drug delivery and tissue repair especially for orthopedic applications.     

Characterization of protein release from hydrolytically degradable poly(ethylene glycol) hydrogels

We present a novel fully hydrophilic, hydrolytically degradable poly(ethylene glycol) (PEG) hydrogel suitable for soft tissue engineering and delivery of protein drugs. The gels were designed to overcome drawbacks associated with current PEG hydrogels (i.e., reaction mechanisms or degradation products that compromise protein stability): the highly selective and mild cross‐linking reaction allowed for encapsulating proteins prior to gelation without altering their secondary structure as shown by circular dichroism experiments. Further, hydrogel degradation and structure, represented by mesh size, were correlated to protein release. It was determined that polymer density had the most profound effect on protein diffusivity, followed by the polymer molecular weight, and finally by the specific chemical structure of the cross‐linker. By examining the diffusion of several model proteins, we confirmed that the protein diffusivity was dependent on protein size as smaller proteins (e.g., lysozyme) diffused faster than larger proteins (e.g., Ig). Furthermore, we demonstrated that the protein physical state was preserved upon encapsulation and subsequent release from the PEG hydrogels and contained negligible aggregation or protein–polymer adducts. These initial studies indicate that the developed PEG hydrogels are suitable for release of stable proteins in drug delivery and tissue engineering applications. Biotechnol. Bioeng. 2011; 108:197–206. © 2010 Wiley Periodicals, Inc.     

Characterization of photo-cross-linked oligo[poly(ethylene glycol) fumarate] hydrogels for cartilage tissue engineering

Photo-cross-linkable oligo[poly(ethylene glycol) fumarate] (OPF) hydrogels have been developed for use in tissue engineering applications. We demonstrated that compressive modulus of these hydrogels increased with increasing polymer concentration, and hydrogels with different mechanical properties were formed by altering the ratio of cross-linker/polymer in precursor solution. Conversely, swelling of hydrogels decreased with increasing polymer concentration and cross-linker/polymer ratio. These hydrogels are degradable and degradation rates vary with the change in cross-linking level. Chondrocyte attachment was quantified as a method for evaluating adhesion of cells to the hydrogels. These data revealed that cross-linking density affects cell behavior on the hydrogel surfaces. Cell attachment was greater on the samples with increased cross-linking density. Chondrocytes on these samples exhibited spread morphology with distinct actin stress fibers, whereas they maintained their rounded morphology on the samples with lower cross-linking density. Moreover, chondrocytes were photoencapsulated within various hydrogel networks. Our results revealed that cells encapsulated within 2-mm thick OPF hydrogel disks remained viable throughout the 3-week culture period, with no difference in viability across the thickness of hydrogels. Photoencapsulated chondrocytes expressed the mRNA of type II collagen and produced cartilaginous matrix within the hydrogel constructs after three weeks. These findings suggest that photo-cross-linkable OPF hydrogels may be useful for cartilage tissue engineering and cell delivery applications.     

Experimental Investigation of Mechanical and Thermal Properties of Silica Nanoparticle-Reinforced Poly(acrylamide) Nanocomposite Hydrogels

Current studies investigating properties of nanoparticle-reinforced polymers have shown that nanocomposites often exhibit improved properties compared to neat polymers. However, over two decades of research, using both experimental studies and modeling analyses, has not fully elucidated the mechanistic underpinnings behind these enhancements. Moreover, few studies have focused on developing an understanding among two or more polymer properties affected by incorporation of nanomaterials. In our study, we investigated the elastic and thermal properties of poly(acrylamide) hydrogels containing silica nanoparticles. Both nanoparticle concentration and size affected hydrogel properties, with similar trends in enhancements observed for elastic modulus and thermal diffusivity. We also observed significantly lower swellability for hydrogel nanocomposites relative to neat hydrogels, consistent with previous work suggesting that nanoparticles can mediate pseudo crosslinking within polymer networks. Collectively, these results indicate the ability to develop next-generation composite materials with enhanced mechanical and thermal properties by increasing the average crosslinking density using nanoparticles.     

