Intraarterial protein delivery via intimally-adherent bilayer hydrogels

Bilayer Hydrogels for Wound Dressing and Tissue Engineering

A large number of different skin diseases such as hits, acute, and chronic wounds dictate the search for alternative and effective treatment options. The wound healing process requires a complex approach, the key step of which is the choice of a dressing with controlled properties. Hydrogel-based scaffolds can serve as a unique class of wound dressings. Presented on the commercial market, hydrogel wound dressings are not found among proposals for specific cases and have a number of disadvantages—toxicity, allergenicity, and mechanical instability. Bilayer dressings are attracting great attention, which can be combined with multifunctional properties, high criteria for an ideal wound dressing (antimicrobial properties, adhesion and hemostasis, anti-inflammatory and antioxidant effects), drug delivery, self-healing, stimulus manifestation, and conductivity, depending on the preparation and purpose. In addition, advances in stem cell biology and biomaterials have enabled the design of hydrogel materials for skin tissue engineering. To improve the heterogeneity of the cell environment, it is possible to use two-layer functional gradient hydrogels. This review summarizes the methods and application advantages of bilayer dressings in wound treatment and skin tissue regeneration. Bilayered hydrogels based on natural as well as synthetic polymers are presented. The results of the in vitro and in vivo experiments and drug release are also discussed.

Smart Bioinks for the Printing of Human Tissue Models

3D bioprinting has tremendous potential to revolutionize the field of regenerative medicine by automating the process of tissue engineering. A significant number of new and advanced bioprinting technologies have been developed in recent years, enabling the generation of increasingly accurate models of human tissues both in the healthy and diseased state. Accordingly, this technology has generated a demand for smart bioinks that can enable the rapid and efficient generation of human bioprinted tissues that accurately recapitulate the properties of the same tissue found in vivo. Here, we define smart bioinks as those that provide controlled release of factors in response to stimuli or combine multiple materials to yield novel properties for the bioprinting of human tissues. This perspective piece reviews the existing literature and examines the potential for the incorporation of micro and nanotechnologies into bioinks to enhance their properties. It also discusses avenues for future work in this cutting-edge field.

3D Bioprinting Photo-Crosslinkable Hydrogels for Bone and Cartilage Repair

Three-dimensional (3D) bioprinting has become a promising strategy for bone manufacturing, with excellent control over geometry and microarchitectures of the scaffolds. The bioprinting ink for bone and cartilage engineering has thus become the key to developing 3D constructs for bone and cartilage defect repair. Maintaining the balance of cellular viability, drugs or cytokines' function, and mechanical integrity is critical for constructing 3D bone and/or cartilage scaffolds. Photo-crosslinkable hydrogel is one of the most promising materials in tissue engineering; it can respond to light and induce structural or morphological transition. The biocompatibility, easy fabrication, as well as controllable mechanical and degradation properties of photo-crosslinkable hydrogel can meet various requirements of the bone and cartilage scaffolds, which enable it to serve as an effective bio-ink for 3D bioprinting. Here, in this review, we first introduce commonly used photo-crosslinkable hydrogel materials and additives (such as nanomaterials, functional cells, and drugs/cytokine), and then discuss the applications of the 3D bioprinted photo-crosslinkable hydrogel scaffolds for bone and cartilage engineering. Finally, we conclude the review with future perspectives about the development of 3D bioprinting photo-crosslinkable hydrogels in bone and cartilage engineering.

Design framework for mechanically tunable soft biomaterial composites enhanced by modified horseshoe lattice structures

Soft biomaterials have a wide range of applications in many areas. However, one material can only cover a specific range of mechanical performance such as the elastic modulus and stretchability. In order to improve the mechanical performance of soft biomaterials, lattice structures are embedded to reinforce the biomaterials. In this paper, rectangular and triangular lattice structures formed by modified horseshoe microstructures are used because their mechanical properties are tunable and can be tailored precisely to match the desired properties by adjusting four geometrical parameters, the length L, radius R, width w and arc angle θ0. A theoretical design framework for the modified horseshoe lattice structures is developed to predict the dependence of the mechanical behaviors on geometrical parameters. Both experiments and finite element simulations on lattice structures are conducted to validate the theoretical models. Results show that a wide range of design space for the elastic modulus (a few kPa to hundreds of MPa), stretchability (strain up to 180%) and Poisson ratio (ranging from -0.5 to 1.2) can be achieved. Experiments on lattice-hydrogel composites are also conducted to verify the reinforcement effect of lattice structures on the hydrogel. This work provides a theoretical method to predict the mechanical behaviors of the lattice structures and aid the rational design of reinforced biomaterials, which has applications in tissue engineering, drug delivery and intraocular lenses.

Translational applications of photopolymerizable hydrogels for cartilage repair

BACKGROUND

Articular cartilage lesions generated by trauma or osteoarthritis are the most common causes of pain and disability in patients. The development of photopolymerizable hydrogels has allowed for significant advances in cartilage repair procedures. Such three-dimensional (3D) networks of polymers that carry large amounts of water can be created to resemble the physical characteristics of the articular cartilage and be delivered into ill-defined cartilage defects as a liquid solution prior to polymerization in vivo for perfect fit with the surrounding native tissue. These hydrogels offer an adapted environment to encapsulate and propagate regenerative cells in 3D cultures for cartilage repair. Among them, mesenchymal stem cells and chondrocytes may represent the most adapted sources for implantation. They also represent platforms to deliver therapeutic, biologically active factors that promote 3D cell differentiation and maintenance for in vivo repair.

