Macroscopic Hydrogels by Self-Assembly of Oligolactate-Grafted Dextran Microspheres

Scaffold-based bone tissue engineering in microgravity: potential, concerns and implications

One of humanity's greatest challenges is space exploration, which requires an in-depth analysis of the data continuously collected as a necessary input to fill technological gaps and move forward in several research sectors. Focusing on space crew healthcare, a critical issue to be addressed is tissue regeneration in extreme conditions. In general, it represents one of the hottest and most compelling goals of the scientific community and the development of suitable therapeutic strategies for the space environment is an urgent need for the safe planning of future long-term manned space missions. Osteopenia is a commonly diagnosed disease in astronauts due to the physiological adaptation to altered gravity conditions. In order to find specific solutions to bone damage in a reduced gravity environment, bone tissue engineering is gaining a growing interest. With the aim to critically investigate this topic, the here presented review reports and discusses bone tissue engineering scenarios in microgravity, from scaffolding to bioreactors. The literature analysis allowed to underline several key points, such as the need for (i) biomimetic composite scaffolds to better mimic the natural microarchitecture of bone tissue, (ii) uniform simulated microgravity levels for standardized experimental protocols to expose biological materials to the same testing conditions, and (iii) improved access to real microgravity for scientific research projects, supported by the so-called democratization of space.

Injectable hydrogels for bone and cartilage tissue engineering: a review

Tissue engineering, using a combination of living cells, bioactive molecules, and three-dimensional porous scaffolds, is a promising alternative to traditional treatments such as the use of autografts and allografts for bone and cartilage tissue regeneration. Scaffolds, in this combination, can be applied either through surgery by implantation of cell-seeded pre-fabricated scaffolds, or through injection of a solidifying precursor and cell mixture, or as an injectable cell-seeded pre-fabricated scaffold. In situ forming and pre-fabricated injectable scaffolds can be injected directly into the defect site with complex shape and critical size in a minimally invasive manner. Proper and homogeneous distribution of cells, biological factors, and molecular signals in these injectable scaffolds is another advantage over pre-fabricated scaffolds. Due to the importance of injectable scaffolds in tissue engineering, here different types of injectable scaffolds, their design challenges, and applications in bone and cartilage tissue regeneration are reviewed.

Fine-Tuning Crystallization-Induced Gelation in Amphiphilic Double-Brush Polymers

A series of amphiphilic double-brush polymers based on itaconate diesters were synthesized with the objective of tailoring the thermal and mechanical properties of hydrogels formed by them; the amphiphilic itaconate diesters carried an MPEG350 segment and an alkyl chain, whose length was varied from C12 to C18. As was reported by us earlier (Macromolecules 2017, 50, 5004), the formation of the hydrogel was due to the crystallization of alkyl segments, as confirmed by the match of the rheological gel-to-sol transition with that of differential scanning calorimetry melting transition of the gel. In an effort to fine-tune the hydrogel-melting temperature and its strength, we varied the length of the alkyl chain length while keeping the hydrophilic segment length constant at MPEG350; apart from varying the alkyl chain length, an oxyethylene spacer was incorporated to examine the effect of decoupling the alkyl side-chain crystallization from the backbone. With these modifications, the melting temperature of the hydrogel was varied from 30 to 56 °C. Likewise, the strength of the hydrogel, as reflected by its storage modulus, varied from around 220 to 970 Pa; the softer gels typically exhibited a slightly larger critical shear strain beyond which the gel transformed into a sol. The thermal and shear-induced gel-to-sol transitions were reversible; however, the modulus after the shear-induced transition did not fully recover instantly (∼80%), suggesting that the formation of the extended gel network is slow. Further fine-tuning could be achieved by copolymerization of two different amphiphilic itaconate monomers, namely, those with C16 and C18, which provided an intermediate gel-melting temperature; however, co-gelation of the two preformed homopolymer gels yielded two distinct gel-melting transitions. Thus, this class of tuneable stimuli-responsive polymeric hydrogels prepared from biobenign components, namely, itaconic acid, 1-alkanols, and MPEGs, could serve as potential candidates for biomedical applications.