Development of a biostable replacement for PEGDA hydrogels

The exceptional tunability of poly(ethylene glycol) (PEG) hydrogel chemical, mechanical, and biological properties enables their successful use in a wide range of biomedical applications. Although PEG diacrylate (PEGDA) hydrogels are often used as nondegradable controls in short-term in vitro studies, it is widely acknowledged that the hydrolytically labile esters formed upon acrylation of the PEG diol make them susceptible to slow degradation in vivo. A PEG hydrogel system that maintains the desirable properties of PEGDA while improving biostability would be valuable in preventing degradation-related failure of gel-based devices in long-term in vivo applications. To this end, PEG diacrylamide (PEGDAA) hydrogels were synthesized and characterized in quantitative comparison to traditional PEGDA hydrogels. It was found that PEGDAA hydrogel modulus and swelling can be tuned over a similar range and to comparable degrees as PEGDA hydrogels with changes in macromer molecular weight and concentration. Additionally, PEGDAA cytocompatibility, low cell adhesion, and capacity for incorporation of bioactivity were analogous to that of PEGDA. In vitro hydrolytic degradation studies showed that the amide-based PEGDAA had significantly increased biostability relative to PEGDA. Overall, these findings indicate that PEGDAA hydrogels are a suitable replacement for PEGDA hydrogels with enhanced hydrolytic resistance. In addition, these studies provide a quantitative measure of the hydrolytic degradation rate of PEGDA hydrogels which was previously lacking in the literature.     

Synthesis and evaluation of novel biodegradable hydrogels based on poly(ethylene glycol) and sebacic acid as tissue engineering scaffolds

Novel biodegradable poly(ethylene glycol) (PEG) based hydrogels, namely, PEG sebacate diacrylate (PEGSDA) were synthesized, and their properties were evaluated. Chemical structures of these polymers were confirmed by Fourier transform infrared and proton nuclear magnetic resonance (1H NMR) spectroscopy. After photopolymerization, the dynamic shear modulus of the hydrogels was up to 0.2 MPa for 50% PEGSDA hydrogel, significantly higher than conventional hydrogels such as PEG diacrylate (PEGDA). The swelling ratios of these macromers were significantly lower than PEGDA. The in vitro degradation study demonstrated that these hydrogels were biodegradable with weight losses about 66% and 32% for 25% and 50% PEGSDA after 8 weeks of incubation in phosphate-buffered saline at 37 degrees C. In vitro biocompatibility was assessed using cultured rat bone marrow stromal cells (MSCs) in the presence of unreacted monomers or degradation products. Unlike conventional PEGDA hydrogels, PEGSDA hydrogel without RGD peptide modification induced MSC cell adhesion similar to tissue culture polystyrene. Finally, complex three-dimensional structures of PEGSDA hydrogels using solid free form technique were fabricated and their structure integrity was better maintained than PEGDA hydrogels. These hydrogels may find use as scaffolds for tissue engineering applications.     

Synthesis and characterization of dendron cross-linked PEG hydrogels as corneal adhesives

In pursuit of a wound-specific corneal adhesive, hydrogels formed by the reaction of propionaldehyde, butyraldehyde, or 2-oxoethyl succinate-functionalized poly(ethylene glycol) (PEG) with a peptide-based dendritic cross-linker (Lys(3)Cys(4)) were characterized. These macromers react within minutes of mixing to form transparent and elastic hydrogels with in vitro degradation times that range from hours to months based on the type of bonds formed during the cross-linking reaction, either thiazolidine or pseudoproline. The mechanical properties of these materials, determined via parallel plate rheology, were dependent on the polymer concentration, as was the hydrogel adhesive strength, which was determined by lap shear adhesive testing. In addition, these hydrogels were efficacious in closing ex vivo 4.1 mm central corneal lacerations: wounds closed with these hydrogel adhesives were able to withstand intraocular pressure values equivalent to, or in excess of, those obtained by closing the wounds with suturing.     

