GMA‐based emulsion‐templated solid foams: Influence of co‐crosslinker on morphology and mechanical properties

Highly open porous polymer foams formed from high internal phase emulsions (polyHIPEs) are attracting significant interest because of their potential applications in many areas of advanced materials science. In this work, the influence of the crosslinker or co‐crosslinkers of different molecular weights on the morphology and mechanical properties of polyHIPEs containing glycidyl methacrylate (GMA) was studied. Several poly(ethylene glycol) dimethacrylate (PEGDMA) crosslinkers were considered. The results show that introducing higher molecular weight crosslinkers into polyHIPEs produces a more open structure, with significantly increased compression strength and deformation at breakage. This eliminated the undesirable brittleness and chalkiness commonly found in polyHIPE materials. The Young's modulus of GMA‐based polyHIPEs containing 40% poly(ethylene glycol) dimethacrylate increased by 50% and the crush strength by 400% when compared with traditional GMA/ethylene glycol dimethylacrylate polyHIPEs. This improvement in mechanical properties is expected to improve the suitability of polyHIPEs for use in a wide range of applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46295.     

Hemocompatibility, swelling and thermal properties of hydrogels based on 2-hydroxyethyl acrylate, itaconic acid and poly(ethylene glycol) dimethacrylate

Two series of novel hydrogels, based on 2-hydroxyethyl acrylate (HEA), itaconic acid (IA), and two poly(ethylene glycol) dimethacrylates (PEGDMA), of different ethylene glycol chain lengths, were prepared by free radical crosslinking copolymerization. The influence of different ethylene glycol chain lengths and concentration in P(HEA/IA/PEGDMA) hydrogels on biocompatibility, swelling and thermal properties was investigated. All samples in contact with blood showed a mean hemolysis value <1.0 % in the direct contact assay, and even <0.5 % in the indirect contact assay, for in vitro testing conditions. Swelling studies, conducted in a physiological pH and temperature range, showed pH sensitivity and relatively small changes of equilibrium swelling with temperature, which varied with PEGDMA molecular weight. The glass transition temperatures (T _g) of P(HEA/IA/PEGDMA) networks were in the range 28.1–36.9 °C, respectively, and also dependent on copolymer composition. Due to good biocompatibility, favorable swelling, and thermal properties these hydrogels show good potential for biomedical uses.     

Gelation and crosslinking characteristics of photopolymerized poly(ethylene glycol) hydrogels

The gelation and crosslinking features of poly(ethylene glycol) (PEG) hydrogels were scrutinized through the UV polymerization processes of poly(ethylene glycol) methacrylate (PEGMA) and poly(ethylene glycol) dimethacrylate (PEGDMA) mixtures. The real‐time evolutions of the elastic moduli of the prepolymerized mixtures with different crosslinking ratios of PEGMA and PEGDMA and the photoinitiator concentrations were measured during photopolymerization. The rheological properties were compared with other properties of the PEG hydrogels, including the relative changes in the C?C amounts in the mixtures before and after UV irradiation, water swelling ratio, gel fraction, mesh size, and mechanical hardness. As the portion of PEGDMA as a crosslinker increased, the final elastic modulus and gel fraction increased, whereas the swelling ratio and scratch penetration depth at the hydrogel film surface decreased because of the formation of compact networks inside the hydrogels. These results indicate that there was a good correlation between the rheological analysis for predicting the crosslinking transition during photopolymerization and the macroscopic properties of the crosslinked hydrogels. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41939.     

Construction and Properties of Double-Crosslinked Hydrogels Based on Host–Guest Interactions/UV Polymerization

