A New Class of Biodegradable Thermosensitive Polymers. I. Synthesis and Characterization of Poly(organophosphazenes) with Methoxy-Poly(ethylene glycol) and Amino Acid Esters as Side Groups

One-Step Preparation of an Injectable Hydrogel Scaffold System Capable of Sequential Dual-Growth Factor Release to Maximize Bone Regeneration

Numerous growth factors are involved in the natural bone healing process, which is precisely controlled in a time- and concentration-dependent manner. Mimicking the secretion pattern of growth factors could be an effective means to maximize the bone regeneration effect. However, achieving the sequential delivery of various growth factors without the use of multiple materials or complex scaffold designs is challenging. Herein, an injectable poly(organophosphazene) hydrogel scaffold (IPS) encapsulating bone morphogenetic protein (BMP)-2 and TGFβ-1 (IPS_BT) is studied to mimic the sequential secretion of growth factors involved in natural bone healing. The IPS_BT system is designed to release TGFβ-1 slowly while retaining BMP-2 for a longer period of time. When IPS_BT is injected in vivo, the hydrogel is replaced by bone tissue. In addition, angiogenic (CD31 and alpha-smooth muscle actin (α-SMA)) and stemness (Nanog and SOX2) markers are highly upregulated in the early stages of bone regeneration. The IPS system developed here has promising applications in tissue engineering because 1) various amounts of the growth factors can be loaded in one step, 2) the release pattern of each growth factor can be controlled via differences in their molecular interactions, and 3) the injected IPS can be degraded and replaced with regenerated bone tissue.

Functional Thermoresponsive Hydrogel Molecule to Material Design for Biomedical Applications

Temperature-induced, rapid changes in the viscosity and reproducible 3-D structure formation makes thermos-sensitive hydrogels an ideal delivery system to act as a cell scaffold or a drug reservoir. Moreover, the hydrogels’ minimum invasiveness, high biocompatibility, and facile elimination from the body have gathered a lot of attention from researchers. This review article attempts to present a complete picture of the exhaustive arena, including the synthesis, mechanism, and biomedical applications of thermosensitive hydrogels. A special section on intellectual property and marketed products tries to shed some light on the commercial potential of thermosensitive hydrogels.

Reversible Speed Regulation of Self-Propelled Janus Micromotors via Thermoresponsive Bottle-Brush Polymers

This work reports a reversible braking system for micromotors that can be controlled by small temperature changes (≈5 °C). To achieve this, gated-mesoporous organosilica microparticles are internally loaded with metal catalysts (to form the motor) and the exterior (partially) grafted with thermosensitive bottle-brush polyphosphazenes to form Janus particles. When placed in an aqueous solution of H2 O2 (the fuel), rapid forward propulsion of the motors ensues due to decomposition of the fuel. Conformational changes of the polymers at defined temperatures regulate the bubble formation rate and thus act as brakes with considerable deceleration/acceleration observed. As the components can be easily varied, this represents a versatile, modular platform for the exogenous velocity control of micromotors.

Self-assembly of peptide amphiphiles by vapor pressure osmometry and dissipative particle dynamics

Peptide amphiphiles are one of the most promising materials in the biomedical field, so much effort has been devoted to characterizing the mechanism of their self-assembly and thermosensitive gelation. In this work, vapor pressure osmometry measurements were carried out to parameterize the thermosensitivity of interactions between peptide amphiphiles in an aqueous solution. The osmometry measurement verified that the peptides became more hydrophobic as temperature increased, which was quantitatively described with the Flory-Huggins χ parameter. Thereafter, a coarse-grained molecular model was used to simulate peptide amphiphiles dissolved in an aqueous solution. The temperature sensitive coarse-grained parameter a HW, which is the repulsive force between the hydrophilic head of the peptide amphiphile and water was estimated from the aforementioned experimentally obtained χ. Furthermore, the effects of concentration and temperature on the self-assembly behavior of peptide amphiphiles were quantitatively studied by dissipative particle dynamics. The simulation results revealed that a HW plays an important role in self-assembly characteristics and in the resulting microstructure of the peptide amphiphiles, which coincides with previous experimental and computational findings. The methodology in quantitatively linking the coarse-grained parameter from experiment and theory provides a sensible foundation for bridging future simulation studies with experimental work on macromolecules.

