Click-crosslinkable and photodegradable gelatin hydrogels for cytocompatible optical cell manipulation in natural environment

Comprehensive Biosafety Profile of Carbomer-Based Hydrogel Formulations Incorporating Phosphorus Derivatives

Determining the safety of a newly developed experimental product is a crucial condition for its medical use, especially for clinical trials. In this regard, four hydrogel-type formulations were manufactured, all of which were based on carbomer (Blank-CP940) and encapsulated with caffeine (CAF-CP940), phosphorus derivatives (phenyl phosphinic (CAF-S1-CP940) and 2-carboxyethyl phenyl phosphinic acids (CAF-S2-CP940)). The main aim of this research was to provide a comprehensive outline of the biosafety profile of the above-mentioned hydrogels. The complex in vitro screening (cell viability, cytotoxicity, morphological changes in response to exposure, and changes in nuclei morphology) on two types of healthy skin cell lines (HaCaT-human keratinocytes and JB6 Cl 41-5a-murine epidermal cells) exhibited a good biosafety profile when both cell lines were treated for 24 h with 150 μg/mL of each hydrogel. A comprehensive analysis of the hydrogel's impact on the genetic profile of HaCaT cells sustains the in vitro experiments. The biosafety profile was completed with the in vivo and in ovo assays. The outcome revealed that the developed hydrogels exerted good biocompatibility after topical application on BALB/c nude mice's skin. It also revealed a lack of toxicity after exposure to the hen's chicken embryo. Further investigations are needed, regarding the in vitro and in vivo therapeutic efficacy and safety for long-term use and potential clinical translatability.

Albumin hydrogels for repeated capture of drugs from the bloodstream and release into the tumor

Despite the efficacy of hydrogels for consistently delivering drugs to targeted areas (primarily tumors), these systems face challenges such as initial burst release, non-refillable drugs, and a lack of dosage control. To address these issues, a novel strategy has been developed to capture and release drugs from the bloodstream, thereby overcoming the limitations of traditional hydrogels. In this study, an innovative albumin hydrogel system was developed through a bioorthogonal reaction using azide-modified albumin and 4-arm PEG-DBCO. This system can repeatedly capture and release drugs over prolonged periods. Inspired by albumin-drug binding in vivo, this hydrogel can be injected intratumorally and acts as a reservoir for capturing drugs circulating in the bloodstream. Drugs captured in hydrogels are released slowly and effectively delivered to tumors through a "capture and release process." Both the in vitro and in vivo results indicated that the hydrogel effectively captured and released drugs, such as indocyanine green and doxorubicin, over repeated cycles without compromising the activity of the drugs. Moreover, implanting the hydrogel at surgical sites successfully inhibited tumor recurrence through its drug capture-release capability. These findings establish the albumin hydrogel system as a promising capture-release platform that leverages drug-binding affinity to effectively deliver drugs to tumors, offering potential advancements in cancer treatment and post-surgery recurrence prevention.

Poly(ethylene oxide)/Gelatin-Based Biphasic Photocrosslinkable Hydrogels of Tunable Morphology for Hepatic Progenitor Cell Encapsulation

Macroporous hydrogels have great potential for biomedical applications. Liquid or gel-like pores were created in a photopolymerizable hydrogel by forming water-in-water emulsions upon mixing aqueous solutions of gelatin and a poly(ethylene oxide) (PEO)-based triblock copolymer. The copolymer constituted the continuous matrix, which dominated the mechanical properties of the hydrogel once photopolymerized. The gelatin constituted the dispersed phase, which created macropores in the hydrogel. The microstructures of the porous hydrogel were determined by the volume fraction of the gelatin phase. When volume fractions were close to 50 v%, free-standing hydrogels with interpenetrated morphology can be obtained thanks to the addition of a small amount of xanthan. The hydrogels displayed Young's moduli ranging from 5 to 30 kPa. They have been found to be non-swellable and non-degradable in physiological conditions. Preliminary viability tests with hepatic progenitor cells embedded in monophasic PEO-based hydrogels showed rapid mortality of the cells, whereas encouraging viability was observed in PEO-based triblock copolymer/gelatin macroporous hydrogels. The latter has the potential to be used in cell therapy.

