Chapter 1: Thermogelling Polymers and Their History

The evolution of the biomedical industry to cope with intricate medical problems has always relied on advances in biomaterials (Figure 1.1).¹  Primary among the properties required of these materials was bioinertness, meaning that they could perform largely mechanical functions well with minimal interaction with the host systems, thus minimizing the chance of biological rejection. However, later developments have seen the phasing out of inert and unresponsive materials, bringing about the era of bioactive materials that can interface with the physiological environment to elicit appropriate biological responses (Figure 1.2).

As biomaterials ascend in increasing sophistication, they have become more responsive to external stimuli. This has resulted in a new class of “smart” biomaterials: biomaterials that respond to changes in environmental pH, temperature, light and more.^2,3  These materials, also known as stimuli-responsive, environmentally-sensitive or intelligent materials, exhibit an observable change in their properties upon the stimulus. Such changes include modification of their shape, solubility, surface characteristics and the ability to self-assemble or undergo sol-to-gel phase transition (Figure 1.2).³ 

Smart thermogelling materials possess both high water content and the tunable properties of hydrogels, with the ability to respond to external temperature changes with a simple, physical and reversible sol-to-gel phase transition.^4–6  Thermogels have been proposed for many uses including drug delivery, gene delivery and scaffolding for tissue engineering.^7–11  The thermogelling Pluronic systems have been an important contributor in the biomedical arena since their introduction about 70 years ago. Also known by their non-proprietary name, poloxamers, Pluronics were first brought onto the 1950s commercial scene by BASF, and have now gained increased industrial footing, being sold by various companies under a variety of other tradenames, such as Kolliphor. First introduced as surface active agents (surfactants), Pluronics, which were under the category of non-ionic triblock copolymers, are characterized by a central hydrophobic propylene oxide block in between hydrophilic ethylene oxide blocks. By arranging the water-soluble hydrophilic blocks and water-insoluble hydrophobic blocks, the molecular weights and the ratio of the weight of the hydrophilic to hydrophobic blocks can be tuned, allowing for the synthesis of a family of surfactants with systematically varying physical properties and similar chemical properties to ether alcohols. This thus allowed for their optimization in a variety of different biomedical applications.¹² 

Compared with other well-known surfactants, such as ethoxylated fatty alcohols or alkyl phenols, Pluronics can be differentiated by four characteristics. Firstly, Pluronics have much higher molecular weights than other surfactants, reaching a range of 1000 to 15 000. Secondly, their triblock structure possesses two hydrophiles, unlike the single hydrophile found in most non-ionic surfactants. The existence of ether oxygen atoms in both their hydrophobic and hydrophilic blocks also allowed Pluronics to form hydrogen bonds in their hydrophobic segments, which is not a feature of ethoxylated fatty alcohols or alkyl phenols. Finally, these copolymers exhibit a micellar arrangement in aqueous solution that is different from the hydrated spherical formations observed in other non-ionic surfactants.¹² 

These unique structural, physical and thermos-responsive properties of thermogelling Pluronics have allowed them to remain very relevant in the biomedical sector decades after their introduction. In this chapter, we aim to examine the Pluronics systems in greater depth and detail their evolution as biomaterials.

The synthetic procedure for making Pluronic copolymers has been detailed by their inventor, Irving Schmolka. It first involves the formation of a polyoxypropylene glycol hydrophobe (minimum molecular weight of 900), by adding propylene oxide to a propylene glycol initiator in the presence of an alkaline catalyst. This must be conducted in an inert and anhydrous environment, under elevated pressure and temperature. Following the complete reaction of all the propylene oxide, two polyoxyethylene blocks, which serve as the hydrophilic segments, are formed through the controlled addition of ethylene oxide. The mixture is then neutralized by adding phosphoric acid or other inorganic or organic acids.¹³  The above process results in the characteristic structure of the Pluronic copolymer which can be represented as:¹³ 

Where a is statistically equal to c, and adds up to 10–90% of the total polymeric weight, and b is at least 15.¹² 

Within the range of possible thermoresponsive properties, one of the most highly researched examples is gelation behaviour. This is due to the fact that thermoresponsive gelation brings with it an entire host of potential applications, including minimally invasive in situ formation of gels, or triggered release of actives (Figure 1.3).