Supramolecular hydrogels based on host–guest interactions have inherent flaw that the host molecules easily slide on or fall off the linear guest molecules, causing collapse of the networks. Hence, a double-crosslinking strategy is introduced in this study. The primary crosslinking formed via host–guest interactions between α-cyclodextrin (α-CD) and poly(ethylene glycol) dimethacrylate (PEGDMA) or between α-CD and four-arm poly(ethylene glycol) methacrylate (4arm PEG-MA). Then, secondary networks among PEGDMA or 4arm PEG-MA formed via UV-induced crosslinking. Results show that the fracture stress and fracture strain of PEGDMA-α-CD double-crosslinked hydrogels (P-C-U) increases up to 0.63 MPa and 71%, respectively, which significantly affected by molecular weight of PEGDMA. The double-crosslinking strategy helps increase the toughness up to 12.9 MJ m⁻³ (P_6k-0.025M-C-U) and 17.23 MJ m⁻³ (4P_10k-0.025M-C-U), as well as impart a certain degree of fatigue resistance to both PEGDMA hydrogels and 4arm-PEG-MA hydrogels, which is believed to be due to the energy dissipation mechanism introduced in the structure. The swelling capacity of double-crosslinked hydrogels is decreased compared to that with single-UV-crosslinked hydrogels, may be because the double-crosslinking strategy increases the crosslinking density of the hydrogel structure. In addition, both the molecular weight and concentration of PEGDMA and 4arm-PEG-MA influences the swelling capacity of the double-crosslinked hydrogels.     

Immobilization of myoglobin from horse skeletal muscle in hydrophilic polymer networks

This work examines the immobilization of myoglobin from horse skeletal muscle in hydrophilic polymer networks. Due to specific changes in the spectroscopic properties of hemoproteins during ligand binding, they could be employed in optical sensing devices. Two immobilization techniques were considered: imbibition and entrapment. Anionic hydrogels composed of methacrylic acid (MAA), cationic hydrogels composed of dimethylamino ethyl methacrylate (DMAEM), and neutral hydrogels composed of poly(ethylene glycol) monomethyl ether monomethacrylate (PEGMA; molecular weight = 200, 400, or 1000), all crosslinked with poly(ethylene glycol) dimethacrylate (PEGDMA) (molecular weight = 200, 600, or 1000), were synthesized by free‐radical solution polymerization. By the imbibition method, MAA‐based hydrogels incorporated the highest amount of myoglobin in comparison with PEGMA or DMAEM polymers. The evaluation of the correlation length of the networks revealed that MAA hydrogels had the highest correlation length in comparison with PEGMA‐containing matrices or DMAEM hydrogels. Release experiments from MAA hydrogels at pHs 5.8 and 7.0 showed that the solute‐transport mechanism was a combination of Fickian and chain relaxation diffusion. Myoglobin‐loaded MAA hydrogels retained their heme reactivity after the immobilization process. The release of myoglobin incorporated by entrapment in MAA–PEGDMA hydrogels was highly influenced by the chain relaxation process. The diffusion coefficients of myoglobin incorporated by entrapment into anionic hydrogels were 2 orders of magnitude smaller (～ 10–13) than those for myoglobin incorporated by imbibition (10–11), both evaluated at pH 7.0. Substrate binding studies indicated that the protein biological activity was not compromised in those hydrogels loaded by the imbibition method, whereas prepolymeric solutions showed detrimental effects on protein stability. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Mesoscale Characterization of Supramolecular Transient Networks Using SAXS and Rheology

Hydrogels and, in particular, supramolecular hydrogels show promising properties for application in regenerative medicine because of their ability to adapt to the natural environment these materials are brought into. However, only few studies focus on the structure-property relationships in supramolecular hydrogels. Here, we study in detail both the structure and the mechanical properties of such a network, composed of poly(ethylene glycol), end-functionalized with ureido-pyrimidinone fourfold hydrogen bonding units. This network is responsive to triggers such as concentration, temperature and pH. To obtain more insight into the sol-gel transition of the system, both rheology and small-angle X-ray scattering (SAXS) are used. We show that the sol-gel transitions based on these three triggers, as measured by rheology, coincide with the appearance of a structural feature in SAXS. We attribute this feature to the presence of hydrophobic domains where cross-links are formed. These results provide more insight into the mechanism of network formation in these materials, which can be exploited for tailoring their behavior for biomedical applications, where one of the triggers discussed might be used.     