Reversible dynamic mechanics of hydrogels for regulation of cellular behavior

The mechanical properties of the native extracellular matrix play a key role in regulating cell behavior during developmental, healing and homeostatic processes. Since these properties change over time, it may be valuable to have the capacity to dynamically vary the mechanical properties of engineered hydrogels used in tissue engineering strategies to better mimic the dynamic mechanical behavior of native extracellular matrix. However, in situ repeatedly reversible dynamic tuning of hydrogel mechanics is still limited. In this study, we have engineered a hydrogel system with reversible dynamic mechanics using a dual-crosslinkable alginate hydrogel. The effect of reversible mechanical signals on encapsulated stem cells in dynamically tunable hydrogels has been demonstrated. In situ stiffening of hydrogels decreases cell spreading and proliferation, and subsequent softening of hydrogels gives way to an increase in cell spreading and proliferation. The hydrogel stiffening and softening, and resulting cellular responses are repeatedly reversible. This hydrogel system provides a promising platform for investigating the effect of repeatedly reversible changes in extracellular matrix mechanics on cell behaviors. STATEMENT OF SIGNIFICANCE: Since the mechanical properties of native extracellular matrix (ECM) change over time during development, healing and homeostatic processes, it may be valuable to have the capacity to dynamically control the mechanics of biomaterials used in tissue engineering and regenerative medicine applications to better mimic this behavior. Unlike previously reported biomaterials whose mechanical properties can be changed by the user only a limited number of times, this system provides the capacity to induce unlimited alterations to the mechanical properties of an engineered ECM for 3D cell culture. This study presents a strategy for on-demand dynamic and reversible control of materials' mechanics by single and dual-crosslinking mechanisms using oxidized and methacrylated alginates. By demonstrating direct changes in encapsulated human mesenchymal stem cell morphology, proliferation and chondrogenic differentiation in response to multiple different dynamic changes in hydrogel mechanics, we have established a repeatedly reversible 3D cellular mechanosensing system. This system provides a powerful platform tool with a wide range of stiffness tunability to investigate the role of dynamic mechanics on cellular mechanosensing and behavioral responses.

Photoinduced Porcine Gelatin Cross-Linking by Homobi- and Homotrifunctional Tetrazoles

Gelatin is a costless polypeptide material of natural origin, able to form hydrogels that are potentially useful in biomaterial scaffold design for drug delivery, cell cultures, and tissue engineering. However, gelatin hydrogels are unstable at physiological conditions, losing their features only after a few minutes at 37 °C. Accordingly, treatments to address this issue are of great interest. In the present work, we propose for the first time the use of bi- and trifunctional tetrazoles, most of them unknown to date, for photoinduced gelatin cross-linking towards the production of physiologically stable hydrogels. Indeed, after UV-B irradiation, aryl tetrazoles generate a nitrilimine intermediate that is reactive towards different functionalities, some of them constitutively present in the amino acid side chains of gelatin. The efficacy of the treatment strictly depends on the structure of the cross-linking agent used, and substantial improved stability was observed by switching from bifunctional to trifunctional cross-linkers.

Mixed Polyplex Micelles with Thermoresponsive and Lysine-Based Zwitterionic Shells Derived from Two Poly(vinyl amine)-Based Block Copolymers

Two series of poly(vinyl amine) (PVAm)-based block copolymers with zwitterionic and thermoresponsive segments were synthesized by the reversible addition-fragmentation chain transfer polymerization. A mixture of the two copolymers, poly(N-acryloyl-l-lysine) (PALysOH) and poly(N-isopropylacrylamide) (PNIPAM), which have the same cationic PVAm chain but different shell-forming segments, were used to prepare mixed polyplex micelles with DNA. Both PVAm-b-PALysOH and PVAm-b-PNIPAM showed low cytotoxicity, with characteristic assembled structures and stimuli-responsive properties. The cationic PVAm segment in both block copolymers showed site-specific interactions with DNA, which were evaluated by dynamic light scattering, zeta potential, circular dichroism, agarose gel electrophoresis, atomic force microscopy, and transmission electron microscopy measurements. The PVAm-b-PNIPAM/DNA polyplexes showed the characteristic temperature-induced formation of assembled structures in which the polyplex size, surface charge, chiroptical property of DNA, and polymer-DNA binding were governed by the nitrogen/phosphate (N/P) ratio. The DNA binding strength and colloidal stability of the PVAm-b-PALysOH/DNA polyplexes could be tuned by introducing an appropriate amount of zwitterionic PALysOH functionality, while maintaining the polyplex size, surface charge, and chiroptical property, regardless of the N/P ratio. The mixed polyplex micelles showed temperature-induced stability originating from the hydrophobic (dehydrated) PNIPAM chains upon heating, and remarkable stability under salty conditions owing to the presence of the zwitterionic PALysOH chain on the polyplex surface.