Chitosan/β-glycerophosphate-based microparticles manufactured by laminar jet break-up technology

This study is about the use of β-glycerophosphate (βGP) to modulate the production of chitosan microparticles through a technology of jet break-up. βGP has been described as capable of producing chitosan gels without additional complexing agents via a thermal transition (inverse gelation). A preliminary assessment on the effect of temperature on the viscosity and gelation of chitosan/βGP precursors demonstrated that the crosslinking process was too slow to afford microparticle production via jet break-up. Instead, βGP was used as a solubilizer to provide stable chitosan solution at neutral pH, which allowed the preparation of microparticles through polyelectrolyte complexation (with triphosphate) under physiological conditions, as opposed to the more conventional method of chitosan solubilisation in acids. Here, the key parameters of the microencapsulation process have been optimized, aiming to produce spherical particle of well-defined size and circularity, as well as toroidal microparticles, with a physico-chemical evaluation of the products.

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.

Microsphere-Based Scaffolds in Regenerative Engineering

Microspheres have long been used in drug delivery applications because of their controlled release capabilities. They have increasingly served as the fundamental building block for fabricating scaffolds for regenerative engineering because of their ability to provide a porous network, offer high-resolution control over spatial organization, and deliver growth factors/drugs and/or nanophase materials. Because they provide physicochemical gradients via spatiotemporal release of bioactive factors and nanophase ceramics, microspheres are a desirable tool for engineering complex tissues and biological interfaces. In this review we describe various methods for microsphere fabrication and sintering, and elucidate how these methods influence both micro- and macroscopic scaffold properties, with a special focus on the nature of sintering. Furthermore, we review key applications of microsphere-based scaffolds in regenerating various tissues. We hope to inspire researchers to join a growing community of investigators using microspheres as tissue engineering scaffolds so that their full potential in regenerative engineering may be realized.

Oppositely Charged Polyurethane Microspheres with Tunable Zeta Potentials as an Injectable Dual-Loaded System for Bone Repair

To effectively repair irregular shaped bone defects by a minimally invasive procedure, the exploration of an injectable gel to fill the defect is desirable. Herein, positively and negatively charged polyurethane microspheres (PU-A and PU-B) with adjustable zeta potentials as well as the hydroxyapatite-loaded PU microsphere (PU-A/HA) and the dexamethasone-loaded PU microsphere (PU-B/Dex) were successfully prepared, and the oppositely charged microspheres could self-assemble into injectable gels with 3D structures by a mutually electrostatic attraction. The self-assembly PU-A/HA+PU-B/Dex gel exhibited a much higher elastic modulus (about 0.20 MPa) and excellent shear-thinning and self-recovery behaviors, which would allow the gel to be injected through a fine syringe to fill the irregular defect. The in vitro and in vivo experiments demonstrated that the coexistence of HA and Dex in PU-A/HA+PU-B/Dex gel had a synergistic effect on cell differentiation and accelerating new bone formation, displaying a good prospect as an injectable gel for bone repair in minimally invasive surgery.

Particle assemblies: toward new tools for regenerative medicine

Regenerative medicine is a demanding field in terms of design and elaboration of materials able to meet the specifications that this application imposes. The regeneration of tissue is a multiscale issue, from the signaling molecule through cell expansion and finally tissue growth requiring a large variety of cues that should be delivered in place and time. Hence, the materials should be able to accommodate cells with respect to their phenotypes, to allow cell division to the right tissue, to maintain the integrity of the surrounding sane tissue, and eventually use their signaling machinery to serve the development of the appropriate neo-tissue. They should also present the ability to deliver growth factors and regulate tissue development, to be degraded into safe products, in order not to impede tissue development, and finally be easily implanted/injected into the patients. In this context, colloid-based materials represent a very promising family of products because one can take advantage of their high specific area, their capability to carry/deliver bio-active molecules, and their capacity of assembling (eventually in vivo) into materials featuring other mechanical, rheological, physicochemical properties. Other benefits of great interest would be their ease of production even via high through-put processes and their potential manufacturing from safe, biodegradable and biocompatible parent raw material. This review describes the state-of-the-art of processes leading to complex materials from the assembly of colloids meeting, at least partially, the above-described specifications for tissue engineering and regenerative medicine.