CONCLUSION

This review presents the benefits of photopolymerization of hydrogels and describes the photoinitiators and materials in current use for enhanced cartilage repair.

Hydrogels: soft matters in photomedicine

Photodynamic therapy (PDT), a shining beacon in the realm of photomedicine, is a non-invasive technique that utilizes dye-based photosensitizers (PSs) in conjunction with light and oxygen to produce reactive oxygen species to combat malignant tissues and infectious microorganisms. Yet, for PDT to become a common, routine therapy, it is still necessary to overcome limitations such as photosensitizer solubility, long-term side effects (e.g., photosensitivity) and to develop safe, biocompatible and target-specific formulations. Polymer based drug delivery platforms are an effective strategy for the delivery of PSs for PDT applications. Among them, hydrogels and 3D polymer scaffolds with the ability to swell in aqueous media have been deeply investigated. Particularly, hydrogel-based formulations present real potential to fulfill all requirements of an ideal PDT platform by overcoming the solubility issues, while improving the selectivity and targeting drawbacks of the PSs alone. In this perspective, we summarize the use of hydrogels as carrier systems of PSs to enhance the effectiveness of PDT against infections and cancer. Their potential in environmental and biomedical applications, such as tissue engineering photoremediation and photochemistry, is also discussed.

Advances in crosslinking strategies of biomedical hydrogels

Biomedical hydrogels as sole repair matrices or combined with pre-seeded cells and bioactive growth factors are extensively applied in tissue engineering and regenerative medicine. Hydrogels normally provide three dimensional structures for cell adhesion and proliferation or the controlled release of the loading of drugs or proteins. Various physiochemical properties of hydrogels endow them with distinct applications. In this review, we present the commonly used crosslinking method for hydrogel synthesis involving physical and chemical crosslinks and summarize their current progress and future perspectives.

Synthesis and Characterization of Injectable Sulfonate-Containing Hydrogels

Sulfonate-containing hydrogels are of particular interest because of their tunable mechanical and swelling properties, as well as their biological effects. Polysulfonate copolymers were synthesized by reacting 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylamide (AM), and acrylic acid (AA). We found that the incorporation rate of sulfonate-containing monomer and the molecular weight of the copolymer were significantly enhanced by increasing the ionic strength of the solution. We introduced thiol groups by modifying the pendant carboxylates or copolymerizing along with a disulfide-containing monomer. The thiol-containing copolymers were reacted with a 4-arm acrylamide-terminated poly(ethylene glycol) via a thiol-ene click reaction, which was mediated by a photoinitiator, a redox initiator, or a base-catalyzed Michael-Addition. We were able to tailor the storage modulus (33-1800 Pa) and swelling capacity (1-91 wt %) of the hydrogel by varying the concentration of the copolymers. We determined that the injectable sulfonate-containing hydrogels were biocompatible up to 20 mg/mL, as observed by an electric cell-substrate impedance sensing (ECIS) technique, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay using three different cell lines: human retinal pigment epithelial cells (ARPE-19), fibroblasts (NIH 3T3), and Chinese hamster ovary cells (CHO).

Toward an understanding of the thermosensitive behaviour of pH-responsive hydrogels based on cyclodextrins

The molecular mechanism responsible for the thermosensitive behaviour exhibited by pH-responsive cyclodextrin-based hydrogels is explored here with the twofold aim of clarifying some basic aspects of H-bond interactions in hydrogel phases and contributing to a future engineering of cyclodextrin hydrogels for targeted delivery and release of bioactive agents. The degree of H-bond association of water molecules entrapped in the gel network and the extent of intermolecular interactions involving the hydrophobic/hydrophilic moieties of the polymer matrix are probed by UV Raman and IR experiments, in order to address the question of how these different and complementary aspects combine to determine the pH-dependent thermal activation exhibited by these hydrogels. Complementary vibrational spectroscopies are conveniently employed in this study with the aim of safely disentangling the spectral response arising from the two main components of the hydrogel systems, i.e. the polymer matrix and water solvent. The experimental evidence suggests that the dominant effects in the mechanism of solvation of cyclodextrin-based hydrogels are due to the changes occurring, upon increasing of temperature, in the hydrophobicity character of specific chemical moieties of the polymer, as triggered by pH variations. The achievements of this work corroborate the potentiality of the UV Raman scattering technique, in combination with more conventional IR experiments, to provide a "molecular view" of complex macroscopic phenomena exhibited in hydrogel phases.

Design and performance of a sericin-alginate interpenetrating network hydrogel for cell and drug delivery

Although alginate hydrogels have been extensively studied for tissue engineering applications, their utilization is limited by poor mechanical strength, rapid drug release, and a lack of cell adhesive ability. Aiming to improve these properties, we employ the interpenetrating hydrogel design rationale. Using alginate and sericin (a natural protein with many unique properties and a major component of silkworm silk), we develop an interpenetrating polymer network (IPN) hydrogel comprising interwoven sericin and alginate double networks. By adjusting the sericin-to-alginate ratios, IPNs' mechanical strength can be adjusted to meet stiffness requirements for various tissue repairs. The IPNs with high sericin content show increased stability during degradation, avoiding pure alginate's early collapse. These IPNs have high swelling ratios, benefiting various applications such as drug delivery. The IPNs sustain controlled drug release with the adjustable rates. Furthermore, these IPNs are adhesive to cells, supporting cell proliferation, long-term survival and migration. Notably, the IPNs inherit sericin's photoluminescent property, enabling bioimaging in vivo. Together, our study indicates that the sericin-alginate IPN hydrogels may serve as a versatile platform for delivering cells and drugs, and suggests that sericin may be a building block broadly applicable for generating IPN networks with other biomaterials for diverse tissue engineering applications.