Precision Control of Programmable Actuation of Thermoresponsive Nanocomposite Hydrogels with Multilateral Engineering

Hydrogels capable of stimuli-responsive deformation are widely explored as intelligent actuators for diverse applications. It is still a significant challenge, however, to “program” these hydrogels to undergo highly specific and extensive shape changes with precision, because the mechanical properties and deformation mechanism of the hydrogels are inherently coupled. Herein, two engineering strategies are simultaneously employed to develop thermoresponsive poly(N-isopropyl acrylamide) (PNIPAm)-based hydrogels capable of programmable actuation. First, PNIPAm is copolymerized with poly(ethylene glycol) diacrylate (PEGDA) with varying molecular weights and concentrations. In addition, graphene oxide (GO) or reduced graphene oxide (rGO) is incorporated to generate nanocomposite hydrogels. These strategies combine to allow the refined control of mechanical and diffusional properties of hydrogels over a broad range, which also directly influences variable thermoresponsive actuation. It is expected that this comprehensive design principle can be applied to a wide range of hydrogels for programmable actuation.     

Effects of pore morphology and size on antimicrobial activity of chitosan/poly(ethylene glycol) diacrylate macromer semi‐IPN hydrogels

The objective of this study was to obtain antibacterial active chitosan/poly(ethylene glycol) diacrylate macromere (CS/PEGM) semi‐IPN hydrogels near a neutral pH level by changing their pore size and morphology. These hydrogels were prepared from CS and PEGM with different molecular weights in the presence of pore‐forming agents, poly (ethylene glycol) (PEG) or sodium bicarbonate (NaHCO₃), by using two different initiator system, namely chemical or UV. A combination of CS with PEG or NaHCO₃ in the presence of PEGM could be able to create desired pore formation in both initiator systems. The antibacterial activity of hydrogels changed with the molecular weight (g/mol) of PEGM in the order 2000>400>8000. A chemical initiation system was found more suitable than the UV initiation system for antibacterial activity. Hydrogels showing the highest antibacterial activity on Staphylococcus aureus and Escherichia coli have medium or distributed pore size and interconnected pores. Hydrogels prepared with PEGM (M_n: 2000 g/mol) were proposed for antibacterial wound dressing and soft tissue regeneration applications owing to their antibacterial activity and elastic modulus. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42707.     

Systematic tailoring of water absorption in photopolymerizable (meth)acrylate networks and its effect on mechanical properties

Photopolymerizable (meth)acrylate networks offer several advantages as biomedical materials including their ability to be formed in situ, fast synthesis rates, and tailorable material properties. The objective of this study was to identify how phosphate buffered saline (PBS) absorption affects the thermomechanical properties of a ternary (meth)acrylate network. Copolymers consisting of 2‐hydroxyethyl (meth)acrylate (2HEMA), benzyl acrylate (BZA), and poly(ethylene glycol) dimethacrylate (PEGDMA; M_n ～ 750) were synthesized under UV with varying weight ratios of 2HEMA to BZA. Each composition underwent dynamic mechanical analysis, tensile strain‐to‐failure testing, Fourier Transform Infrared (FTIR) analysis, and swelling measurements after 24‐h immersion in PBS. Networks with higher 2HEMA concentrations absorbed larger amounts of PBS resulting in a larger decrease in the glass transition temperature. PBS absorption affects the mechanical properties of BZA‐2HEMA‐PEGDMA networks in a manner dependent upon the amount of PBS absorbed into the network. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of Molecular Weight of Poly(Ethylene Glycol) Dicarboxylate on the Properties of Cross-Linked Hydrogel Film as an Antiadhesion Barrier

Poly(ethylene glycol) dicarboxylate (PEGDC)/poly(ethylene oxide) (PEO) cross-linked hydrogel films were developed as an antiadhesion barrier using an e-beam. The effects of molecular weight of PEGDC on hydrogel properties were investigated. The decrease in molecular weight of PEGDC increased the gel fraction and tissue adhesion, whereas the mechanical strength did not change considerably. On the other hand, the swelling ratio decreased rapidly with decreasing molecular weight of PEGDC. The cytotoxicity of PEGDC (2000 or 3000) was low, whereas that of PEGDC (1000) was higher. In animal studies, all hydrogels showed a better antiadhesive effect compared to the control.     