Photopatterned biomolecule immobilization to guide three-dimensional cell fate in natural protein-based hydrogels

Hydrogel biomaterials derived from natural biopolymers (e.g., fibrin, collagen, decellularized extracellular matrix) are regularly utilized in three-dimensional (3D) cell culture and tissue engineering. In contrast to those based on synthetic polymers, natural materials permit enhanced cytocompatibility, matrix remodeling, and biological integration. Despite these advantages, natural protein-based gels have lagged behind synthetic alternatives in their tunability; methods to selectively modulate the biochemical properties of these networks in a user-defined and heterogeneous fashion that can drive encapsulated cell function have not yet been established. Here, we report a generalizable strategy utilizing a photomediated oxime ligation to covalently decorate naturally derived hydrogels with bioactive proteins including growth factors. This bioorthogonal photofunctionalization is readily amenable to mask-based and laser-scanning lithographic patterning, enabling full four-dimensional (4D) control over protein immobilization within virtually any natural protein-based biomaterial. Such versatility affords exciting opportunities to probe and direct advanced cell fates inaccessible using purely synthetic approaches in response to anisotropic environmental signaling.

Visible light-degradable supramolecular gels comprising cross-linked polyrotaxanes capped with trithiocarbonate groups

Visible light-degradable supramolecular gels were designed using polyrotaxanes (PRXs) containing bulky trithiocarbonate groups as stopper molecules that are cleaved by visible light irradiation.

Spatiotemporally Controlled Photoresponsive Hydrogels: Design and Predictive Modeling from Processing through Application

Photoresponsive hydrogels (PRHs) are soft materials whose mechanical and chemical properties can be tuned spatially and temporally with relative ease. Both photo-crosslinkable and photodegradable hydrogels find utility in a range of biomedical applications that require tissue-like properties or programmable responses. Progress in engineering with PRHs is facilitated by the development of theoretical tools that enable optimization of their photochemistry, polymer matrices, nanofillers, and architecture. This review brings together models and design principles that enable key applications of PRHs in tissue engineering, drug delivery, and soft robotics, and highlights ongoing challenges in both modeling and application.

Fabrication of Hollow Structures in Photodegradable Hydrogels Using a Multi-Photon Excitation Process for Blood Vessel Tissue Engineering

Engineered blood vessels generally recapitulate vascular function in vitro and can be utilized in drug discovery as a novel microphysiological system. Recently, various methods to fabricate vascular models in hydrogels have been reported to study the blood vessel functions in vitro; however, in general, it is difficult to fabricate hollow structures with a designed size and structure with a tens of micrometers scale for blood vessel tissue engineering. This study reports a method to fabricate the hollow structures in photodegradable hydrogels prepared in a microfluidic device. An infrared femtosecond pulsed laser, employed to induce photodegradation via multi-photon excitation, was scanned in the hydrogel in a program-controlled manner for fabricating the designed hollow structures. The photodegradable hydrogel was prepared by a crosslinking reaction between an azide-modified gelatin solution and a dibenzocyclooctyl-terminated photocleavable tetra-arm polyethylene glycol crosslinker solution. After assessing the composition of the photodegradable hydrogel in terms of swelling and cell adhesion, the hydrogel prepared in the microfluidic device was processed by laser scanning to fabricate linear and branched hollow structures present in it. We introduced a microsphere suspension into the fabricated structure in photodegradable hydrogels, and confirmed the fabrication of perfusable hollow structures of designed patterns via the multi-photon excitation process.

Stepwise construction of dynamic microscale concentration gradients around hydrogel-encapsulated cells in a microfluidic perfusion culture device

Inside living organisms, concentration gradients dynamically change over time as biological processes progress. Therefore, methods to construct dynamic microscale concentration gradients in a spatially controlled manner are needed to provide more realistic research environments. Here, we report a novel method for the construction of dynamic microscale concentration gradients in a stepwise manner around cells in micropatterned hydrogel. In our method, cells are encapsulated in a photodegradable hydrogel formed inside a microfluidic perfusion culture device, and perfusion microchannels are then fabricated in the hydrogel by micropatterned photodegradation. The cells in the micropatterned hydrogel can then be cultured by perfusing culture medium through the fabricated microchannels. By using this method, we demonstrate the simultaneous construction of two dynamic concentration gradients, which allowed us to expose the cells encapsulated in the hydrogel to a dynamic microenvironment.