Water and polymer dynamics in a model polysaccharide hydrogel: the role of hydrophobic/hydrophilic balance

The role of hydrophobicity/hydrophilicity balance in the gelation phenomena in water-swollen polymers is explored in a model polysaccharide hydrogel.

Miniature probe for the delivery and monitoring of a photopolymerizable material

Photopolymerization is a common method to cure materials initially in a liquid state, such as dental implants or bone or tissue fillers. Recent advances in the development of biocompatible gel- and cement-systems open up an avenue for in situ photopolymerization. For minimally invasive surgery, such procedures require miniaturized surgical endoscopic probes to activate and control photopolymerization in situ. We present a miniaturized light probe in which a photoactive material can be (1) mixed, pressurized, and injected, (2) photopolymerized/photoactivated, and (3) monitored during the chemical reaction. The device is used to implant and cure poly(ethylene glycol) dimethacrylate-hydrogel-precursor in situ with ultraviolet A (UVA) light (365 nm) while the polymerization reaction is monitored in real time by collecting the fluorescence and Raman signals generated by the 532-nm excitation light source. Hydrogels could be delivered, photopolymerized, and monitored by the probe up to a curing depth of 4 cm. The size of the photopolymerized samples could be correlated to the fluorescent signal collected by the probe, and the reproducibility of the procedure could be demonstrated. The position of the probe tip inside a bovine caudal intervertebral disc could be estimated in vitro based on the collected fluorescence and Raman signal.

Photopolymerizable hydrogels for implants: Monte-Carlo modeling and experimental in vitro validation

Photopolymerization is commonly used in a broad range of bioapplications, such as drug delivery, tissue engineering, and surgical implants, where liquid materials are injected and then hardened by means of illumination to create a solid polymer network. However, photopolymerization using a probe, e.g., needle guiding both the liquid and the curing illumination, has not been thoroughly investigated. We present a Monte Carlo model that takes into account the dynamic absorption and scattering parameters as well as solid-liquid boundaries of the photopolymer to yield the shape and volume of minimally invasively injected, photopolymerized hydrogels. In the first part of the article, our model is validated using a set of well-known poly(ethylene glycol) dimethacrylate hydrogels showing an excellent agreement between simulated and experimental volume-growth-rates. In the second part, in situ experimental results and simulations for photopolymerization in tissue cavities are presented. It was found that a cavity with a volume of 152  mm3 can be photopolymerized from the output of a 0.28-mm2 fiber by adding scattering lipid particles while only a volume of 38  mm3 (25%) was achieved without particles. The proposed model provides a simple and robust method to solve complex photopolymerization problems, where the dimension of the light source is much smaller than the volume of the photopolymerizable hydrogel.

Hydrogel scaffolds for tissue engineering: Progress and challenges

Designing of biologically active scaffolds with optimal characteristics is one of the key factors for successful tissue engineering. Recently, hydrogels have received a considerable interest as leading candidates for engineered tissue scaffolds due to their unique compositional and structural similarities to the natural extracellular matrix, in addition to their desirable framework for cellular proliferation and survival. More recently, the ability to control the shape, porosity, surface morphology, and size of hydrogel scaffolds has created new opportunities to overcome various challenges in tissue engineering such as vascularization, tissue architecture and simultaneous seeding of multiple cells. This review provides an overview of the different types of hydrogels, the approaches that can be used to fabricate hydrogel matrices with specific features and the recent applications of hydrogels in tissue engineering. Special attention was given to the various design considerations for an efficient hydrogel scaffold in tissue engineering. Also, the challenges associated with the use of hydrogel scaffolds were described.

Interfacial thiol-ene photoclick reactions for forming multilayer hydrogels

Interfacial visible light-mediated thiol-ene photoclick reactions were developed for preparing step-growth hydrogels with multilayer structures. The effect of a noncleavage type photoinitiator eosin-Y on visible-light-mediated thiol-ene photopolymerization was first characterized using in situ photorheometry, gel fraction, and equilibrium swelling ratio. Next, spectrophotometric properties of eosin-Y in the presence of various relevant macromer species were evaluated using ultraviolet-visible light (UV-vis) spectrometry. It was determined that eosin-Y was able to reinitiate the thiol-ene photoclick reaction, even after light exposure. Because of its small molecular weight, most eosin-Y molecules readily leached out from the hydrogels. The diffusion of residual eosin-Y from preformed hydrogels was exploited for fabricating multilayer step-growth hydrogels. Interfacial hydrogel coating was formed via the same visible-light-mediated gelation mechanism without adding fresh initiator. The thickness of the thiol-ene gel coating could be easily controlled by adjusting visible light exposure time, eosin-Y concentration initially loaded in the core gel, or macromer concentration in the coating solution. The major benefits of this interfacial thiol-ene coating system include its simplicity and cytocompatibility. The formation of thiol-ene hydrogels and coatings neither requires nor generates any cytotoxic components. This new gelation chemistry may have great utilities in controlled release of multiple sensitive growth factors and encapsulation of multiple cell types for tissue regeneration.