New hybrid system: Poly(ethylene glycol) hydrogel with covalently bonded pegylated nanotubes

Hydrogels containing carbon nanotubes (CNTs) are expected to be promising conjugates because they might show a synergic combination of properties from both materials. Most of the hybrid materials containing CNTs only entrap them physically, and the covalent attachment has not been properly addressed yet. In this study, single‐walled carbon nanotubes (SWNTs) were successfully incorporated into a poly(ethylene glycol) (PEG) hydrogel by covalent bonds to form a hybrid material. For this purpose, SWNTs were functionalized with poly(ethylene glycol) methacrylate (PEGMA) to obtain water‐soluble pegylated SWNTs (SWNT–PEGMA). These functionalized SWNTs were covalently bonded through their PEG moieties to a PEG hydrogel. The hybrid network was obtained from the crosslinking reaction of poly(ethylene glycol) diacrylate prepolymer and the SWNT–PEGMA by dual photo‐UV and thermal initiations. The mechanical and swelling properties of the new hybrid material were studied. In addition, the material and lixiviates were analyzed to elucidate any kind of SWNT release and to evaluate a possible in vitro cytotoxic effect. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Study on the biocomposites with poly(ethylene glycol) dimethacrylate and surfaced‐grafted hydroxyapatite nanoparticles

In composites of hydroxyapatite (HA) nanoparticles with a polymer matrix, the aggregation of nanoparticles would induce structural defects. In order to improve the dispersibility of HA nanoparticles in poly(ethylene glycol) dimethacrylate (PEGDMA) matrix and enhance mechanical properties of the HA/PEGDMA composite as potential bone substitute material, surface‐grafted HA nanoparticles with poly(ethylene glycol) monomethacrylate (PEGMA) were prepared, and crosslinked with PEGDMA under UV light to form a composite. The structure of HA‐g‐PEGMA was characterized by X‐ray diffraction (XRD) and thermal gravimetric analysis (TGA). The dispersibility of HA‐g‐PEGMA nanoparticles in poly(PEGDMA) was evaluated by SEM. The mechanical properties of the composites were investigated by compressive test. The dispersibility of HA‐g‐PEGMA nanoparticles in poly(PEGDMA) matrix was better than the bare HA. At a 1 wt % content of loading, the strength of composites increased by 14%, and the modulus increased by 9%. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Preparation of hydrophilic polymer networks by post-curing reaction

Hydrophilic gels are a very important class of polymeric materials with extensive applications as biomedical products. The critical properties of hydrogels, such as sorption and desorption, mechanical behavior, swelling properties, etc., are controlled by network characteristics, i.e. degree of crosslinking and the density, distribution and length of crosslinks. Hydrogels prepared by copolymerization of 2-hydroxyethyl methacrylate (HEMA) with ethylene glycol dimethacrylate (EGDMA) have already been studied in detail. In this work, hydrophilic networks were prepared by crosslinking HEMA with EGDMA, and poly(2-hydroxy-ethyl methacrylate) (PHEMA) with diphenylmethane-4,4′-diisocyanate (MDI). The swelling properties of both types of networks were studied and the differences in behavior were attributed to the different techniques applied for network formation.     

Synthesis,characterization, and cytotoxicity of PCL–PEG–PCL diacrylate and agarose interpenetrating network hydrogels for cartilage tissue engineering

Hydrogels are suitable biomaterials for cartilage tissue engineering due to the excellent ability to retain water to provide suitable environment for the tissue, however, the insufficient mechanical properties often prevent their wider applications. The objective of this study was to fabricate biocompatible hydrogels with good mechanical performance, high-water content, and porous microstructure for cartilage regeneration. Photocrosslinked hydrogels are one of the most widely used systems in tissue engineering due to the superior mechanical properties. In this study, block copolymer, poly(ε -caprolactone)-poly(ethylene)-poly(ε -caprolactone) diacrylate (PCL–PEG–PCL; PEC), was prepared by ring-opening polymerization, and PEC hydrogels were made through free radical crosslinking mechanism. Agarose network is chosen as another component of the hydrogels, because of the high-swelling behavior and cartilage-like microstructure, which is helpful for chondrocytes growth. Interpenetrating networks (IPN) were fabricated by diffusing PEC into agarose network followed by photo-crosslinking process. It was noted that incorporating PEC into the agarose network increased the elastic modulus and the compressive failure properties of individual component networks. In addition, high-swelling ratio and uniform porosity microstructures were found in the IPN hydrogels. IPN and PEC showed low cytotoxicity and good biocompatibility in elution test method. The results suggest promising characteristics of IPN hydrogels as a potential biomaterial for cartilage tissue engineering.     