Transiently thermoresponsive polymers and their applications in biomedicine

The focus of this review is on the class of transiently thermoresponsive polymers. These polymers are thermoresponsive, but gradually lose this property upon chemical transformation - often a hydrolysis reaction - in the polymer side chain or backbone. An overview of the different approaches used for the design of these polymers along with their physicochemical properties is given. Their amphiphilic properties and degradability into fully soluble compounds make this class of responsive polymers attractive for drug delivery and tissue engineering applications. Examples of these are also provided in this review.

LCST-Type Phase Separation of Poly[poly(ethylene glycol) methyl ether methacrylate]s in Hydrofluorocarbon

Poly[poly(ethylene glycol) methyl ether methacrylate]s [poly(PEGMA)s] sharply and reversibly exhibited lower critical solution temperature (LCST)-type phase separation in 2H,3H-perfluoropentane (2HPFP). The cloud points decreased from 52 to 41 °C with increasing the PEG pendant length [-(CH₂CH₂O)_mCH₃: m = 4.5, 9, 19]. The cloud point was precisely controlled via the addition of perfluoroalkanes (e.g., perfluorooctane) to the 2HPFP solution: typically, it was inversely proportional to the amount of perfluorooctane in the mixture. The unique thermoresponsive solubility further afforded the temperature-mediated micellization of a block copolymer of PEG19MA and methyl methacrylate (MMA) in 2HPFP to uniquely give a PEG-core micelle with PMMA shell. Therefore, the LCST phase separation properties in the hydrofluorocarbon would open new vistas for thermoresponsive polymeric materials.

Efficient Design for Stimuli-Responsive Polymers with Quantitative Acid-Degradability: Specifically Designed Alternating Controlled Cationic Copolymerization and Facile Complete Degradation

Specifically designed alternating cationic copolymerization produced well-defined thermo- or pH-responsive polymers with complete acid-degradability. For example, a thermosensitive alternating copolymer with acid-labile acetal linkages in the main chain was obtained from the controlled cationic copolymerization of p-methoxybenzaldehyde (pMeOBzA) and a vinyl ether (VE) with an oxyethylenic side chain. The resulting copolymer exhibited a sharp thermosensitive phase transition in water. The same strategy but using different VEs with esters and benzaldehyde (BzA) yielded pH-responsive copolymers with nearly alternating sequences and narrow molecular weight distributions (MWDs). The alternately arranged acetal bonds in the copolymers allowed complete facile and rapid degradation under acidic conditions, which selectively produced low-molecular-weight compounds (MW ∼ 1-2 × 10²).

Succinylated polysaccharide-based thermosensitive polyelectrostatic complex for protein drug delivery

The objective of this study was to develop a thermosensitive polyelectrostatic complex, based on polysaccharides, as carriers for long-term protein delivery. We developed a thermosensitive polyelectrostatic complex formed through combined electrostatic and hydrophobic interactions. The copolymer (succinylated pullulan -poly(l-lactide)) showed thermosensitivity in aqueous solution and complexed with protein (lysozyme) via electrostatic attractions and hydrophobic interactions at physiological temperature which formed a thermosensitive polyelectrostatic complex. The particle size of the thermosensitive polyelectrostatic complex was decreased from ~520 nm at 4°C to ~190 nm at 37.5°C. These thermosensitive polyelectrostatic complexes were stable in serum and salt conditions, and maintained the bioactivity of encapsulated protein for 36 days. The thermosensitive polyelectrostatic complex had prolonged in vivo stability that was greater than the polyelectrostatic complex. Based on stability and bioactivity tests for the lysozyme-loaded thermosensitive polyelectrostatic complexes, the potential of the long-term protein delivery carrier in physiological conditions was confirmed.

Design, synthesis and ex-vivo release studies of colon-specific polyphosphazene-anticancer drug conjugates

Colon-specific azo based polyphosphazene-anticancer drug conjugates (11-18) have been synthesized and evaluated by ex-vivo release studies. The prepared polyphosphazene drug conjugates (11-18) are stable in acidic (pH=1.2) buffer which showed that these polymer drug conjugates are protected from acidic environment which is the primary requirement of colon specific targeted drug delivery. The ex-vivo release profiles of polyphosphazene drug conjugates (11-18) have been performed in the presence as well as in the absence of rat cecal content. The results showed that more than 89% of parent drugs (methotrexate and gemcitabine) are released from polymeric backbone of polyphosphazene drug conjugates (14 and 18) having n-butanol (lipophilic moiety). The in-vitro cytotoxicity assay has also been performed which clearly indicated that these polymeric drug conjugates are active against human colorectal cancer cell lines (HT-29 and COLO 320 DM). The drug release kinetic study demonstrated that Higuchi's equation is found to be best fitted equation which showed that release of drug from polymeric backbone as square root of time dependent process based on non-fickian diffusion. Therefore, the synthesized polyphosphazene azo based drug conjugates of methotrexate and gemcitabine are the potential candidates for colon targeted drug delivery system with minimal undesirable side effects.