Preparation and characterization of elastin-like polypeptide scaffolds for local delivery of antibiotics and proteins

Tissue engineering applications could benefit from simultaneous release of growth factors, signaling molecules, and antibiotics to obtain optimal healing of tissues. Elastin-like polypeptides (ELPs) are genetically engineered polymers that possess good biocompatibility, are biodegradable, and exhibit mechanical properties similar to natural elastin. In addition, ELPs exhibit a characteristic inverse phase transition temperature (T(t)). This T(t) behavior is widely exploited in hyperthermia mediated drug delivery. The objectives of this research were to prepare ELP hydrogel scaffolds using a novel ultrasonication method and to investigate the release of a model protein (bovine serum albumin, BSA) and a commonly used antibiotic in periodontal therapy (doxycycline) from the scaffolds at two different temperatures (25 °C <T(t) vs. 37 °C >T(t)). Both BSA and doxycycline showed a gradual time dependent release and showed a trend of higher release fractions with higher loading doses. Based on the comparison between the release profiles at the two selected temperatures, the release was higher at 37 °C compared to that at 25 °C for both the loading concentrations of doxycycline (0.05 and 0.1 % v/v) and only one of the loading concentrations of BSA (0.5 % v/v) studied, while the release was higher at 25 °C compared to that at 37 °C only for the other loading concentration of BSA (1 % v/v) studied. These results suggested that the drug molecular weight and loading concentration were significant factors that affected the release kinetics. The experiments in this study demonstrated that the ELP hydrogel scaffolds can successfully release proteins and antibiotics critical to tissue engineering.

Responsive micromolds for sequential patterning of hydrogel microstructures

Microscale hydrogels have been shown to be beneficial for various applications such as tissue engineering and drug delivery. A key aspect in these applications is the spatial organization of biological entities or chemical compounds within hydrogel microstructures. For this purpose, sequentially patterned microgels can be used to spatially organize either living materials to mimic biological complexity or multiple chemicals to design functional microparticles for drug delivery. Photolithographic methods are the most common way to pattern microscale hydrogels but are limited to photocrosslinkable polymers. So far, conventional micromolding approaches use static molds to fabricate structures, limiting the resulting shapes that can be generated. Herein, we describe a dynamic micromolding technique to fabricate sequentially patterned hydrogel microstructures by exploiting the thermoresponsiveness of poly(N-isopropylacrylamide)-based micromolds. These responsive micromolds exhibited shape changes under temperature variations, facilitating the sequential molding of microgels at two different temperatures. We fabricated multicompartmental striped, cylindrical, and cubic microgels that encapsulated fluorescent polymer microspheres or different cell types. These responsive micromolds can be used to immobilize living materials or chemicals into sequentially patterned hydrogel microstructures which may potentially be useful for a range of applications at the interface of chemistry, materials science and engineering, and biology.

Experimental studies and modeling of drug release from a tunable affinity-based drug delivery platform

An affinity-based drug delivery platform for controlling drug release is analyzed by a combination of experimental studies and mathematical modeling. This platform has the ability to form selective interactions between a therapeutic agent and host matrix that yields advantages over systems that employ nonselective methods. The incorporation of molecular interactions in drug delivery can increase the therapeutic lifetime of drug delivery implants and limit the need for multiple implants in treatment of chronic illnesses. To analyze this complex system for rational design of drug delivery implants, we developed a mechanistic mathematical model to quantify the molecular events and processes. With a β-cyclodextrin hydrogel host matrix, defined release rates were obtained using a fluorescent model drug. The key processes were the complexation between the drug and cyclodextrin and diffusion of the drug in the hydrogel. Optimal estimates of the model parameters were obtained by minimizing the difference between model simulation and experimentally measured drug release kinetics. Model simulations could predict the drug release dynamics under a wide range of experimental conditions.

Formation of three-dimensional hydrogel multilayers using enzyme-mediated redox chain initiation

Enzyme-mediated redox chain initiation involving glucose oxidase (GOX) was employed in an iterative solution dip-coating technique to polymerize multiple, three-dimensional hydrogel layers using mild aqueous conditions at ambient temperature and oxygen levels. To the best of our knowledge, sequential enzyme-mediated dip-coating resulting in an interfacial radical chain polymerization and subsequent formation of three-dimensional hydrogel layers has not been previously explored. Conformal, micrometer-scale, uniform poly(ethylene glycol) (PEG)-based hydrogel layers were polymerized within seconds and remained securely associated after incubation in water for 16 weeks. Incorporation of either small molecules (i.e., rhodamine-B acrylate, fluorescein acrylate) or fluorescent nanoparticles into crosslinked hydrogel layers during the polymerization reaction was also achieved. The encapsulation of 0.2 microm-diameter nanoparticles into hydrogels during polymerization of a 2-hydroxyethyl acrylate (HEA)/PEG(575) diacrylate monomer formulation, using the GOX-mediated initiation, resulted in minimal effects on polymerization kinetics, with final acrylate conversions of 95% (+/- 1%) achieved within minutes. The temporal control and spatial localization afforded by this interfacial redox approach resulted in the polymerization of uniform secondary layers ranging between 150 (+/- 10) microm and 650 (+/- 10) microm for 15 and 120 s immersion times, respectively. Moreover, increasing the PEG(575)-fraction within the initial hydrogel substrate from 10 to 50% decreased the subsequent layer thicknesses from 690 (+/- 30) microm to 490 (+/- 10) microm because of lowered glucose concentration at the hydrogel interface. The ability to sequentially combine differing initiation mechanisms with this coating approach was achieved by using GOX-mediated interfacial polymerization on hydrogel substrates initially photopolymerized in the presence of glucose. The strict control of layer thicknesses combined with the rapid, water-soluble, and mild polymerization will readily benefit applications requiring formation of stratified, complex, and three-dimensional polymer structures.