Fabrication of interpenetrating polymer network to enhance the biological activity of synthetic hydrogels

A three dimensional porous hydrogel with suitable biological and mechanical properties are required for bone tissue engineering. Hydrogels of poly(lactic-ethylene oxide fumarate) (PLEOF), crosslinked with poly(ethylene glycol)-diacrylate (PEG-da) have desirable mechanical properties, however, their application for bone regeneration is limited due to the lack of cell motif sites within their structure. The aim of this study was to incorporate a naturally derived polymer such as gelatin into PLEOF hydrogels to promote their biological properties. Interpenetrating polymer network (IPN) was used as an efficient technique to acquire uniform mixture of these two polymers. Additionally gas foaming agents were used to create pores with average diameter of 250 μm in these IPN hydrogels. The concentrations of PEG-da and gelatin were optimized to tune the mechanical strength and degradation properties of these hydrogels. A compression modulus of 500 kPa was achieved for hydrogel fabricated with 400 mg/ml PLEOF, 200 mg/ml PEG-da and 150 mg/ml gelatin. The addition of gelatin to PLEOF elevated the compression modulus by two-fold and decreased the energy loss by 40%. The result of protein analysis demonstrated that IPN substantially enhanced the retention of physically crosslinked gelatin in the 3D structure of hydrogel. More than 50% of gelatin was retained in IPN hydrogel after two weeks of incubation in simulated physiological environment. Preserving gelatin in the hydrogel structure provides cell motif sites for a longer period of time, which is desirable for uniform cell proliferation. In vitro studies showed that primary human osteoblast cells adhered and proliferated in PLEOF-gelatin hydrogel. These results demonstrated the potential of using this IPN hydrogel for bone tissue engineering.     

Double‐network hydrogel with high mechanical strength prepared from two biocompatible polymers

Novel double‐network (DN) hydrogels with high mechanical strength have been fabricated with two biocompatible polymers, poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG), through a simple freezing and thawing method. Some properties of the obtained hydrogels, such as the mechanical strength, rheological and thermodynamic behavior, drug release, and morphology, have been characterized. The results reveal that in sharp contrast to most common hydrogels made with simple natural or synthetic polymers, PVA/PEG hydrogels can sustain a compressive pressure as high as several megapascals, highlighting their potential application as biomedical materials. In addition, a model for describing the structural formation of PVA/PEG DN hydrogels is proposed: the condensed PVA‐rich phase forms microcrystals first, which bridge with one another to form a rigid and inhomogeneous net backbone to support the shape of the hydrogel, and then the dilute PEG‐rich phase partially crystallizes among the cavities or voids of the backbone; meanwhile, there are entanglements of molecular chains between the two polymers. Moreover, a mechanism is also proposed to explain the high mechanical strength of PVA/PEG DN hydrogels. It is suggested that the free motion of PEG clusters in the cavities of PVA networks can prevent the crack from growing to a macroscopic level because the linear PEG chains in the cavities effectively absorb the crack energy and relax the local stress either by viscous dissipation or by large deformation of the PEG chains. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Stereocomplexed 8-armed poly(ethylene glycol)–poly(lactide) star block copolymer hydrogels: Gelation mechanism,mechanical properties and degradation behavior

Mixing aqueous poly(ethylene glycol)-poly(d-lactide) and poly(ethylene glycol)-poly(l-lactide) star block copolymer solutions resulted in the formation of stereocomplexed hydrogels within 1 min. A study towards the mechanism of the temperature dependent formation of stereocomplexes in the hydrogels using rheology and nuclear magnetic resonance experiments revealed that the formation of stereocomplexes is facilitated at higher temperatures, due to rearrangement in the micellar aggregates thereby exposing more PLA units available for stereocomplexation. The formed gels became temperature irreversible due to the presence of highly stable semi-crystalline stereocomplexed PLA domains. An enantiomeric mixture of 8-armed star block copolymers linked by an amide group between the poly(ethylene glycol) core and the poly(lactide) arms (PEG–(NHCO)–(PLA)₈) yielded hydrogels with improved mechanical properties and stability at 37 °C in PBS compared to 8-armed star block copolymers linked by an ester group. The possibility to be formed in situ in combination with their robustness make PEG–(NHCO)–(PLA)₈ hydrogels appealing materials for various biomedical applications.     