Synthesis of oligo(poly(ethylene glycol) fumarate)

This protocol describes the synthesis of oligo(poly(ethylene glycol) fumarate) (OPF; 1-35 kDa; a polymer useful for tissue engineering applications) by a one-pot reaction of poly(ethylene glycol) (PEG) and fumaryl chloride. The procedure involves three parts: dichloromethane and PEG are first dried; the reaction step follows, in which fumaryl chloride and triethylamine are added dropwise to a solution of PEG in dichloromethane; and finally, the product solution is filtered to remove by-product salt, and the OPF product is twice crystallized, washed and dried under vacuum. The reaction is affected by the molecular weight of PEG and reactant molar ratio. The OPF product is cross-linked by radical polymerization by either a thermally induced or ultraviolet-induced radical initiator, and the physical properties of the OPF oligomer and resulting cross-linked hydrogel are easily tailored by varying PEG molecular weight. OPF hydrogels are injectable, they polymerize in situ and they undergo biodegradation by hydrolysis of ester bonds. The expected time required to complete this protocol is 6 d.     

An approach to modulate degradation and mesenchymal stem cell behavior in poly(ethylene glycol) networks

A simple, sequential approach for creation of hydrolytically degradable poly(ethylene glycol) (PEG) hydrogels has been developed and characterized. The chemistry involves an initial step growth polymerization reaction between PEG-diacrylate and dithiothreitol (DTT) to form acrylate-terminated (-PEG-DTT-)n PEG chains, followed by photocross-linking to form a hydrogel network. Varying the extent of step growth polymerization prior to photocross-linking allowed for control over the equilibrium swelling ratio, degradation, and erosion of PEG hydrogels. Hydrogel degradability had a significant effect on behavior of human mesenchymal stem cells (hMSCs) encapsulated within PEG hydrogels, both in the presence and absence of an RGDSP cell adhesion ligand. In particular, enhanced network degradability resulted in enhanced hMSC viability and spreading during in vitro culture. Comparison of degradable and nondegradable hydrogels with similar physical properties (e.g., equilibrium swelling ratio) demonstrated that hMSC viability and spreading were dependent on network degradability. This study demonstrates that hydrolytically degradable PEG hydrogels can be formed via a sequential step growth polymerization and photocross-linking process and the resulting materials may serve as promising matrices for 3-dimensional stem cell culture and tissue engineering applications.     

Combined influence of substrate stiffness and surface topography on the antiadhesive properties of Acr-sP(EO-stat-PO) hydrogels

Biomaterials that prevent nonspecific protein adsorption and cell adhesion are of high relevance for diverse applications in tissue engineering and diagnostics. One of the most widely applied materials for this purpose is Poly(ethylene glycol) (PEG). We have investigated how micrometer line topography and substrate elasticity act upon the antiadhesive properties of PEG-based hydrogels. In our studies we apply bulk hydrogel cross-linked from star-shaped poly(ethylene oxide-stat-propylene oxide) macromonomers. Substrate surfaces were topographically patterned via replica molding. Additionally, the mechanical properties were altered by variations in the cross-linking density. Surface patterns with dimensions in the range of the cells' own size, namely 10 μm wide grooves, induced significant cell adhesion and spreading on the Acr-sP(EO-stat-PO) hydrogels. In contrast, there was only little adhesion to smaller and larger pattern sizes and no adhesion at all on the smooth substrates, regardless the rigidity of the gel. The effect of varied substrate stiffness on cell behavior was only manifest in combination with topography. Softer substrates with line patterns lead to significantly higher cell adhesion and spreading than stiff substrates. We conclude that the physical and mechanical surface characteristics can eliminate the nonadhesive properties of PEG-based hydrogels to a large extent. This has to be taken into account when designing surfaces for biomedical application such as scaffolds for tissue engineering which rely on the inertness of PEG.     