Azido-functionalized gelatin via direct conversion of lysine amino groups by diazo transfer as a building block for biofunctional hydrogels

Gelatin is one of the most prominent biopolymers in biomedical material research and development. It is frequently used in hybrid hydrogels, which combine the advantageous properties of bio-based and synthetic polymers. To prevent the biological component from leaching out of the hydrogel, the biomolecules can be equipped with azides. Those groups can be used to immobilize gelatin covalently in hydrogels by the highly selective and specific azide-alkyne cycloaddition. In this contribution, we functionalized gelatin with azides at its lysine residues by diazo transfer, which offers the great advantage of only minimal side-chain extension. Approximately 84-90% of the amino groups are modified as shown by ¹ H-NMR spectroscopy, 2,4,6-trinitrobenzenesulfonic acid assay as well as Fourier-transform infrared spectroscopy, rheology, and the determination of the isoelectric point. Furthermore, the azido-functional gelatin is incorporated into hydrogels based on poly(ethylene glycol) diacrylate (PEG-DA) at different concentrations (0.6, 3.0, and 5.5%). All hydrogels were classified as noncyctotoxic with significantly enhanced cell adhesion of human fibroblasts on their surfaces compared to pure PEG-DA hydrogels. Thus, the new gelatin derivative is found to be a very promising building block for tailoring the bioactivity of materials.

Spun Biotextiles in Tissue Engineering and Biomolecules Delivery Systems

Nowadays, tissue engineering is described as an interdisciplinary field that combines engineering principles and life sciences to generate implantable devices to repair, restore and/or improve functions of injured tissues. Such devices are designed to induce the interaction and integration of tissue and cells within the implantable matrices and are manufactured to meet the appropriate physical, mechanical and physiological local demands. Biodegradable constructs based on polymeric fibers are desirable for tissue engineering due to their large surface area, interconnectivity, open pore structure, and controlled mechanical strength. Additionally, biodegradable constructs are also very sought-out for biomolecule delivery systems with a target-directed action. In the present review, we explore the properties of some of the most common biodegradable polymers used in tissue engineering applications and biomolecule delivery systems and highlight their most important uses.

Biomedical applications of copper-free click chemistry: in vitro, in vivo, and ex vivo

Recently, click chemistry has provided important advances in biomedical research fields. Particularly, copper-free click chemistry including strain-promoted azide-alkyne cycloaddition (SPAAC) and inverse-electron-demand Diels-Alder (iEDDA) reactions enable fast and specific chemical conjugation under aqueous conditions without the need for toxic catalysts. Click chemistry has resulted in a change of paradigm, showing that artificial chemical reactions can occur on cell surfaces, in cell cytosol, or within the body, which is not easy with most other chemical reactions. Click chemistry in vitro allows specific labelling of cellular target proteins and studying of drug target engagement with drug surrogates in live cells. Furthermore, cellular membrane lipids and proteins could be selectively labelled with click chemistry in vitro and cells could be adhered together using click chemistry. Click chemistry in vivo enables efficient and effective molecular imaging and drug delivery for diagnosis and therapy. Click chemistry ex vivo can be used to develop molecular tools to understand tissue development, diagnosis of diseases, and therapeutic monitoring. Overall, the results from research to date suggest that click chemistry has emerged as a valuable tool in biomedical fields as well as in organic chemistry.

Radiation-Induced Targeted Nanoparticle-Based Gene Delivery for Brain Tumor Therapy

Targeted therapy against the programmed cell death ligand-1 (PD-L1) blockade holds considerable promise for the treatment of different tumor types; however, little effect has been observed against gliomas thus far. Effective glioma therapy requires a delivery vehicle that can reach tumor cells in the central nervous system, with limited systemic side effect. In this study, we developed a cyclic peptide iRGD (CCRGDKGPDC)-conjugated solid lipid nanoparticle (SLN) to deliver small interfering RNAs (siRNAs) against both epidermal growth factor receptor (EGFR) and PD-L1 for combined targeted and immunotherapy against glioblastoma, the most aggressive type of brain tumors. Building on recent studies showing that radiation therapy alters tumors for enhanced nanotherapeutic delivery in tumor-associated macrophage-dependent fashion, we showed that low-dose radiation primes targeted SLN uptake into the brain tumor region, leading to enhanced downregulation of PD-L1 and EGFR. Bioluminescence imaging revealed that radiation therapy followed by systemic administration of targeted SLN leads to a significant decrease in glioblastoma growth and prolonged mouse survival. This study combines radiation therapy to prime the tumor for nanoparticle uptake along with the targeting effect of iRGD-conjugated nanoparticles to yield a straightforward but effective approach for combined EGFR inhibition and immunotherapy against glioblastomas, which can be extended to other aggressive tumor types.