Dextran vesicular carriers for dual encapsulation of hydrophilic and hydrophobic molecules and delivery into cells

Dextran vesicular nanoscaffolds were developed based on polysaccharide and renewable resource alkyl tail for dual encapsulation of hydrophilic and hydrophobic molecules (or drugs) and delivery into cells. The roles of the hydrophobic segments on the molecular self-organization of dextran backbone into vesicles or nanoparticles were investigated in detail. Dextran vesicles were found to be a unique dual carrier in which water-soluble molecules (like Rhodamine-B, Rh-B) and polyaromatic anticancer drug (camptothecin, CPT) were selectively encapsulated in the hydrophilic core and hydrophobic layer, respectively. The dextran vesicles were capable of protecting the plasma-sensitive CPT lactone pharmacophore against the hydrolysis by 10× better than the CPT alone in PBS. The aliphatic ester linkage connecting the hydrophobic tail with dextran was found to be cleaved by esterase under physiological conditions for fast releasing of CPT or Rh-B. Cytotoxicity of the dextran vesicle and its drug conjugate were tested on mouse embryonic fibroblast cells (MEFs) using MTT assay. The dextran vesicular scaffold was found to be nontoxic to living cells. CPT loaded vesicles were found to be 2.5-fold more effective in killing fibroblasts compared to that of CPT alone in PBS. Confocal microscopic images confirmed that both Rh-B and CPT loaded vesicles to be taken up by fibroblasts compared to CPT alone, showing a distinctly perinuclear localization in cells. The custom designed dextran vesicular provides new research opportunities for dual loading and delivering of hydrophilic and hydrophobic drug molecules.

Comparison of micro- vs. nanostructured colloidal gelatin gels for sustained delivery of osteogenic proteins: Bone morphogenetic protein-2 and alkaline phosphatase

Colloidal gels have recently emerged as a promising new class of materials for regenerative medicine by employing micro- and nanospheres as building blocks to assemble into integral scaffolds. To this end, physically crosslinked particulate networks are formed that are injectable yet cohesive. By varying the physicochemical properties of different particle populations, the suitability of colloidal gels for programmed delivery of multiple therapeutic proteins is superior over conventional monolithic gels that lack this strong capacity for controlled drug release. Colloidal gels made of biodegradable polymer micro- or nanospheres have been widely investigated over the past few years, but a direct comparison between micro- vs. nanostructured colloidal gels has not been made yet. Therefore, the current study has compared the viscoelastic properties and capacity for drug release of colloidal gels made of oppositely charged gelatin microspheres vs. nanospheres. Viscoelastic properties of the colloidal gelatin gels were characterized by rheology and simple injectability tests, and in vitro release of two selected osteogenic proteins (i.e. bone morphogenetic protein-2 (BMP-2) and alkaline phosphatase (ALP)) from the colloidal gelatin gels was evaluated using radiolabeled BMP-2 and ALP. Nanostructured colloidal gelatin gels displayed superior viscoelastic properties over microsphere-based gels in terms of elasticity, injectability, structural integrity, and self-healing behavior upon severe network destruction. In contrast, microstructured colloidal gelatin gels exhibited poor gel strength and integrity, unfavorable injectability, and did not recover after shearing, resulting from the poor gel cohesion due to insufficiently strong interparticle forces. Regarding the capacity for drug delivery, sustained growth factor (BMP-2) release was obtained for both micro- and nanosphere-based gels, the kinetics of which were mainly depending on the particle size of gelatin spheres with the same crosslinking density. Therefore, the optimal gelatin carrier for drug delivery in terms of particle size and crosslinking density still needs to be established for specific clinical indications that require either short-term or long-term release. It can be concluded that nanostructured colloidal gelatin gels show great potential for sustained delivery of therapeutic proteins, whereas microstructured colloidal gelatin gels are not sufficiently cohesive as injectables for biomedical applications.