Cytocompatibility studies of an in situ photopolymerized thermoresponsive hydrogel nanoparticle system using human aortic smooth muscle cells

We have been investigating thermoresponsive hydrogel nanoparticle composite networks to develop photopolymerized hydrogels to deliver drugs for prevention of restenosis after angioplasty. These composite systems can form a gel under physiological conditions and release drugs in response to temperature changes. Our novel system, consisting of poly(N-isopropylacrylamide) thermoresponsive nanoparticles, photo cross-linker poly(ethylene glycol) diacrylate, and UV photoinitiator, 2-hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl-1-propanone-1-one (Irgacure 2959), would be photopolymerized in situ in the presence of UV light. The focus of this study was to evaluate the effects of photoinitiator and UV exposure on human aortic smooth muscle cells (HASMCs). We found that the exposure to UV light did not significantly affect the cellular survival within doses required for photopolymerization. The photoinitiator was cytocompatible at low concentrations (< or = 0.015% w/v); however, cytotoxicity increased with increasing photoinitiator concentrations. In addition, free radicals formed in the presence of a photoinitiator and UV light caused significant levels of cell death. An antioxidant (free radical scavenger), ascorbic acid, added to the cell media, significantly improved relative cell survival but increased the hydrogel gelation time. Finally, HASMC survival when exposed to all potential cytotoxic components was also evaluated by exposing HASMCs to media incubated with our composite hydrogels. In summary, our studies show that the photoinitiator and free radicals are responsible for the cytotoxicity on HASMCs, and the addition of antioxidants can significantly reduce these harmful effects.

Graft healing in anterior cruciate ligament reconstruction

Successful anterior cruciate ligament reconstruction with a tendon graft necessitates solid healing of the tendon graft in the bone tunnel. Improvement of graft healing to bone is crucial for facilitating an early and aggressive rehabilitation and ensuring rapid return to pre-injury levels activity. Tendon graft healing in a bone tunnel requires bone ingrowth into the tendon. Indirect Sharpey fiber formation and direct fibrocartilage fixation confer different anchorage strength and interface properties at the tendon-bone interface. For enhancing tendon graft-to-bone healing, we introduce a strategy that includes the use of periosteum, hydrogel supplemented with periosteal progenitor cells and bone morphogenetic protein-2, and a periosteal progenitor cell sheet. Future studies include the use of cytokines, gene therapy, stem cells, platelet-rich plasma, and mechanical stress for tendon-to-bone healing. These strategies are currently under investigation, and will be applied in the clinical setting in the near future.

Hydrogels in regenerative medicine

Hydrogels, due to their unique biocompatibility, flexible methods of synthesis, range of constituents, and desirable physical characteristics, have been the material of choice for many applications in regenerative medicine. They can serve as scaffolds that provide structural integrity to tissue constructs, control drug and protein delivery to tissues and cultures, and serve as adhesives or barriers between tissue and material surfaces. In this work, the properties of hydrogels that are important for tissue engineering applications and the inherent material design constraints and challenges are discussed. Recent research involving several different hydrogels polymerized from a variety of synthetic and natural monomers using typical and novel synthetic methods are highlighted. Finally, special attention is given to the microfabrication techniques that are currently resulting in important advances in the field.

Micropatterning of Poly (Ethylene Glycol)-Diacrylate (PEG-DA) Hydrogel by Soft-Photolithography for Analysis of Cell-Biomaterial Interactions

Poly (ethylene glycol) hydrogel (PEG) micropatterns fabricated by photolithography and various other microfabrication techniques have been used as a platform to analyze cell-biomaterial interactions in cell culture studies. Numerous innovative techniques have been described about photolithography and the use of Poly (dimethyl siloxane) stamp (PDMS) based pressure moulding technique for the microfabrication of PEG hydrogel micropatterns. We herein this literature describe a simple and a versatile method for fabricating Poly (ethylene glycol) hydrogel-diacrylate (PEG-DA) hydrogel micropatterns using the ‘Soft-photolithography’ technique which is a combination of pressure moulding using a PDMS stamp and photolithography. Using this simple technique, PEG-DA hydrogel micropatterns were fabricated on a silicon substrate of varying dimensions from 40μm to 10μm within the same substrate. Such a three-dimensional microenvironment with varying sizes can serve as an excellent platform to study cell behaviour in culture. These PEG-DA hydrogel micropatterns can further be functionalized by adding a variety of biomolecular cues within the PEG-DA hydrogel matrix or these biomolecules can be patterned on the PEG-DA micropatterns after photopolymerization using micro-contact printing for analysis of cell-biomaterial interactions and tissue engineering purposes.