During gelation, a solution of macromolecules or particles that exists in a liquid phase as a flowing fluid can be converted into a solid with elastic properties that can maintain its structural integrity and remain non-flowing throughout the duration of the experimental time scale.^14,15  Conventional gelation behaviour can be observed in many systems, such as gelatin solutions, which gel upon being cooled down to a certain temperature. Typically, gelation occurs through the random increase of the number of either physical or chemical bonds that exist between the particles or molecules, which eventually results in the formation of a continuous connected network. In the case of physical bonds such as hydrogen bonds or Van der Waals forces, the gels formed can be re-converted back into their liquid state through modifications to the physical environment, such as through a change in temperature or pH. By contrast, chemical bonds are covalent in nature, and tend to result in permanent, non-reversible cross-linkage.¹⁴ 

The aqueous gelatin solution has been a commonly explored example that exhibits thermoreversible gelation behaviour. It is able to undergo this process at concentrations above 2–3% (w/w), at a gelation temperature of approximately 30 °C. Thus, the typical procedure for the fabrication of gelatin gels involves preparing a hot gelation solution at around 50 °C and thereafter allowing it to cool to about 0 °C. At the start of the process, the solvated gelatin chains predominantly exist in random coil conformations of either α-type or β-type chains. However, the formation of the gel arises from the intertwining of the gelatin chains through the creation of hydrogen bonds to form a triple-helix structure. The dynamics involved are currently thought to consist of three discrete steps: the formation of aggregates from monomers, the disorder-to-order transition of random coils to single helices, and the order-to-order transition of the single helix to triple helix transition.¹⁶ 

So far we have described the gelation behavior of conventional gels. However, thermogels, the class of hydrogels to which the Pluronics series belongs, are thought to be a separate category from these. Although these copolymer systems can transition from a solution to a gel phase by virtue of the supramolecular interactions of their polymeric blocks, they differ from conventional gels, as this happens during temperature elevation. Thus, these systems exist as aqueous polymeric solutions in a non-viscous fluid phase when temperatures are low, and, upon an increase in temperature, such as to body temperature, are able to form a solid hydrogel. This phenomenon, at first impression, seems to contradict conventional sense, whereby the elevation material temperature should result in a solid-to-liquid transition due to melting. However, this conversion, which is thermoreversible, occurs via an entirely different mechanism of polymeric chain hydration and dehydration.¹⁷ 

To begin to look into this mechanism, the general properties of thermogelling copolymers must first be examined. Thermogelling copolymers are known to be amphiphilic macromolecules with a delicate balance of hydrophobic and hydrophilic properties of their different segments. This balance allows for the solvation of the copolymers in an aqueous solution during low temperatures via the occurrence of hydrogen bonding between water and the copolymers. These hydrogen bonds, however, are weakened upon the introduction of increased thermal activity through heating, which causes random motion of the molecules. This thus provides the chance for the random association of the copolymers’ hydrophobic parts, which results in the formation of crosslink points in the form of nano-domains. Thus, at low concentrations with heated settings, the self-assembly of the copolymer chains results in their aggregation into micelles. This forms a corona of hydrophilic components and a core of hydrophobic components in the supramolecular structure. The above phenomenon, however, is not sufficient to induce gelation at low polymer concentration. An increase in polymer concentration brings about an increase in the number of micelles, which results in a denser packing within the solution. Only at the critical gelation concentration (CGC) are the micelles packed closely enough to cause the formation of a gel state. Thus, this is why the sol-to-gel phase transition of a thermogel is both temperature and concentration dependent.¹⁷ 