An in situ forming collagen-PEG hydrogel for tissue regeneration

There are limited options for surgeons to repair simple or complex tissue defects due to injury, illness or disease. Consequently, there are few treatments for many serious ailments, including neural-related injuries, myocardial infarction and focal hyaline cartilage defects. Tissue-engineered scaffolds offer great promise for addressing these wide-ranging indications; however, there are many considerations that need to be made when conceptualizing a product. For many applications, an in situ forming scaffold that could completely fill defects with complex geometries, adhere to adjacent tissues and foster cell proliferation would be ideal. Additionally, the scaffold would preferably have tailored mechanical properties similar to native tissues and highly controllable gelation kinetics, and would not require an external trigger, such as ultraviolet light, for gelation. We have developed a unique injectable hydrogel system composed of collagen and multi-armed poly(ethylene glycol) (PEG) that meets all of these criteria. The collagen component enables cellular adhesion and permits enzymatic degradation, while the multi-armed PEG component has amine-reactive chemistry that also binds proteins/tissue and is hydrolytically degradable. We have characterized the mechanical properties, swelling, degradation rates and cytocompatibility of these novel hydrogels. The hydrogels demonstrated tunable mechanics, variable swelling and suitable degradation profiles. Cells adhered and proliferated to near confluence on the hydrogels over 7 days. These data suggest that these collagen and PEG hydrogels exhibit the mechanical, physical and biological properties suitable for use as an injectable tissue scaffold for the treatment of a variety of simple and complex tissue defects.

The use of micro- and nanospheres as functional components for bone tissue regeneration

During the last decade, the use of micro- and nanospheres as functional components for bone tissue regeneration has drawn increasing interest. Scaffolds comprising micro- and nanospheres display several advantages compared with traditional monolithic scaffolds that are related to (i) an improved control over sustained delivery of therapeutic agents, signaling biomolecules and even pluripotent stem cells, (ii) the introduction of spheres as stimulus-sensitive delivery vehicles for triggered release, (iii) the use of spheres to introduce porosity and/or improve the mechanical properties of bulk scaffolds by acting as porogen or reinforcement phase, (iv) the use of spheres as compartmentalized microreactors for dedicated biochemical processes, (v) the use of spheres as cell delivery vehicle, and, finally, (vi) the possibility of preparing injectable and/or moldable formulations to be applied by using minimally invasive surgery. This article focuses on recent developments with regard to the use of micro- and nanospheres for bone regeneration by categorizing micro-/nanospheres by material class (polymers, ceramics, and composites) as well as summarizing the main strategies that employ these spheres to improve the functionality of scaffolds for bone tissue engineering.

Stereocomplexes Formed From Select Oligomers of Polymer d-lactic Acid (PDLA) and l-lactate May Inhibit Growth of Cancer Cells and Help Diagnose Aggressive Cancers-Applications of the Warburg Effect

It is proposed that select oligomers of polymer d-lactic acid (PDLA) will form a stereocomplex with l-lactate in vivo, producing lactate deficiency in tumor cells. Those cancer cells that utilize transport of lactate to maintain electrical neutrality may cease to multiply or die because of lactate trapping, and those cancer cells that benefit from utilization of extracellular lactate may be impaired. Intracellular trapping of lactate produces a different physiology than inhibition of LDH because the cell loses the option of shuttling pyruvate to an alternative pathway to produce an anion. Conjugated with stains or fluorescent probes, PDLA oligomers may be an agent for the diagnosis of tissue lactate and possibly cell differentiation in biopsy specimens. Preliminary experimental evidence is presented confirming that PDLA in high concentrations is cytotoxic and that l-lactate forms a presumed stereocomplex with PDLA. Future work should be directed at isolation of biologically active oligomers of PDLA.

Liquid-liquid two-phase systems for the production of porous hydrogels and hydrogel microspheres for biomedical applications: A tutorial review

Macroporous hydrogels may have direct applications in regenerative medicine as scaffolds to support tissue formation. Hydrogel microspheres may be used as drug-delivery vehicles or as building blocks to assemble modular scaffolds. A variety of techniques exist to produce macroporous hydrogels and hydrogel microspheres. A subset of these relies on liquid-liquid two-phase systems. Within this subset, vastly different types of polymerization processes are found. In this review, the history, terminology and classification of liquid-liquid two-phase polymerization and crosslinking are described. Instructive examples of hydrogel microsphere and macroporous scaffold formation by precipitation/dispersion, emulsion and suspension polymerizations are used to illustrate the nature of these processes. The role of the kinetics of phase separation in determining the morphology of scaffolds and microspheres is also delineated. Brief descriptions of miniemulsion, microemulsion polymerization and ionotropic gelation are also included.