Poly(ethylene glycol)-poly(lactic-co-glycolic acid) based thermosensitive injectable hydrogels for biomedical applications

Stimuli triggered polymers provide a variety of applications related with the biomedical fields. Among various stimuli triggered mechanisms, thermoresponsive mechanisms have been extensively investigated, as they are relatively more convenient and effective stimuli for biomedical applications. In a contemporary approach for achieving the sustained action of proteins, peptides and bioactives, injectable depots and implants have always remained the thrust areas of research. In the same series, Poloxamer based thermogelling copolymers have their own limitations regarding biodegradability. Thus, there is a need to have an alternative biomaterial for the formulation of injectable hydrogel, which must remain biocompatible along with safety and efficacy. In the same context, poly(ethylene glycol) (PEG) based copolymers play a crucial role as a biomedical material for biomedical applications, because of their biocompatibility, biodegradability, thermosensitivity and easy controlled characters. This review stresses on the physicochemical property, stability and composition prospects of smart PEG/poly(lactic-co-glycolic acid) (PLGA) based thermoresponsive injectable hydrogels, recently utilized for biomedical applications. The manuscript also highlights the synthesis scheme and stability characteristics of these copolymers, which will surely help the researchers working in the same area. We have also emphasized the applied use of these smart copolymers along with their formulation problems, which could help in understanding the possible modifications related with these, to overcome their inherent associated limitations.

Temperature-modulated noncovalent interaction controllable complex for the long-term delivery of etanercept to treat rheumatoid arthritis

The clinical applications of etanercept (Enbrel), an emerging therapeutic protein for rheumatoid arthritis (RA), are limited by its instability and low bioavailability. In this study, a long-term and efficient therapeutic nanocomplex formulation for RA treatment was developed in the form of a temperature-modulated noncovalent interaction controllable (TMN) complex based on a temperature-sensitive amphiphilic polyelectrolyte (succinylated pullulan-g-oligo(L-lactide); SPL). The TMN complexes were prepared by simply mixing the negatively charged SPL copolymer and the positively charged etanercept via electrostatic interaction at 4 °C below the polymer's clouding temperature (CT), and the resulting complex demonstrated significantly improved salt and serum stability with increased hydrophobic interactions at temperatures (physiological condition, 37.5 °C) above the CT. An in vitro study of the bioactivity of etanercept indicated that the TMN complex improves the long-term stability of etanercept in an aqueous environment because of the exposure of the functional active site and the molecular chaperone-like effect of the hydrophobic copolymer. This formulation possessed prolonged in vivo pharmacokinetic parameters. In a collagen-induced arthritis RA rat model, we verified the outstanding therapeutic effect of the TMN complexes. These results imply that this approach would be widely applied to protein and peptide delivery systems.

Thermoresponsive hydrogels for cellular delivery

The delivery of living cells into a host body has emerged as a promising approach to treating a variety of different diseases and for tissue repair. However, one of the major obstacles for clinical success is to deliver the cells to the target tissue without losing control of cell fate and function after transplantation. Temperature-responsive biomaterials represent a promising vehicle to deliver cells noninvasively by injection of a liquid precursor, which undergoes a reversible phase transition at body temperature, thus, forming temperature-induced hydrogels in situ. The final material provides transplanted cells with a synthetic extracellular matrix, which retains the cells at the injection site, supports cell growth and mitigates migration. This mini review is intended to cover the fundamental physicochemical characteristics of these thermoresponsive biomaterials, and to examine the applications, with a focus on the recently developed cell-delivery systems for tissue engineering and cell therapy, including advantages, limitations and future challenges.

New directions in thermoresponsive polymers

Interest in thermoresponsive polymers has steadily grown over many decades, and a great deal of work has been dedicated to developing temperature sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of previously reported temperature-responsive polymers are based on poly(N-isopropylacrylamide) (PNIPAM), despite the fact that a wide range of other thermoresponsive polymers have demonstrated similar promise for the preparation of adaptive materials. Herein, we aim to highlight recent results that involve thermoresponsive systems that have not yet been as fully considered. Many of these (co)polymers represent clear opportunities for advancements in emerging biomedical and materials fields due to their increased biocompatibility and tuneable response. By highlighting recent examples of newly developed thermoresponsive polymer systems, we hope to promote the development of new generations of smart materials.