The popularity of the Pluronics series can be directly attributed to its capability for exhibiting this thermogelling behaviour through self-assembly upon a temperature change. Below a specific temperature and/or concentration, known respectively as the critical micelle concentration and temperature (CMC and CMT), the copolymers exist as unimers, which are unaggregated individual coils in an aqueous solution. This phase, known as the sol phase, is characterized by fluidic flow. The increase in temperature of the solution and/or copolymeric concentration allows for the formation of thermodynamically stable micelles. However, the mechanism of gelation in Pluronics systems has been a point of contention between researchers over the years. Conflicting results observed using ultracentrifugation or light-scattering techniques indicated the lack of micellization in Pluronic solutions, while dye-solubilization and surface tension techniques could effectively determine CMC values.¹⁸  Later studies featuring static-light-scattering techniques also reported a lack of aggregation, which was contradicted by micellization associations detected through photon correlation spectroscopy and viscosity measurements. It was in 1987 that a study suggested three different regions of the micellization process: a unimer region, an equilibrium region consisting of a mixture of unimers and micelles, and a micelle region, suggesting that early controversies were due to observations made in only one of the three regions.¹⁹  However, among the studies that managed to observe micellization, different gelation mechanisms were proposed by different groups. Studies reported in 1983 observed a decrease in the CMC with increasing temperature. This led the researchers to conclude that the gelation could be attributed to an intrinsic change in the properties of the micelle, such as in their aggregation number or symmetry. However, a year later, a similar study observed a change in the ¹³C nuclear magnetic resonance chemical shift and peak broadening of the poly(phenylene oxide) (PPO) methyl group at the transition temperature, concluding that this was the result of dehydration of PPO from the existing micelles, which caused increased amounts of friction between the chains of copolymers, resulting in an increased solution viscosity and formation of a gel phase.¹⁵  In the same year, further studies resulted in the proposition of an entropically driven gelation mechanism involving the increase in overall disorder upon the squeezing out of ordered water molecules caused by interactions between the hydrophobic segments of the copolymers.¹⁹  It was also later proposed that the gelation was driven by the dehydration of the polyethylene oxide segments upon temperature increase.¹⁵ 

Many varying theories culminated in a paper in 1995 that proposed that the gelation was driven by the reduced polarity of ethylene oxide and propylene oxide segments upon temperature increase, as well as the entropically favourable hydrophobic effect, which is described as a gain in entropy in the surrounding water upon the aggregation of the unimers.²⁰  A recent review thus proposed the following system. At a low temperature above the CMC, the copolymers begin to form micelles until they reach equilibrium with the unimers. Further temperature increase sees an increase in the volume fraction of the micelles due to the equilibrium shift towards the micelles, which correspondingly results in the reduction of unaggregated unimers. When the micelle volume fraction has increased beyond a certain limit (0.53), hard-sphere crystallization, or micelle packing, can occur and cause the system to become a gel.³ 

The kinetics behind block copolymer micelles were thought to be different from surfactant micelles and observed through experiments that looked at the dynamics at equilibrium and dynamics of micellization. From these experiments, dynamics at equilibrium were attributed to an insertion-expulsion mechanism based on randomization kinetics that were independent of the concentration of polymer. By contrast, the dynamics of micellization were similar to surfactant micelle kinetics and thought to exhibit two different processes, namely a fast and a slow process. The former process was associated with the insertion of free copolymers into existing micelles, resulting in the aggregation of metastable micelles, while the latter was associated with either insertion-expulsion or fusion-fragmentation.²¹ 

As mentioned in the introduction, micelles associated with Pluronic systems have the ability to form shapes different from the conventionally observed spherical shapes. In particular, there have been observations of sphere to rod micelle transitions attributed to the random fusion and fragmentation of the micelles instead of an ordered successive addition of micelle spheres to rod-like micelles.²¹  This difference in aggregate morphology can affect their suitability for different applications and thus further expands the range of purposes Pluronic thermogels can serve. For example, the potential to form vesicular and spherical micelles is often associated with drug delivery matrices, whereas worm-like micelles are applicable to areas such as oil extraction (Figure 1.4).²² 

Pluronics in early medical research manifested in a variety of different end applications. They were largely used for purposes related to issues with fat and blood, serving to prevent thrombolysis, diminish fat emboli and hemolysis during cardio-pulmonary bypasses, reducing platelet adhesiveness and blood viscosity and to emulsify fat.²³  While these purposes were limited to the copolymers themselves, and not in view of their capabilities as stimuli-responsive materials, they formed a foundation of research that established the Pluronics family as almost nontoxic materials (Figure 1.5).

The first notable entry of Pluronic thermogelling systems into the biomedical arena was proposed by their original inventor. In his paper, Irving Schmolka detailed the potential for Pluronic F-127 to be made into an “artificial skin” suitable for the healing of burns by incorporating silver salts and other medicines into cold aqueous F-127 solutions before increasing the temperature and allowing the gels to form. These gels could be useful for topical application onto burnt or abraded skin. In determining their suitability for such purposes, Schmolka examined the toxicological properties of the Pluronics and concluded that they ranged from having very low toxicity at a very low molecular weight to being completely nontoxic at a higher molecular weight and concluded that there should be no hazardous effects if these systems were used for this application.²³ 