Modular scaffolds assembled around living cells using poly(ethylene glycol) microspheres with macroporation via a non-cytotoxic porogen

Modular, bioactive, macroporous scaffolds were formed by crosslinking poly(ethylene glycol) (PEG) microspheres around living cells. Hydrogel microspheres were produced from reactive PEG derivatives in aqueous sodium sulfate solutions without the use of surfactants or copolymers. Microspheres were formed following thermally induced phase separation if the gel point was reached prior to extensive coarsening of the PEG-rich domains. Three types of PEG microspheres with different functionalities were used to form scaffolds: one type provided mechanical support, the second type provided controlled delivery of the angiogenesis-promoting molecule, sphingosine 1-phosphate (S1P) and the third type served as a slowly dissolving non-cytotoxic porogen. Scaffolds were formed by centrifuging microspheres in the presence of HepG2 hepatoma cells, resulting in a homogenous distribution of cells. During overnight incubation at 37 degrees C, the microspheres reacted with serum proteins in cell culture medium to stabilize the scaffolds. Within 2 days in culture, macropores formed due to the dissolution of the porogenic PEG microspheres, without affecting cell viability. Gradients in porosity were produced by varying the buoyancy of the porogenic microspheres. Conjugated RGD cell adhesion peptides and the delivery of S1P promoted endothelial cell infiltration through macropores in the scaffolds. The scaffolds presented here differ from previous hydrogel scaffolds in that: (i) cells are not encapsulated in hydrogel; (ii) macropores form in the presence of cells; and (iii) scaffold properties are controlled by the modular assembly of different microspheres that perform distinct functions.

Factors affecting size and swelling of poly(ethylene glycol) microspheres formed in aqueous sodium sulfate solutions without surfactants

The LCST behavior of poly(ethylene glycol) (PEG) in aqueous sodium sulfate solutions was exploited to fabricate microspheres without the use of other monomers, polymers, surfactants or organic solvents. Reactive PEG derivatives underwent thermally induced phase separation to produce spherical PEG-rich domains that coarsened in size pending gelation, resulting in stable hydrogel microspheres between approximately 1 and 100 microns in size. The time required to reach the gel point during the coarsening process and the extent of crosslinking after gelation both affected the final microsphere size and swelling ratio. The gel point could be varied by pre-reaction of the PEG derivatives below the cloud point, or by controlling pH and temperature above the cloud point. Pre-reaction brought the PEG derivatives closer to the gel point prior to phase separation, while the pH and temperature influenced the rate of reaction. Dynamic light scattering indicated a percolation-to-cluster transition about 3-5 min following phase separation. The mean radius of PEG-rich droplets subsequently increased with time to the 1/4th power until gelation. PEG microspheres produced by these methods with controlled sizes and densities may be useful for the production of modular scaffolds for tissue engineering.

Self-assembly of chemically engineered hydrophilic dextran into microscopic tubules

Although macromolecular self-assemblies are mostly fabricated from amphiphilic copolymers, here we report a tubular structure self-assembled solely from hydrophilic dextran-derived homopolymers via electrostatic interaction. To obtain tubular structures, we prepared two oppositely charged dextran derivatives by incorporating 2-bromoethylamine (Dex-BH) and chloroacetic acid (Dex-CA) into dextran, and their structures were confirmed using Fourier transform infrared spectroscopy. The two oppositely charged dextran derivatives self-assembled into microsize tubules when mixed in a pH 4.0 buffer solution. The tubular self-assemblies were sensitive to both pH and salt concentrations. Scanning electron microscopy and light microscopy confirmed that the tubules have hollow structures up to 100 microm long with a diameter between 600 nm and 2 microm. The X-ray study did not reveal any ordered molecular organization. This paper explores the mechanism of the tubule self-assembly and suggests a model.