Polymerization Behavior and Polymer Properties of Eosin-Mediated Surface Modification Reactions

Surface modification by surface-mediated polymerization necessitates control of the grafted polymer film thicknesses to achieve the desired property changes. Here, a microarray format is used to assess a range of reaction conditions and formulations rapidly in regards to the film thicknesses achieved and the polymerization behavior. Monomer formulations initiated by eosin conjugates with varying concentrations of poly(ethylene glycol) diacrylate (PEGDA), N-methyldiethanolamine (MDEA), and 1-vinyl-2-pyrrolidone (VP) were evaluated. Acrylamide with MDEA or ascorbic acid as a coinitiator was also investigated. The best formulation was found to be 40 wt% acrylamide with MDEA which yielded four to eight fold thicker films (maximum polymer thickness increased from 180 nm to 1420 nm) and generated visible films from 5-fold lower eosin surface densities (2.8 vs. 14 eosins/µm(2)) compared to a corresponding PEGDA formulation. Using a microarray format to assess multiple initiator surface densities enabled facile identification of a monomer formulation that yields the desired polymer properties and polymerization behavior across the requisite range of initiator surface densities.

Photoencapsulation of bone morphogenetic protein-2 and periosteal progenitor cells improve tendon graft healing in a bone tunnel

BACKGROUND

Tissue-engineered solutions for promoting the tendon graft incorporation within the bone tunnel appear to be promising.

HYPOTHESIS

To determine the feasibility that conjugation of hyaluronic acid-tethered bone morphogenetic protein-2 can be used to stimulate periosteal progenitor cells direct fibrocartilagenous attachment and new bone formation in an extra-articular tendon-bone healing model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 42 mature New Zealand White rabbits were used. The long digitorum extensor tendon was transplanted into a bone tunnel of the proximal tibia. The tendon was pulled through a drill hole in the proximal tibia and attached to the medial aspect of the tibia. Photopolymerizable hydrogel based on poly (ethylene glycol) diacrylate with hyaluronic acid-tethered bone morphogenetic protein-2 was injected and photogelated in a bone tunnel. Histological and biomechanical examination of the tendon-bone interface was evaluated at postoperative weeks 3 and 6.

RESULTS

Histological analysis showed an interface fibrocartilage and new bone formed by photoencapsulation of bone morphogenetic protein-2 and periosteal progenitor cells at 6 weeks. Biomechanical testing revealed higher maximum pullout strength and stiffness in experimental groups with a statistically significant difference at 3 and 6 weeks after tendon transplantation.

CONCLUSION

The healing tendon-bone interface undergoes a gradual remodeling process; it appears that photoencapsulation of bone morphogenetic protein-2 and periosteal progenitor cells possesses a powerful inductive ability between the tendon and the bone to incorporate the healing in a rabbit model.

CLINICAL RELEVANCE

Novel technologies, such as those described in this study, including photopolymerization and tissue engineering, may provide minimally invasive therapeutic procedures via arthroscopy to enhance biological healing after reconstruction of the anterior cruciate ligament.

Thermal gellation and photo-polymerization of di-acrylated Pluronic F 127

Thermal gellation and photo-cross-linking of di-acrylated Pluronic PF 127 (DA-PF 127) were characterized. First, thermo-hysteresis of Pluronic F 127 (PF 127) solution was characterized. Upon heating, thermal gellation was observed at higher temperature with faster heating rate, whereas upon cooling, the re-melting was rather insensitive to cooling rate. Thus, the net thermo-hysteresis effect was more enhanced by increasing heating rate and was more evident at lower concentrations of PF 127. The hysteresis behavior was not affected at all by the presence of acrylated end-groups of di-acrylated DA-PF 127. Next, photo-polymerization of DA-PF 127 was compared with that of non-thermo-sensitive, di-acrylated PEG (DA-PEG). The micellar nature of DA-PF 127 resulted in the beneficial effect on photo-cross-linking reaction. DA-PF 127 showed faster and more effective cross-linking reaction than DA-PEG, characterized by rheometry and FT-IR. Also, by controlling temperature and concentration, morphology of the cross-linked DA-PF 127 hydrogels was varied more significantly than that of DA-PEG hydrogels.

Fabrication of poly(ethylene glycol) hydrogel micropatterns with osteoinductive growth factors and evaluation of the effects on osteoblast activity and function

The aims of this study were to fabricate poly(ethylene glycol) (PEG) hydrogel micropatterns on a biomaterial surface to guide osteoblast behaviour and to study how incorporating vascular endothelial growth factor (VEGF) within the adhered hydrogel influenced cell morphology. Standard photolithographic procedures or photopolymerization through a poly(dimethyl siloxane) (PDMS) mould were used to fabricate patterned PEG hydrogels on the surface of silanized silicon wafers. Hydrogel patterns were evaluated by light microscopy and surface profilometry. Rat osteoblasts were cultured on these surfaces and cell morphology investigated by fluorescence microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Release of protein trapped in the polymerized PEG was evaluated and VEGF-PEG surfaces were characterized for their ability to support cell growth. These studies show that photopolymerized PEG can be used to create anti-adhesive structures on the surface of silicon that completely control where cell interaction with the substrate takes place. Using conventional lithography, structures down to 50 microm were routinely fabricated with the boundaries exhibiting sloping sides. Using the PDMS mould approach, structures were fabricated as small as 10 microm and boundaries were very sharp and vertical. Osteoblasts exhibiting typical morphology only grew on the silicon wafer surface that was not coated with PEG. Adding BSA to the monomer solution showed that protein could be released from the hydrogel for up to 7 days in vitro. Incorporating VEGF in the hydrogel produced micropatterns that dramatically altered osteoblast behaviour. At boundaries with the VEGF-PEG hydrogel, there was striking formation of cellular processes and membrane ruffling indicative of a change in cell morphology. This study has explored the morphogenetic properties of VEGF and the applications of nano/microfabrication techniques for guided tissue (bone) regeneration in dental and orthopaedic applications using osteoinductive PEG hydrogel micropatterns.