Several other studies have since explored the possibility of Pluronic systems as candidates for wound healing applications. The use of Pluronic F-127 in the standardized treatment of third-degree burns as a skin substitute was investigated. This study observed enhanced rates of healing at burn sites where the F-127, with propylene glycol added as a humectant, was applied as a reversible sol to gel “bandage” and investigated the potential for the addition of bacteriostatic or bactericidal agents to delay infection.²⁴ 

Later revelations with respect to wound healing indicated it to be a complex, localized process that involves many aspects, notably inflammation, wound cell mitosis and migration, and neovascularization, as well as the regeneration of the extracellular matrix. This led to an interest in improving wound healing systems beyond anti-infection, moving towards promoting wound healing. Studies thus began to look at Pluronic gel encapsulation of various growth factors that were thought to play significant roles in wound healing.²⁵  Encapsulation of growth factors such as epidermal growth factors and transforming growth factors were investigated in several studies in the 1990s, and pointed towards the growing prominence of Pluronic thermogels as topical growth factor delivery systems (Figure 1.6).^25,26 

The beginning of the twenty-first century saw scientific research looking to fine-tune the properties of the Pluronic thermogels through mixtures of family members. Se et al. sought to develop a polymer solution that could exist as a gel coating on the injured surface to prevent postsurgical tissue adhesion. They combined Pluronic F-127 with Pluronic F-68. F-127 was notable for transitioning from a sol to gel state when heated to human physiological temperature, but was consequently difficult to handle at ambient temperatures because of this propensity for low-temperature gelling. On the other hand, F-68 only underwent transition at 40 °C and existed in sol state at physiological temperatures. Hence, the combination of the two at different ratios, together with mildly crosslinked alginate and ibuprofen resulted in Pluronic mixtures with controllable transition temperatures that could be applied in their liquid form to sites of injury, where they gelled and were stably maintained in that state.²⁷ 

The interest in Pluronic thermogels as wound healing systems has lasted well into current times, triggering the study of a Pluronic F-127 gel as a wound healer in itself. This work made use of the F-127 gel in normal saline solution without additional actives, and discovered that the mere topical application of these gels could enhance the healing of cutaneous wounds in rats, significantly increasing the rate of wound closure. This was observed along with the increased expressions of vascular endothelial growth factor (VEGF) and transforming growth factor-beta 1 (TGF-β₁), growth factors which are thought to be key players in the wound healing process, serving to recruit inflammatory cells and ultimately aid in in angiogenesis and granulation tissue formation. This thus led the authors to suggest that F-127 was mildly inflammatory in nature, and served to quicken wound healing by stimulating the expression of the aforementioned growth factors to take part during the inflammatory and proliferative stages of healing.²⁸ 

Although early research has already long advocated the use of Pluronic gels as candidates for wound healing, this was before the discovery that the systems in themselves could stimulate the quickening of the process. The systems were thus only thought of as advanced “bandages”, or vessels for the delivery of actives. However, the discovery of the inherent ability of Pluronic F-127 to aid in wound healing will lead to more investigations into the other members of the Pluronic family. This also brings about more reasons for the employment of the gels in such purposes, as their innate ability can be used synergistically with their other capabilities, such as encapsulation or preventing tissue adhesion, to bring about greater advances in this area.

The potential for micellization in Pluronic systems meant that they were capable of acting as encapsulants for drug delivery (Figure 1.4). The possibility of Pluronic systems incorporating other compounds had already been investigated early in their development when Schmolka successfully incorporated silver salts and other medicines into his gels. There is an overlap between drug delivery and wound healing methods, as the latter also seeks to deliver drugs and growth factors to accelerate the process of healing, and might hence be considered a subset of the former. The two areas, however, do diverge in several ways (Figure 1.7).

The introduction of medicine-holding Pluronic gel systems for burn healing sparked many investigations that followed largely similar paths. It also drew attention to the capabilities of these systems as drug carriers, beyond traditional dermal applications. Their micellar aggregations rendered them highly attractive as encapsulants, wherein their hydrophobic core could be used to hold a cargo of significant amounts (up to 20–30 wt.%) of water-insoluble encapsulates. At the same time, their hydrophilic corona allows for the micelles to exist in a dispersion, and thus reduces undesirable drug interactions with the biological system. It was thus thought that the incorporation of drugs into Pluronic systems could aid in increasing drug solubility and stability, and as a result bring about better drug pharmacokinetics and biodistribution. Furthermore, these systems offered advantages in specific applications such as anticancer chemotherapy, as the micelles enabled a passive drug-targeting route to tumors. This is due to the enhanced permeability and retention effect, which occurs due to the unusually high blood vessel permeability in tumors, as well as the extended circulation periods of the micelles within the body.²⁹  There was hence increasing amounts of focus being placed on Pluronics in the role of delivery vessels to meet a variety of medical needs.