Behavior of semi IPN hydrogels composed of PEG and AAm/SMA copolymers in swelling and uptake of Janus Green B from aqueous solutions

Semi-interpenetrating polymer network (semi IPN) hydrogels of poly(ethylene glycol; PEG) were prepared as a water adsorbent for dye (Janus Green B) sorption. For this, PEG and copolymer of acrylamide/sodium methacrylate (AAm/SMA) were prepared by polymerization of aqueous solution of acrylamide (AAm), sodium methacrylate (SMA) using ammonium persulfate (APS)/N,N,N′,N′-tetramethylethylenediamine (TEMED) as redox initiating pair in presence of PEG and poly(ethylene glycol)dimethacrylate (PEGDMA) as crosslinker. FTIR spectroscopy was used to identify the presence of different repeating units in the semi IPNs. Some swelling and diffusion characteristics were calculated for different semi IPNs and hydrogels prepared under various formulations. Water uptake and dye sorption properties of AAm/SMA hydrogels and AAm/SMA/PEG semi IPNs were investigated as a function of chemical composition of the hydrogels. Janus Green B have used in sorption studies. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Poly(ethylene glycol)‐based hydrogels as cartilage substitutes: Synthesis and mechanical characteristics

Cartilage substitutes are needed to replace cartilage tissue, damaged in accidents or by pathologies (e.g., osteoarthritis). Treatment by total hip replacement has disadvantages, particularly due to immunological reaction to the implant's wear debris. One promising alternative is to replace damaged cartilage with substitutes based on hydrogel‐type material, designed to mimic the structure and properties of cartilage. The development of such a substitute must consider a wide spectrum of requirements. In this study, we addressed one aspect of this development namely the preparation and investigation of hydrogels exhibiting the required mechanical characteristics. To this aim, poly(ethylene glycol) (PEG) hydrogels and amphiphilic interpenetrating polymer networks (IPNs) of PEG with poly(methyl methacrylate) (PMMA) were prepared and characterized for their mechanical and swelling properties. Twenty‐seven types of hydrogels were synthesized, differing in their composition: PEG molecular weight, crosslink density, and PMMA volume fraction. The properties measured were water content, compression modulus, strength, fatigue durability, and poroelastic properties (hydraulic permeability and equilibrium modulus). All were investigated as functions of hydrogel's composition. Results show that lower PEG M_w, higher crosslink densities and higher PMMA fraction, all lead to higher modulus and lower water content, and that these properties can be controlled independently by proper choice of ingredients. Introduction of IPN greatly improved the hydrogels' strength. No reduction in the compression modulus resulting from fatigue damage was evident. Poroelastic properties varied nonmonotonously with structural characteristics. Seven types of the hydrogels were found to fit cartilage in their water content, modulus, and poroelastic properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of macromer synthesis time on the properties of the resulting poly(β‐amino ester) degradable hydrogel

Poly(β‐amino ester) biodegradable hydrogels are common in biomedical applications because of their tunable properties and similarities to natural soft tissue. Previous work has shown property adjustments through the choice of monomers, the ratio between monomers and the addition of a crosslinking component. Here, we show that the reaction time for the creation of the macromer can affect the resulting hydrogel properties, and thus provides another method of tuning properties. Macromer was created through the reaction of isobutylamine with poly(ethylene glycol) diacrylate (n = 400). The reaction progress was analyzed using IR and GPC analysis. Hydrogels were created through UV photopolymerization from macromers synthesized for 24, 36, and 48 h. The degradation, compressive moduli, and swelling were measured in an aqueous solution. All showed significant differences between hydrogels of different macromer synthesis times. These differences likely stem from the incomplete macromer synthesis reaction and resulting PEG‐rich regions in hydrogels from shorter synthesis times. These regions will not readily degrade, but do increase the mechanical properties and extent of swelling. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011