Gelatin-Based Hydrogels through Homobifunctional Triazolinediones Targeting Tyrosine Residues

Gelatin is a biopolymer with interesting properties that can be useful for biomaterial design for different applications such as drug delivery systems, or 3D scaffolds for tissue engineering. However, gelatin suffers from poor mechanical stability at physiological temperature, hence methods for improving its properties are highly desirable. In the present work, a new chemical cross-linking strategy based on triazolinedione ene-type chemistry towards stable hydrogel is proposed. Two different homobifunctional 1,2,4-triazoline-3,5(4H)-diones, namely 4,4'-hexane-1,6-diylbis(3H-1,2,4-triazoline-3,5(4H)-dione) 1 and 4,4'-[methylenebis(4,1-phenylene)]bis(3H-1,2,4-triazoline-3,5(4H)-dione) 2 were used as cross-linkers in different ratio to tyrosine residues in gelatin. The reaction was proved effective in all experimented conditions and hydrogels featured with different thermal stability were obtained. In general, the higher the cross-linker/tyrosine ratio, the more thermostable the hydrogel. The swelling properties are strictly dependent upon the chemical nature of the cross-linker.

Mechanically robust photodegradable gelatin hydrogels for 3D cell culture and in situ mechanical modification

Highly conjugated, hydrophobically modified gelatin hydrogels were synthesized, polymerized and degraded with orthogonal wavelengths of light.

A Modular Approach to Sensitized Two-Photon Patterning of Photodegradable Hydrogels

Photodegradable hydrogels have emerged as useful platforms for research on cell function, tissue engineering, and cell delivery as their physical and chemical properties can be dynamically controlled by the use of light. The photo-induced degradation of such hydrogel systems is commonly based on the integration of photolabile o-nitrobenzyl derivatives to the hydrogel backbone, because such linkers can be cleaved by means of one- and two-photon absorption. Herein we describe a cytocompatible click-based hydrogel containing o-nitrobenzyl ester linkages between a hyaluronic acid backbone, which is photodegradable in the presence of cells. It is demonstrated for the first time that by using a cyclic benzylidene ketone-based small molecule as photosensitizer the efficiency of the two-photon degradation process can be improved significantly. Biocompatibility of both the improved two-photon micropatterning process as well as the hydrogel itself is confirmed by cell culture studies.

Achieving Controlled Biomolecule-Biomaterial Conjugation

The conjugation of biomolecules can impart materials with the bioactivity necessary to modulate specific cell behaviors. While the biological roles of particular polypeptide, oligonucleotide, and glycan structures have been extensively reviewed, along with the influence of attachment on material structure and function, the key role played by the conjugation strategy in determining activity is often overlooked. In this review, we focus on the chemistry of biomolecule conjugation and provide a comprehensive overview of the key strategies for achieving controlled biomaterial functionalization. No universal method exists to provide optimal attachment, and here we will discuss both the relative advantages and disadvantages of each technique. In doing so, we highlight the importance of carefully considering the impact and suitability of a particular technique during biomaterial design.

Silane coatings of metallic biomaterials for biomedical implants: A preliminary review

In response to increased attention in literature, this work provides a qualitative review surrounding the application of silane-based coatings of metallic biomaterials for biomedical implants. Included herein is both a brief summary of existing knowledge and concepts regarding silane-based thin films, along with an analysis of recent peer-reviewed publications and advances towards their practical application for biomedical coatings. Specifically, the review identifies innovative silane-based coatings according to their molecular identity and film structure and analyses their impact on the biocorrosion resistance, protein adsorption, cell viability, and antimicrobial properties of the overall coated implant. It is shown that a range of common silanes clearly exhibit promising properties for biomedical implant coatings, but further work is needed, particularly on mechanisms of physiological interaction and characteristic effects of silane functional groups, before seeing clinical use. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2901-2918, 2018.