Polyphosphazenes: Multifunctional, Biodegradable Vehicles for Drug and Gene Delivery

Poly[(organo)phosphazenes] are a unique class of extremely versatile polymers with a range of applications including tissue engineering and drug delivery, as hydrogels, shape memory polymers and as stimuli responsive materials. This review aims to divulge the basic principles of designing polyphosphazenes for drug and gene delivery and portray the huge potential of these extremely versatile materials for such applications. Polyphosphazenes offer a number of distinct advantages as carriers for bioconjugates; alongside their completely degradable backbone, to non-toxic degradation products, they possess an inherently and uniquely high functionality and, thanks to recent advances in their polymer chemistry, can be prepared with controlled molecular weights and narrow polydispersities, as well as self-assembled supra-molecular structures. Importantly, the rate of degradation/hydrolysis of the polymers can be carefully tuned to suit the desired application. In this review we detail the recent developments in the chemistry of polyphosphazenes, relevant to drug and gene delivery and describe recent investigations into their application in this field.

Injectable and biodegradable hydrogels: gelation, biodegradation and biomedical applications

Injectable hydrogels with biodegradability have in situ formability which in vitro/in vivo allows an effective and homogeneous encapsulation of drugs/cells, and convenient in vivo surgical operation in a minimally invasive way, causing smaller scar size and less pain for patients. Therefore, they have found a variety of biomedical applications, such as drug delivery, cell encapsulation, and tissue engineering. This critical review systematically summarizes the recent progresses on biodegradable and injectable hydrogels fabricated from natural polymers (chitosan, hyaluronic acid, alginates, gelatin, heparin, chondroitin sulfate, etc.) and biodegradable synthetic polymers (polypeptides, polyesters, polyphosphazenes, etc.). The review includes the novel naturally based hydrogels with high potential for biomedical applications developed in the past five years which integrate the excellent biocompatibility of natural polymers/synthetic polypeptides with structural controllability via chemical modification. The gelation and biodegradation which are two key factors to affect the cell fate or drug delivery are highlighted. A brief outlook on the future of injectable and biodegradable hydrogels is also presented (326 references).

Polyphosphazenes containing lactic acid ester and methoxyethoxyethoxy side groups — Thermosensitive properties and, in vitro degradation, and biocompatibility

The thermosensitive and hydrolytic properties and biocompatibility of polyphosphazenes containing lactic acid ester and methoxyethoxyethoxy as co-substitutents were investigated. Depending on the type of lactic acid ester, these polyphosphazenes exhibited lower critical solution temperatures (LCST) (from 33 to 52 °C), which were almost concentration-independent in the range of 1.5 to 15 wt% of the polymers in aqueous solution. The salt effect on the thermosensitivity of the polymers was studied by measuring their LCST in aqueous solutions containing various salts. Bu4NBr and KI showed salting-in effects among the tested six salts, but NH4Br, NaBr, NH4Cl, and NaCl showed salting-out effects. Hydrolysis studies showed the rate of polymer hydrolysis decreased in the order of basic > acidic > neutral solution. The polyphosphazene with bulkier and more hydrophobic ester groups was more stable in hydrolysis. The results of a cytotoxicity study using an MTT assay method with HepG2 cell and K562/VCR cell showed that these polyphosphazenes and their degradation products were biocompatible. The thermoresponsiveness and biocompatibility of these biodegradable polyphosphazenes may favor the polymers as potential stimuli-responsive materials in biomedical applications.

Cyclotriphosphazene-Pt-DACH Conjugates with Dipeptide Spacers for Drug Delivery Systems

An intelligent drug delivery system that exhibits a thermosensitive phase transition from a soluble to insoluble state was developed to deliver drugs into a tumor. Cyclotriphosphazene-Pt-DACH ( trans(±)-1,2-diaminocyclohexane) conjugates with dipeptide spacers were synthesized by successive reaction steps. The conjugates exhibited a reversible and thermosensitive phase transition with lower critical solution temperature in aqueous media that was varied by the substitution of different hydrophilic/hydrophobic side groups. The conjugate had similar cytotoxicity as cisplatin, but much greater than carboplatin against various human cancer cell lines. The conjugate was much more potent than cisplatin against the cisplatin-resistant cell line, A2780/CP70. The biodistribution and pharmacokinetic studies indicate that the present thermosensitive conjugate exhibits locally controlled drug delivery feature that could circumvent surgery treatment and reduce toxic side effects due to systemic administration.