Development of macroporous poly(ethylene glycol) hydrogel arrays within microfluidic channels

The mass transport of solutes through hydrogels is an important design consideration in materials used for tissue engineering, drug delivery, and protein arrays used to quantify protein concentration and activity. We investigated the use of poly(ethylene glycol) (PEG) as a porogen to enhance diffusion of macromolecules into the interior of polyacrylamide and PEG hydrogel posts photopatterned within microfluidic channels. The diffusion of GST-GFP and dextran-FITC into hydrogels was monitored and effective diffusion coefficients were determined by fitting to the Fickian diffusion equations. PEG-diacrylate (M(r) 700) with porogen formed a macroporous structure and permitted significant penetration of 250 kDa dextran. Proteins copolymerized in these macroporous hydrogels retained activity and were more accessible to antibody binding than proteins copolymerized in nonporous gels. These results suggest that hydrogel macroporosity can be tuned to regulate macromolecular transport in applications such as tissue engineering and protein arrays.     

Synthesis of in situ cross-linkable macroporous biodegradable poly(propylene fumarate-co-ethylene glycol) hydrogels

This study describes a synthesis method of biodegradable macroporous hydrogels suitable as in situ cross-linkable biomaterials. Macroporous hydrogels were based on poly(propylene fumarate-co-ethylene glycol) and prepared via coupled free radical and pore formation reactions. Cross-linking was initiated by a pair of redox initiators, ammonium persulfate and L-ascorbic acid. Pores were formed by the reaction between L-ascorbic acid and sodium bicarbonate, a basic component, which evolved carbon dioxide. Sol fraction of the hydrogels was varied from 0.06 +/- 0.01 to 0.64 +/- 0.01. A stereological approach was used to analyze the morphological properties of the macroporous hydrogels by relating the morphological properties of thin sections to the original three-dimensional macroporous hydrogel. Prepared macroporous hydrogels had porosities between 0.43 +/- 0.08 and 0.84 +/- 0.02 and surface area densities between 55 +/- 3 and 108 +/- 7 cm(-1). Sodium bicarbonate concentration had the greatest effect on both the porosity and surface area density. The effect of copolymer formulation on the porosity and surface area density was insignificant. From thin sections of the macroporous hydrogels, the profile size distributions were determined as an estimate of the pore size distribution. Two formulations synthesized with varying L-ascorbic acid concentration of 0.05 and 0.1 M had median profile sizes of 50-100 and 150-200 microm, respectively. This novel synthesis method allows for the in situ cross-linking of biodegradable macroporous hydrogels with morpholological properties suitable for consideration as an injectable tissue engineering scaffold.     

Synthesis and properties of thermo- and pH-sensitive poly(N-isopropylacrylamide)/polyaspartic acid IPN hydrogels

Various interpenetrating polymer network (IPN) hydrogels with sensitivity to temperature and pH were prepared by introducing the pH-sensitive polymer polyaspartic acid (PASP) hydrogel, into the poly(N-isopropylacrylamide) (PNIPAAm) hydrogel system for the purpose of improving its response rate to temperature. The morphologies and thermal behavior of the prepared IPN hydrogels were studied by both scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The IPN hydrogels showed a large and uneven porous network structure, without showing the common PNIPAAm hydrogel structure. The paper moreover studied their swelling properties, such as temperature dependence of equilibrium swelling ratio, shrinking kinetics, re-swelling kinetics and oscillatory swelling behavior in water. The swelling experiment results revealed that IPN hydrogels exhibited much faster shrinking and re-swelling in function of the composition ratio of the two network components. These fast responsive hydrogels foster potential applications in biomedical and biotechnology fields.     

Hydrogels for osteochondral repair based on photocrosslinkable carbamate dendrimers