Toward Light-Regulated Living Biomaterials

Living materials are an emergent material class, infused with the productive, adaptive, and regenerative properties of living organisms. Property regulation in living materials requires encoding responsive units in the living components to allow external manipulation of their function. Here, an optoregulated Escherichia coli (E. coli)-based living biomaterial that can be externally addressed using light to interact with mammalian cells is demonstrated. This is achieved by using a photoactivatable inducer of gene expression and bacterial surface display technology to present an integrin-specific miniprotein on the outer membrane of an endotoxin-free E. coli strain. Hydrogel surfaces functionalized with the bacteria can expose cell adhesive molecules upon in situ light-activation, and trigger cell adhesion. Surface immobilized bacteria are able to deliver a fluorescent protein to the mammalian cells with which they are interacting, indicating the potential of such a bacterial material to deliver molecules to cells in a targeted manner.

Recent Advances in Edible Polymer Based Hydrogels as a Sustainable Alternative to Conventional Polymers

The over increasing demand of eco-friendly materials to counter various problems, such as environmental issues, economics, sustainability, biodegradability, and biocompatibility, open up new fields of research highly focusing on nature-based products. Edible polymer based materials mainly consisting of polysaccharides, proteins, and lipids could be a prospective contender to handle such problems. Hydrogels based on edible polymer offer many valuable properties compared to their synthetic counterparts. Edible polymers can contribute to the reduction of environmental contamination, advance recyclability, provide sustainability, and thereby increase its applicability along with providing environmentally benign products. This review is highly emphasizing on toward the development of hydrogels from edible polymer, their classification, properties, chemical modification, and their potential applications. The application of edible polymer hydrogels covers many areas including the food industry, agricultural applications, drug delivery to tissue engineering in the biomedical field and provide more safe and attractive products in the pharmaceutical, agricultural, and environmental fields, etc.

Elastomeric and pH-responsive hydrogels based on direct crosslinking of the poly(glycerol sebacate) pre-polymer and gelatin

The synthesis and biomedical applications of novel elastomeric, pH-responsive, biocompatible and biodegradable copolymer hydrogels based on poly(glycerol sebacate) and gelatin.

Spatiotemporal hydrogel biomaterials for regenerative medicine

Hydrogels mimic many of the physical properties of soft tissue and are widely used biomaterials for tissue engineering and regenerative medicine. Synthetic hydrogels have been developed to recapitulate many of the healthy and diseased states of native tissues and can be used as a cell scaffold to study the effect of matricellular interactions in vitro. However, these matrices often fail to capture the dynamic and heterogenous nature of the in vivo environment, which varies spatially and during events such as development and disease. To address this deficiency, a variety of manufacturing and processing techniques are being adapted to the biomaterials setting. Among these, photochemistry is particularly well suited because these reactions can be performed in precise three-dimensional space and at specific moments in time. This spatiotemporal control over chemical reactions can also be performed over a range of cell- and tissue-relevant length scales with reactions that proceed efficiently and harmlessly at ambient conditions. This review will focus on the use of photochemical reactions to create dynamic hydrogel environments, and how these dynamic environments are being used to investigate and direct cell behavior.

BMP2-modified injectable hydrogel for osteogenic differentiation of human periodontal ligament stem cells

This is the first report on the development of a covalently bone morphogenetic protein-2 (BMP2)-immobilized hydrogel that is suitable for osteogenic differentiation of human periodontal ligament stem cells (hPLSCs). O-propargyl-tyrosine (OpgY) was site-specifically incorporated into BMP2 to prepare BMP2-OpgY with an alkyne group. The engineered BMP2-OpgY exhibited osteogenic characteristics after in vitro osteogenic differentiation of hPLSCs, indicating the osteogenic ability of BMP2-OpgY. A methoxy polyethylene glycol-(polycaprolactone-(N₃)) block copolymer (MC-N₃) was prepared as an injectable in situ-forming hydrogel. BMP2 covalently immobilized on an MC hydrogel (MC-BMP2) was prepared quantitatively by a simple biorthogonal reaction between alkyne groups on BMP2-OpgY and azide groups on MC-N₃ via a Cu(I)-catalyzed click reaction. The hPLSCs-loaded MC-BMP2 formed a hydrogel almost immediately upon injection into animals. In vivo osteogenic differentiation of hPLSCs in the MC-BMP2 formulation was confirmed by histological staining and gene expression analyses. Histological staining of hPLSC-loaded MC-BMP2 implants showed evidence of mineralized calcium deposits, whereas hPLSC-loaded MC-Cl or BMP2-OpgY mixed with MC-Cl, implants showed no mineral deposits. Additionally, MC-BMP2 induced higher levels of osteogenic gene expression in hPLSCs than in other groups. In conclusion, BMP2-OpgY covalently immobilized on MC-BMP2 induced osteogenic differentiation of hPLSCs as a noninvasive method for bone tissue engineering.