The fabrication of an effective delivery system requires the understanding of the encapsulant-encapsulate behaviour to tune their properties as desired. Studies thus began to look into the effects solutes had on the Pluronic systems to better understand their behaviour in these interactions. Gilbert et al., for example, inversely correlated solute diffusion coefficient with the concentration of Pluronic F-127 that could be described by an exponential equation: y=7213 exp(−0.39 x)+1.84 (r=0.984). They proposed that this result could be explained by an increased micelle size and number with increasing F-127 concentration, constricting the water channel sizes and increasing the path of diffusion.³⁰ 

Further understanding of the drug delivery thermogel systems was achieved by a study highlighting the effect solutes and polymers had on F-127's gelation properties. This investigation determined that the addition of solutes resulted in a concentration-dependent decrease in gelation temperature. Moreover, the inclusion of esters to the polymeric solution also decreased the gelation temperature possibly by binding to the polymeric chains, resulting in increased dehydration and hence increased micelle entanglement.³¹ 

The effect of salts and electrolytes on the micellization behaviour of the systems was also investigated. Urea addition increased the CMC and decreased enthalpy of micellization, with increasing urea concentration.²⁰  Studies such as these formed the basis for understanding these gels as drug delivery depots for other potential applications. Shortly after Schmolka's publication, a patent detailing the use of Pluronics thermogels as pharmaceutical vehicles for delivering drugs to mucous membranes was filed. This patent was concerned with a persistent problem in the treatment of ocular diseases, whereby aqueous solutions made poor contact when applied to mucous membranes, and this contact was not therefore sustained.^32,33  Sustained release in ocular applications was difficult to achieve because solution-based dosage delivery could easily be diluted in the ocular tear film and spill over the lid margin.³⁴ 

Until a certain point in time, the polymeric carrier systems were used only as an inert tool that passively facilitated the delivery of actives, such as by offering protection, or sustained release. However, this notion changed upon the discovery that certain synthetic polymers could induce a change in specific cellular response by acting as biological response modifiers. The Pluronic systems were found to be capable of this, acting to provide other benefits, such as sensitizing multidrug-resistant cancer cells, or promoting the transport of drugs across cellular barriers. The research, however, concluded that these functions were attributed to the polymeric unimers that are able to integrate into and translocate across the cellular membranes rather than the micelles associated with the thermogelling systems.²⁹  A thermogelling drug delivery system that can make use of this to its advantage might thus have to establish a finely-tuned balance between micellar aggregations used for drug transport and the unimers to elicit an appropriate response.

Although there has been much research directed into Pluronic drug delivery systems starting from early in its introduction, many of the studies can still be considered topical, and while injectable drug delivery depots involving the Pluronic systems have been known since the 1970s, subcutaneous use of Pluronic thermogelling systems was only reported much later.

The reason behind the lack of subcutaneous applications for Pluronic systems despite their reported biocompatibility can be attributed to several of their properties. Firstly, the Pluronic systems were not biodegradable³⁵  and were thus unable to be eliminated from the body after their temporary use. This resulted in side effects from accumulation, such as the toxic enhancement of triglycerol and plasma cholesterol.³⁶  Secondly, Pluronic gel systems demonstrated poor mechanical properties, and were easily eroded after administration.³⁵  It was found that the gel formed could not be stably maintained in a subcutaneous environment for more than a day, and hence limited its applications to short-term systems.²⁰  Later research efforts that were directed to designing biodegradable thermogelling systems with more suitable mechanical properties then paved the way for the Pluronic systems to be used in implant systems, which offered the benefits of minimally invasive entry via injection.

The disadvantages of Pluronic thermogels have necessitated their modification to fully exploit their potential in therapeutic devices. Research has seen the emergence of many Pluronic based systems that offer biodegradability and/or improved mechanical properties compared to the original gel systems.