First generation, photocrosslinkable dendrimers consisting of natural metabolites (i.e., succinic acid, glycerol, and beta-alanine) and nonimmunogenic poly(ethylene glycol) (PEG) were synthesized divergently in high yields using ester and carbamate forming reactions. Aqueous solutions of these dendrimers were photocrosslinked with an eosin-based photoinitiator to afford hydrogels. The hydrogels displayed a range of mechanical properties based on their structure, generation size, and concentration in solution. All of the hydrogels showed minimal swelling characteristics. The dendrimer solutions were then photocrosslinked in situ in an ex vivo rabbit osteochondral defect (3 mm diameter and 10 mm depth), and the resulting hydrogels were subjected to physiologically relevant dynamic loads. Magnetic resonance imaging (MRI) showed the hydrogels to be fixated in the defect site after the repetitive loading regimen. The ([G1]-PGLBA-MA) 2-PEG hydrogel was chosen for the 6 month pilot in vivo rabbit study because this hydrogel scaffold could be prepared at low polymer weight (10 wt %) and possessed the largest compressive modulus of the 10% formulations, a low swelling ratio, and contained carbamate linkages, which are more hydrolytically stable than the ester linkages. The hydrogel-treated osteochondral defects showed good attachment in the defect site and histological analysis showed the presence of collagen II and glycosaminoglycans (GAGs) in the treated defects. By contrast, the contralateral unfilled defects showed poor healing and negligible GAG or collagen II production. Good mechanical properties, low swelling, good attachment to the defect site, and positive in vivo results illustrate the potential of these dendrimer-based hydrogels as scaffolds for osteochondral defect repair.     

Synthetically simple, highly resilient hydrogels

Highly resilient synthetic hydrogels were synthesized by using the efficient thiol-norbornene chemistry to cross-link hydrophilic poly(ethylene glycol) (PEG) and hydrophobic polydimethylsiloxane (PDMS) polymer chains. The swelling and mechanical properties of the hydrogels were controlled by the relative amounts of PEG and PDMS. The fracture toughness (G(c)) was increased to 80 J/m(2) as the water content of the hydrogel decreased from 95% to 82%. In addition, the mechanical energy storage efficiency (resilience) was more than 97% at strains up to 300%. This is comparable with one of the most resilient materials known: natural resilin, an elastic protein found in many insects, such as in the tendons of fleas and the wings of dragonflies. The high resilience of these hydrogels can be attributed to the well-defined network structure provided by the versatile chemistry, low cross-link density, and lack of secondary structure in the polymer chains.     

Porphyrin-cross-linked hydrogel for fluorescence-guided monitoring and surgical resection

We demonstrate that porphyrins can be used as efficient cross-linkers to generate a new class of hydrogels with enabling optical properties. Tetracarboxylic acid porphyrins reacted with PEG diamines to form a condensation polyamide in a range of appropriate conditions, with respect to reaction time, diisopropylethylamine initiator concentration, porphyrin-to-PEG ratio, porphyrin concentration, and PEG size. The network structure of the hydrogel maintained a porphyrin spacing that prevented excessive fluorescence self-quenching despite high porphyrin density. The near-infrared properties readily enabled low background, noninvasive fluorescence monitoring of the implanted hydrogel in vivo, as well as its image-guided surgical removal in real time using a low-cost fluorescence camera prototype. Emission could be tuned by incorporating copper metalloporphyrins into the network. The approach of creating hydrogels using cross-linking porphyrin comonomers creates opportunities for new polymer designs with strong optical character.     

Polysaccharide-poly(ethylene glycol) star copolymer as a scaffold for the production of bioactive hydrogels

The production of polysaccharide-derivatized surfaces, polymers, and biomaterials has been shown to be a useful strategy for mediating the biological properties of materials, owing to the importance of polysaccharides for the sequestration and protection of bioactive proteins in vivo. We have therefore sought to combine the benefits of polysaccharide derivatization of polymers with unique opportunities to use these polymers for the production of bioactive, noncovalently assembled hydrogels. Accordingly, we report the synthesis of a heparin-modified poly(ethylene glycol) (PEG) star copolymer that can be used in the assembly of bioactive hydrogel networks via multiple strategies and that is also competent for the delivery of bioactive growth factors. A heparin-decorated polymer, synthesized by the reaction of thiol end-terminated four-arm star PEG (M(n) = 10 000) with maleimide functionalized low molecular weight heparin (LMWH, M(r) = 3000), has been characterized via (1)H NMR spectroscopy and size-exclusion chromatography; results indicate attachment of the LMWH with at least 73% efficiency. Both covalently and noncovalently assembled hydrogels can be produced from the PEG-LMWH conjugate. Viscoelastic noncovalently assembled hydrogels have been formed on the basis of the interaction of the PEG-LMWH with a PEG polymer bearing multiple heparin-binding peptide motifs. The binding and release of therapeutically important proteins from the assembled hydrogels have also been demonstrated via immunochemical assays, which demonstrate the slow release of basic fibroblast growth factor (bFGF) as a function of matrix erosion. The combination of these results suggests the opportunities for producing polymer-polysaccharide conjugates that can assemble into novel hydrogel networks on the basis of peptide-saccharide interactions and for employing these materials in delivery applications.     