Tethering Growth Factors to Collagen Surfaces Using Copper-Free Click Chemistry: Surface Characterization and in Vitro Biological Response

Surface modifications with tethered growth factors have mainly been applied to synthetic polymeric biomaterials in well-controlled, acellular settings, followed by seeding with cells. The known bio-orthogonality of copper-free click chemistry provides an opportunity to not only use it in vitro to create scaffolds or pro-migratory tracks in the presence of living cells, but also potentially apply it to living tissues directly as a coupling modality in situ. In this study, we studied the chemical coupling of growth factors to collagen using biocompatible copper-free click chemistry and its effect on the enhancement of growth factor activity in vitro. We verified the characteristics of modified epidermal growth factor (EGF) using mass spectrometry and an EGF/EGF receptor binding assay, and evaluated the chemical immobilization of EGF on collagen by copper-free click chemistry using surface X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR) spectroscopy, and enzyme-linked immunosorbent assay (ELISA). We found that the anchoring was noncytotoxic, biocompatible, and rapid. Moreover, the surface-immobilized EGF had significant effects on epithelial cell attachment and proliferation. Our results demonstrate the possibility of copper-free click chemistry as a tool for covalent bonding of growth factors to collagen in the presence of living cells. This approach is a novel and potentially clinically useful application of copper-free click chemistry as a way of anchoring growth factors to collagen and foster epithelial wound healing.

Bio-orthogonal conjugation and enzymatically triggered release of proteins within multi-layered hydrogels

Hydrogels are facile architectures for the controlled presentation of proteins with far-reaching applications, from fundamental biological studies in three-dimensional culture to new regenerative medicine and therapeutic delivery strategies. Here, we demonstrate a versatile approach for spatially-defined presentation of engineered proteins within hydrogels through i) immobilization using bio-orthogonal strain-promoted alkyne-azide click chemistry and ii) dynamic protease-driven protein release using exogenously applied enzyme. Model fluorescent proteins were expressed using nonsense codon replacement to incorporate azide-containing unnatural amino acids in a site-specific manner toward maintaining protein activity: here, cyan fluorescent protein (AzCFP), mCherry fluorescent protein (AzmCh), and mCh decorated with a thrombin cut-site. (AzTMBmCh). Eight-arm poly(ethylene glycol) (PEG) was modified with dibenzylcyclooctyne (DBCO) groups and reacted with azide functionalized PEG in aqueous solution for rapid formation of hydrogels. Azide functionalized full-length fluorescent proteins were successfully incorporated into the hydrogel network by reaction with PEG-DBCO prior to gel formation. Temporal release and removal of select proteins (AzTMBmCh) was triggered with the application of thrombin and monitored in real-time with confocal microscopy, providing a responsive handle for controlling matrix properties. Hydrogels with regions of different protein compositions were created using a layering technique with thicknesses of hundreds of micrometers, affording opportunities for the creation of complex geometries on size scales relevant for controlling cellular microenvironments.

STATEMENT OF SIGNIFICANCE

Controlling protein presentation within biomaterials is important for modulating interactions with biological systems. For example, native tissues are composed of subunits with different matrix compositions (proteins, stiffness) that dynamically interact with cells, influencing function and fate. Toward mimicking such temporally-regulated and spatially-defined microenvironments, we utilize bio-orthogonal click chemistry and protein engineering to create hydrogels with distinct regions of proteins and modify them over time. Through nonsense codon replacement, we site-specifically functionalize large proteins with i) azides for covalent conjugation and ii) an enzymatic cleavage site for user-defined release from hydrogels. Our results exemplify not only the ability to create unique bio-functionalized hydrogels with controlled mechanical properties, but also the potential for creating interesting interfaces for cell culture and tissue engineering applications.