Enhanced Protein Delivery from Photopolymerized Hydrogels Using a Pseudospecific Metal Chelating Ligand

PURPOSE

This study was conducted to investigate the cause of incomplete protein release from photopolymerized poly(ethylene glycol) (PEG) hydrogels and verify the protein-protection mechanism provided by iminodiacetic acid (IDA).

METHODS

The in vitro release of bovine serum albumin (BSA) from PEG hydrogels prepared under different conditions was studied. Photoinitiator and initial protein concentrations were varied as well as the addition of IDA and metal ions. Protein immobilization within the nondegradable networks via free-radical reaction was demonstrated by gel electrophoresis.

RESULTS

Protein release efficiency was shown to be dependent on photoinitiator and initial protein concentration. Gel electrophoresis results revealed immobilization of protein to the polymer network and further indicated the detrimental role of free radicals in lowering protein-release efficiency. Adding IDA to the prepolymer solution enhanced total protein release from the subsequently photopolymerized network in a dose-dependent manner. The addition of metal ions including Cu2+, Zn2+, and Ni2+ further increased BSA release efficiency. Agreement between the protein release data and theoretical model predictions accounting for reversible protein-IDA binding further validated the protection effect provided by IDA and IDA-transition metal complexes.

CONCLUSIONS

The protection effect described in this study offers a novel strategy for increasing the delivery efficiencies of many therapeutically valuable proteins.

Light-activated hydrogel formation via the triggered folding and self-assembly of a designed peptide

Photopolymerization can be used to construct materials with precise temporal and spatial resolution. Applications such as tissue engineering, drug delivery, the fabrication of microfluidic devices and the preparation of high-density cell arrays employ hydrogel materials that are often prepared by this technique. Current photopolymerization strategies used to prepare hydrogels employ photoinitiators, many of which are cytotoxic and require large macromolecular precursors that need to be functionalized with moieties capable of undergoing radical cross-linking reactions. We have developed a simple light-activated hydrogelation system that employs a designed peptide whose ability to self-assemble into hydrogel material is dependent on its intramolecular folded conformational state. An iterative design strategy afforded MAX7CNB, a photocaged peptide that, when dissolved in aqueous medium, remains unfolded and unable to self-assemble; a 2 wt % solution of freely soluble unfolded peptide is stable to ambient light and has the viscosity of water. Irradiation of the solution (260 < lambda < 360 nm) releases the photocage and triggers peptide folding to produce amphiphilic beta-hairpins that self-assemble into viscoelastic hydrogel material. Circular dichroic (CD) spectroscopy supports this folding and self-assembly mechanism, and oscillatory rheology shows that the resulting hydrogel is mechanically rigid (G' = 1000 Pa). Laser scanning confocal microscopy imaging of NIH 3T3 fibroblasts seeded onto the gel indicates that the gel surface is noncytotoxic, conducive to cell adhesion, and allows cell migration. Lastly, thymidine incorporation assays show that cells seeded onto decaged hydrogel proliferate at a rate equivalent to cells seeded onto a tissue culture-treated polystyrene control surface.

In situ-forming hydrogels--review of temperature-sensitive systems

In the past few years, an increasing number of in situ-forming systems have been reported in the literature for various biomedical applications, including drug delivery, cell encapsulation, and tissue repair. There are several possible mechanisms that lead to in situ gel formation: solvent exchange, UV-irradiation, ionic cross-linkage, pH change, and temperature modulation. The thermosensitive approach can be advantageous for particular applications as it does not require organic solvents, co-polymerization agents, or an externally applied trigger for gelation. In the last 2 decades, several thermosensitive formulations have been proposed. This manuscript focuses on aqueous polymeric solutions that form implants in situ in response to temperature change, generally from ambient to body temperature. It mainly reviews the characterization and use of polysaccharides, N-isopropylacrylamide copolymers, poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (poloxamer) and its copolymers, poly(ethylene oxide)/(D,L-lactic acid-co-glycolic acid) copolymers, and thermosensitive liposome-based systems.

Rapid prototyping of tissue-engineering constructs, using photopolymerizable hydrogels and stereolithography

One of the most important aspects of tissue engineering is the design of the scaffold providing the mechanical strength and access to nutrients for the new tissue. For customized tissue engineering, it is essential to be able to fabricate three-dimensional scaffolds of various geometric shapes, in order to repair defects caused by accidents, surgery, or birth. Rapid prototyping or solid free-form fabrication (SFF) techniques hold great promise for designing three-dimensional customized scaffolds, yet traditional cell-seeding techniques may not provide enough cell mass for larger constructs. This article presents a novel attempt to fabricate three-dimensional scaffolds, using hydrogels combined with cell encapsulation to fabricate high-density tissue constructs. A commercially available stereolithography technique was applied to fabricate scaffolds using poly(ethylene oxide) and poly(ethylene glycol)dimethacrylate photopolymerizable hydrogels. Mechanical characterization shows the constructs to be comparable with soft tissues in terms of elasticity. High cell viability was achieved and high-density constructs fabricated.

Hydrogels for tissue engineering: scaffold design variables and applications

Polymer scaffolds have many different functions in the field of tissue engineering. They are applied as space filling agents, as delivery vehicles for bioactive molecules, and as three-dimensional structures that organize cells and present stimuli to direct the formation of a desired tissue. Much of the success of scaffolds in these roles hinges on finding an appropriate material to address the critical physical, mass transport, and biological design variables inherent to each application. Hydrogels are an appealing scaffold material because they are structurally similar to the extracellular matrix of many tissues, can often be processed under relatively mild conditions, and may be delivered in a minimally invasive manner. Consequently, hydrogels have been utilized as scaffold materials for drug and growth factor delivery, engineering tissue replacements, and a variety of other applications.