With regards to achieving more desirable mechanical properties, many studies were aimed at allowing for the prevention of the rapid dissolution of Pluronic gels upon implantation. This instability of the hydrogels was thought to be attributed to the exposure of the gels to large solution volumes, which resulted in the immediate dilution of the polymer concentration and the consequent deterioration of gel structure and integrity (Figure 1.8).³⁷ 

The use of chemical crosslinks was thus one of the methods employed in combating this phenomenon. In a study by Chun et al., hydrogels were formed through the photo-crosslinking of di-acrylated Pluronic macromers through UV irradiation. Prolonging the length of UV irradiation resulted in higher amounts of crosslinks within the gel, and allowed for a lower rate of degradation, which occurred through the cleavage of an ester linkage in the polymerized site. The effects of the increased crosslinking could also be observed in the decreased swelling of the gels. Furthermore, the improvement in mechanical properties of the gels could be observed through the increase in dynamic moduli values, which also increased with increased UV irradiation, compared to those of physical Pluronic hydrogels (Figure 1.9).³⁷ 

In another study making use of photo-crosslinking, supramolecular hydrogels that possessed highly elastic as well as thermoresponsive properties were developed through the use of a synthesized Pluronic F-68/poly(ε-caprolactone) block copolymer end-capped with acryloyl groups as a macromer with aqueous α-cyclodextrin (α-CD) for inclusion complexation. This produced a physical hydrogel precursor with thermosensitive properties that could be tuned to the varying molar feed ratio of the two components. Upon in situ UV photo-crosslinking, the hydrogels displayed markedly improved viscoelastic properties with high elastic moduli.³⁸ 

Apart from photo-crosslinking, studies have also delved into the introduction of stereocomplexed crystalline domains within hydrogel structures to render them more physically stable. This was achieved through the inclusion of monomers such as poly(lactic acid) which possesses a chiral carbon atom and thus exhibits stereocomplexed crystalline behaviour. By linking multi-block Pluronic copolymers with d-lactide and l-lactide oligomers, stereocomplexed hydrogels capable of in situ formation could be formed through the mixing of the two enantiomeric copolymers. These hydrogels retained the temperature-responsiveness of Pluronic gel systems, and were shown to demonstrate increased mechanical strength through rheological assessments.³⁹ 

Improvement of Pluronic gel mechanical properties have also been reported by simply using it in blends with other polymers such as Carbopol or alginate. Studies observing a marked increase in gel strength when Pluronic was mixed with alginate attributed this to the formation of crosslinks between the polymers by virtue of water molecules acting as crosslinking agents and allowing for the formation of hydrogen bonds between the alginate carboxyl groups and the Pluronic ether groups.⁴⁰ 

The improvement of Pluronic gel stability and mechanical properties is crucial in anchoring its role in the biomedical industry. Without the ability to offer extended usage, the applicability of Pluronic systems in many biomedical issues that require long-term effects, such as in drug and gene delivery and tissue engineering, will be severely restricted.

Another property naturally lacking in Pluronic systems that is of utmost importance in the biomedical scene is biodegradability. Despite the rapid dissolution of the gel systems due to their instability, Pluronic systems cannot be processed, degraded and eliminated by the body. There have thus been numerous efforts to combat this problem in a variety of ways (Figure 1.10).

Early in 2002, it was shown that Pluronics could be employed as the hydrophilic segments in polyurethanes, with poly(ε-caprolactone) diol as the hydrophobic segment, to achieve polyurethanes with different hydrophobic-to-hydrophilic segment ratios that could undergo in vitro degradation with tunable rates. This resulted in mass loss, a lowering in the molecular weight and a decrease in polydispersity, as well as a loss of mechanical strength with degradation. However, this work focused on the polyurethanes themselves, instead of a hydrogel system.⁴¹ 

A later study also performed in the early 2000s then saw the production of novel amphiphilic PLA-F127-PLA block copolymers through the grafting of poly(lactic acid) (PLA) onto both ends of Pluronic F-127. This resulted in the conferring of biodegradability to the hydrogel through hydrolytic degradation at two possible sites: in the middle of the PLA block and at the interface between the PLA and F-127 blocks. The copolymer described in this study, however, formed nanoparticles in PBS solutions, and did not maintain the stimuli-responsiveness of Pluronic gelling systems.⁴² 

Further studies such as the one conducted by Liu et al., which involved the development of amphiphilic PCL-Pluronic-PCL block copolymers through ring opening polymerization, saw greater success in retaining the thermosensitivity of the resulting gel systems while achieving biodegradability. Investigations made into the effects of varying the total molecular weight of the PCL block further revealed that the thermoresponsiveness and gelation requirements of the system could be tuned in such a way. By increasing that value, the team observed the lowering of the critical gel temperature, as well as the critical gel concentration, which effectively meant a wider temperature and concentration range for phase transition could be achieved to suit various purposes.⁴³ 