Morphology-based optical separation of subpopulations from a heterogeneous murine breast cancer cell line

Understanding tumor heterogeneity is an urgent and unmet need in cancer research. In this study, we used a morphology-based optical cell separation process to classify a heterogeneous cancer cell population into characteristic subpopulations. To classify the cell subpopulations, we assessed their morphology in hydrogel, a three-dimensional culture environment that induces morphological changes according to the characteristics of the cells (i.e., growth, migration, and invasion). We encapsulated the murine breast cancer cell line 4T1E, as a heterogeneous population that includes highly metastatic cells, in click-crosslinkable and photodegradable gelatin hydrogels, which we developed previously. We observed morphological changes within 3 days of encapsulating the cells in the hydrogel. We separated the 4T1E cell population into colony- and granular-type cells by optical separation, in which local UV-induced degradation of the photodegradable hydrogel around the target cells enabled us to collect those cells. The obtained colony- and granular-type cells were evaluated in vitro by using a spheroid assay and in vivo by means of a tumor growth and metastasis assay. The spheroid assay showed that the colony-type cells formed compact spheroids in 2 days, whereas the granular-type cells did not form spheroids. The tumor growth assay in mice revealed that the granular-type cells exhibited lower tumor growth and a different metastasis behavior compared with the colony-type cells. These results suggest that morphology-based optical cell separation is a useful technique to classify a heterogeneous cancer cell population according to its cellular characteristics.

Microfluidic Technology for the Generation of Cell Spheroids and Their Applications

A three-dimensional (3D) tissue model has significant advantages over the conventional two-dimensional (2D) model. A 3D model mimics the relevant in-vivo physiological conditions, allowing a cell culture to serve as an effective tool for drug discovery, tissue engineering, and the investigation of disease pathology. The present reviews highlight the recent advances and the development of microfluidics based methods for the generation of cell spheroids. The paper emphasizes on the application of microfluidic technology for tissue engineering including the formation of multicellular spheroids (MCS). Further, the paper discusses the recent technical advances in the integration of microfluidic devices for MCS-based high-throughput drug screening. The review compares the various microfluidic techniques and finally provides a perspective for the future opportunities in this research area.

Visible Laser-Induced In Situ Cell Detachment from Gold Nanoparticle-Embedded Collagen Gel

Cell sorting is important for cell biology and regenerative medicine. A visible light-responsive cell scaffold is produced using gold nanoparticles and collagen gel. Various kinds of cells are cultured on the visible light-responsive cell scaffold, and the target cells are selectively detached by photoirradiation without any cytotoxicity. This is a new image-guided cell sorting system.

Programmable biomaterials for dynamic and responsive drug delivery

Biomaterials are continually being designed that enable new methods for interacting dynamically with cell and tissues, in turn unlocking new capabilities in areas ranging from drug delivery to regenerative medicine. In this review, we explore some of the recent advances being made in regards to programming biomaterials for improved drug delivery, with a focus on cancer and infection. We begin by explaining several of the underlying concepts that are being used to design this new wave of drug delivery vehicles, followed by examining recent materials systems that are able to coordinate the temporal delivery of multiple therapeutics, dynamically respond to changing tissue environments, and reprogram their bioactivity over time.

Hydrogel microfabrication technology toward three dimensional tissue engineering

The development of biologically relevant three-dimensional (3D) tissue constructs is essential for the alternative methods of organ transplantation in regenerative medicine, as well as the development of improved drug discovery assays. Recent technological advances in hydrogel microfabrication, such as micromolding, 3D bioprinting, photolithography, and stereolithography, have led to the production of 3D tissue constructs that exhibit biological functions with precise 3D microstructures. Furthermore, microfluidics technology has enabled the development of the perfusion culture of 3D tissue constructs with vascular networks. In this review, we present these hydrogel microfabrication technologies for the in vitro reconstruction and cultivation of 3D tissues. Additionally, we discuss current challenges and future perspectives of 3D tissue engineering.

In situ-forming click-crosslinked gelatin based hydrogels for 3D culture of thymic epithelial cells

In situ-forming gelatin based hydrogels, which are crosslinked using an efficient nitrile oxide-norbornene click reaction, provide a suitable 3D culture environment for thymic epithelial cells.

Multi-Functional Macromers for Hydrogel Design in Biomedical Engineering and Regenerative Medicine

Contemporary biomaterials are expected to provide tailored mechanical, biological and structural cues to encapsulated or invading cells in regenerative applications. In addition, the degradative properties of the material also have to be adjustable to the desired application. Oligo- or polymeric building blocks that can be further cross-linked into hydrogel networks, here addressed as macromers, appear as the prime option to assemble gels with the necessary degrees of freedom in the adjustment of the mentioned key parameters. Recent developments in the design of multi-functional macromers with two or more chemically different types of functionalities are summarized and discussed in this review illustrating recent trends in the development of advanced hydrogel building blocks for regenerative applications.