Tailoring the degradation of hydrogels formed from multivinyl poly(ethylene glycol) and poly(vinyl alcohol) macromers for cartilage tissue engineering

Tuning the degradation profiles of polymer cell carriers to match cell and tissue growth is an important design parameter for (cartilage) tissue engineering. In this study, degradable hydrogels were fabricated from divinyl, tetrafunctional poly(ethylene glycol) (PEG) and multivinyl, multifunctional poly(vinyl alcohol) (PVA) macromers to form homopolymer and copolymer gels. These gels were characterized by their volumetric swelling ratio and mass loss profiles as a function of degradation time. By variation of the macromer chemistry and functionality, the degradation time changed from less than 1 day for homopolymer PVA gels to 34 days for pure PEG gels. Furthermore, the degrading medium influenced mass loss, and a marked decrease in degradation time, from 34 to 12 days, was observed with the PEG gels when a chondrocyte-specific medium containing fetal bovine serum was employed. Interestingly, when copolymer gels of PEG and PVA were formed, PVA was released throughout the degradation (as determined by gel permeation chromatography) suggesting that covalent cross-linking of the PVA in the network was facilitated by copolymerizing with the PEG macromer. To assess these novel gels for cartilage tissue engineering applications, chondrocytes were photoencapsulated in the copolymer networks and cultured in vitro for up to 6 weeks. DNA, glycosaminoglycan (GAG), and total collagen contents increased with culture time, and the resulting neocartilaginous tissue at 6 weeks was homogeneously distributed as seen histologically. Biochemical analysis revealed that the constructs were comprised of 0.66 +/- 0.04 microg of DNA/mg wet weight (ww), 1.0 +/- 0.05% GAG/ww, and 0.29 +/- 0.07% total collagen/ww at 6 weeks. Furthermore, the compressive modulus increased during culture from 7 to 97 kPa as the neocartilaginous tissue evolved and the gel degraded. In summary, fabricating hydrogels through the copolymerization of PEG and PVA macromers is an effective tool for encapsulating chondrocytes, controlling gel degradation profiles, and generating cartilaginous tissue.     

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.     

Thermo-responsive PNiPAAm-g-PEG films for controlled cell detachment

A series of graft copolymers consisting of either poly(N-isopropylacrylamide) (PNiPAAm) or poly(N,N-diethylacrylamide) (PDEAAm) as a thermo-responsive component in the polymer backbone and poly(ethyleneglycol) (PEG) were immobilized as thin films and cross-linked on a fluoropolymer substrate using low-pressure argon plasma treatment. The surface-immobilized hydrogels exhibit a transition from partially collapsed to completely swollen, which is in the range of 32-35 degrees C and corresponds to the lower critical solution temperature of the soluble polymers. The hydrogels were used as cell carriers in culture experiments with L929 mouse fibroblast cells to probe for cell adhesion, proliferation, and temperature-dependent detachment of cell layers. The fibroblast cells adhere, spread, and proliferate on the hydrogel layers at 37 degrees C and become completely detached after reducing the temperature by 3 K. The cell release characteristics were further correlated to the swelling and collapsing behavior of the hydrogel films and the polymer solutions as measured in PBS solution and RPMI cell cultivation medium. It could be shown that, long before the swelling has completed upon temperature reduction, the cells detach. This can be attributed to the large content of PEG present in the hydrogel, which weaken the cell adhesion strength to the hydrogel layers.