The stereocomplexed hydrogels described in the previous section, which made use of the linking of Pluronic copolymers by d-lactide and l-lactide oligomers to allow for improved mechanical properties through additional crosslinking brought about by their stereocomplex crystalline domains, were also proven to be biodegradable. This was made possible through hydrolytically cleaving the oligo(lactic acid) spacers that linked the copolymers. This system was proposed not only to achieve controlled protein delivery, but also for the possibility of subcutaneous and percutaneous delivery.³⁶ 

The constructs containing more F-127 displayed a considerably reduced size. As the amount of X-HA increased, the constructs carried a swollen morphology, seemingly due to the uptake of surrounding fluids. Only if an appropriate ratio was met could the composite hydrogel maintain its original size without a significant volume change.

The advent of tissue engineering, along with increased development of biodegradability and improved mechanical properties for Pluronic systems, brought about an increased interest in utilizing such systems as cell carriers and biocompatible scaffolds. Notably, Jung et al. demonstrated in 2010 the achievement of a composite hydrogel that made use of Pluronic F-127 derivatives and crosslinked hyaluronic acid with thermo-reversible gelling properties and biodegradability. Prior to this, studies investigating F-127 had shown it to have potential for providing an optimal 3D environment for cell growth and differentiation.⁴⁴  The Jung study, however, was novel in physically conjugating bioactive growth factors on to F-127 itself, instead of simply adding them in their free forms.⁴⁵ 

Although the use of Pluronics mainly as topical aids in the past has meant that biodegradability was a lesser concern compared with other properties, such as biocompatibility, growing demands to cope with today's needs are causing a rising importance in this property. The increasing research directed to achieving biodegradability has thus allowed Pluronic systems to be applicable in new areas.

The applications we have looked at so far involve the modification of Pluronics to counter general problems that are largely faced in their biomedical usage. However, there has also been a large body of research directed at the using Pluronic systems to cope with specific and targeted needs (Figure 1.11).

For one, 3D printing has been a rapidly developing field in recent years, finding its footing in various fields. Notably, it has been applied in tissue engineering, and Pluronic copolymers have been one of the materials involved in serving as a bioink due to their thermo-responsive and rheological properties. Furthermore, although it was found that the high gelation concentration required for Pluronics caused reduced long-term cell viability, it was possible to make use of a high concentration only during printing and subsequently removing and reducing the concentration to allow for high cell viability in the printed prototypes (Figure 1.12).⁴⁶ 

Pluronics have also been additionally studied for use in cell printing applications due to their bio-inertness towards many cell types, their range of viscosities that induce lower cell stress, and the ability for them to be easily rinsed and removed post-printing through exposure to a temperature below their critical gelation temperature. They have thus been utilized in the biofabrication of cellularized hydrogel scaffolds, in which cells are first dispersed in the solution, and an additive-manufacturing printer can induce sol-to-gel phase transition and subsequently perform a layer-by-layer extrusion of the scaffold based on a prior computer design. This will thus form a cellularized construct for tissue engineering purposes.⁴⁷ 

With the advancement in modern technology, the smart properties of Pluronic thermogels may find a new range of applications, as there could be an increasing demand on selective material properties for emerging specific purposes.

Pluronic thermogelling systems have experienced increased and sustained attention over recent years because of their tunable thermogelling properties and biocompatibility. As efforts are made to overcome their disadvantages, there will be an expansion of their range of applications. They will then play an increasingly significant role in drug delivery. Originally thought to be rather inert vessels for delivery, Pluronic systems seem to have the potential to play a nuanced role in controlled release.²⁹  In addition, recent studies have also sparked interest in their abilities to act as biological response modifiers, and in particular, aid in combating multidrug resistance in cancer therapy. This has placed Pluronic copolymers under the scrutiny of the nanomedicine lens, which is starting to play a more significant role in drug delivery. With this, their potential in areas such as tissue engineering and wound healing applications will be affected. However, a large portion of research seem to be looking into these properties independently of Pluronic systems’ other benefits. Given that these hydrogels also offer stimuli-responsiveness, it will be interesting to observe if all of these advantages will in future be combined synergistically to solve medical problems.²⁹