A method to protect sensitive molecules from a light-induced polymerizing environment

Systems that can be polymerized in situ upon exposure to light radiation may have significant applications in tissue engineering and drug delivery. However, the light-induced polymerization step, which is the requisite for this technology, could be potentially deleterious to sensitive bioactive agents (e.g., enzymes, cytokines, matrix metalloproteinases) being entrapped. In this study, a method to protect sensitive molecules from a light-induced polymerizing environment is proposed. This method is based on the idea that nonaccessible substances cannot interact with the polymerizing species. To examine this concept, two model enzymes-namely, horseradish peroxidase and alpha-glucosidase-were protected by gelatin-based wet granulation and incorporated within a cured polyethylene glycol dimethacrylate, a photocurable monomer, under different conditions. Unprotected enzymes were used as controls. Enzymes were then allowed to diffuse out of the polymerized matrices. The activity and total enzyme recovered from these matrices by passive diffusion were compared to ascertain the extent of activity retention. Matrix assisted laser desorption ionization mass spectrometry combined with time of flight mass spectrometry (MALDI-TOF) was used to determine changes in enzyme molecular weight. During the first 24 h of diffusion from the polymerized matrices, unprotected enzymes consistently showed a loss of activity ranging from 10-66%, depending on the matrix composition and enzyme properties. In contrast, protected enzymes retained over 94% of their activity irrespective of the experimental setting. The loss of activity appears to be a direct consequence of the polymerizing environment.

Photopolymerizable hydrogels for tissue engineering applications

Photopolymerized hydrogels are being investigated for a number of tissue engineering applications because of the ability to form these materials in situ in a minimally invasive manner such as by injection. In addition, hydrogels, three-dimensional networks of hydrophilic polymers that are able to swell large amounts of water, can be made to resemble the physical characteristics of soft tissues. Hydrogel materials also generally exhibit high permeability and good biocompatibility making, these materials attractive for use in cell encapsulation and tissue engineering applications. A number of hydrogel materials can be formed via photopolymerization processes mild enough to be carried out in the presence of living cells. This allows one to homogeneously seed cells throughout the scaffold material and to form hydrogels in situ. This review presents advantages of photopolymerization of hydrogels and describes the photoinitiators and materials in current use. Applications of photopolymerized hydrogels in tissue engineering that have been investigated are summarized.

Growth factor release from tissue engineering scaffolds

Synthetic scaffold materials are used in tissue engineering for a variety of applications, including physical supports for the creation of functional tissues, protective gels to aid in wound healing and to encapsulate cells for localized hormone-delivery therapies. In order to encourage successful tissue growth, these scaffold materials must incorporate vital growth factors that are released to control their development. A major challenge lies in the requirement for these growth factor delivery mechanisms to mimic the in-vivo release profiles of factors produced during natural tissue morphogenesis or repair. This review highlights some of the major strategies for creating scaffold constructs reported thus far, along with the approaches taken to incorporate growth factors within the materials and the benefits of combining tissue engineering and drug delivery expertise.

Emerging protein delivery methods

The efficient and safe delivery of therapeutic proteins is the key to commercial success and, in some cases, the demonstration of efficacy in current and future biotechnology products. Numerous delivery technologies and companies have evolved over the past year. To critically evaluate the available options, each method must be assessed in terms of how easily it can be manufactured, impact on protein quality, bioavailability, and toxicity. Recent advances in depot delivery systems have, for the most part, overcome all of these obstacles except for complex and costly manufacturing. On the other hand, pulmonary delivery usually involves efficient manufacturing, but low protein bioavailability resulting in higher doses compared with injections. Although recent advances in transdermal and oral delivery have been significant, both of these delivery routes require logarithmic increases in bioavailability to make them viable candidates for commercialization. In the next few years, protein delivery for commercial products will probably be limited to injection devices, depot systems and pulmonary administration.

Toward new biomaterials

Polymers are widely used for a large range of medical devices used as biomaterials on a temporary, intermittent, and long-term basis. It is now well accepted that the initial rapid adsorption of proteins to polymeric surfaces affects the performance of these biomaterials. However, protein adsorption to a polymer surface can be modulated by an appropriate design of the interface. Extensive study has shown that these interactions can be minimized by coating with a highly hydrated layer (hydrogel), by grafting on the surface different biomolecules, or by creating domains with chemical functions (charges, hydrophilic groups). Our laboratory has investigated the latter approach over the past 2 decades, in particular the synthesis and the biological activities of polymers to improve the biocompatibility of blood-contacting devices. These soluble and insoluble polymers were obtained by chemical substitution of macromolecular chains with suitable groups able to develop specific interactions with biological components. Applied to compatibility with the blood and the immune systems, this concept has been extended to interactions of polymeric biomaterials with eukaryotic and prokaryotic cells. The design of new biomaterials with low bacterial attachment is thus under intensive study. After a brief overview of current trends in the surface modifications of biocompatible materials, we will describe how biospecific polymers can be obtained and review our recent results on the inhibition of bacterial adhesion using one type of functionalized polymer obtained by random substitution. This strategy, applied to existing or new materials, seems promising for the limitation of biomaterial-associated infections.