Thermosensitive sol?gel reversible hydrogels

Thermosensitive sol?gel reversible hydrogels

(Parte 1 de 3)

Thermosensitive sol–gel reversible hydrogels ab b,*Byeongmoon Jeong , Sung Wan Kim , You Han Bae aPacific Northwest National Laboratory (PNNL), 902 Battelle Blvd. P.O. Box 9, K2-4, Richland, WA 99352, USA bCenter for Controlled Chemical Delivery, Department of Pharmaceutics and Pharmaceutical Chemistry, University of Utah, Salt Lake City, UT 84112, USA

Received 23 July 2001; accepted 10 September 2001


Aqueous polymer solutions that are transformed into gels by changes in environmental conditions, such as temperature and pH, thus resulting in in situ hydrogel formation, have recently attracted the attention of many investigators for scientific interest and for practical biomedical or pharmaceutical applications. When the hydrogel is formed under physiological conditions and maintains its integrity for a desired period of time, the process may provide various advantages over conventional hydrogels. Because of the simplicity of pharmaceutical formulation by solution mixing, biocompatibility with biological systems, and convenient administration, the pharmaceutical and biomedical uses of the water-based sol–gel transition include solubilization of low-molecular-weight hydrophobic drugs, controlled release, labile biomacromolecule delivery, such as proteins and genes, cell immobilization, and tissue engineering. When the formed gel is proven to be biocompatible and biodegradable, producing non-toxic degradation products, it will provide further benefits for in vivo applications where degradation is desired. It is timely to summarize the polymeric systems that undergo sol–gel transitions, particularly due to temperature, with emphasis on the underlying transition mechanisms and potential delivery aspects. This review stresses the polymeric systems of natural or modified natural polymers, N-isopropylacrylamide copolymers, poly(ethylene oxide)/poly(propylene oxide) block copolymers, and poly(ethylene glycol)/poly(D,L-lactide-co-glycolide) block copolymers. Ó 2002 Elsevier Science B.V. All rights reserved.

Keywords: Aqueous polymer solution; Sol–gel transition; In situ hydrogel formation; Temperature; Drug delivery

1. Introduction38
2. Natural and modified natural polymers38
3. N-Isopropylacrylamide copolymers40
4. PEG/PPO block copolymers and related derivatives41
5. PEG/PLGA block copolymers43
6. Summary48


1. Introduction sol–gel transition condition [7]. When a small heavy ball resting on top of a solution (gel phase) begins to

Hydrogels preformed by chemical or physical penetrate into the gel under specific conditions, it can crosslinking are a special class of polymers that be regarded as a gel–sol transition, again being imbibe a considerable amount of water while main- dependent on the relative ball density compared to taining their shape. The research on hydrogels with the gel strength. When gelation is induced by temrespect to drug delivery and biomedical devices has perature, the endothermic peak during heating obbeen extensive over the last few decades because of tained from differential scanning calorimetry (DSC) their biocompatible properties and easy control of determines the transition temperature as well as the solute transport. One of the more recent trends in enthalpy of gelation [8]. Recently, a dynamic mehydrogel research is in situ hydrogel formation by chanical analysis was used to determine the sol–gel photopolymerization [1] or by phase transition [2,3]. transition in a more reproducible manner [9]. An In situ hydrogel formation makes it more feasible to abrupt change in the storage modulus or viscosity apply hydrogels for macromolecular drug delivery, reflects the sol–gel transition. tissue barriers, and tissue engineering. A particularly In this review, the sol-to-gel transition of aqueous interesting and important polymeric system is hydro- polymer solutions primarily induced by temperature gel forming solutions by a simple phase transition will be stressed, covering the natural or seminatural (sol–gel transition) in water without any chemical polymeric systems, N-isopropylacrylamide reaction or external stimulation. This system pro- (NiPAAM) copolymers, poly(ethylene glycol-b-provides simplicity and safety in in vivo situations. pylene glycol-b-ethylene glycol) (Poloxamer) and its

The sol phase is defined as a flowing fluid, analogs, and poly(ethylene glycol)/poly(D,L-lactic whereas the gel phase is non-flowing on an ex- acid-co-glycolic acid) block copolymers. perimental time scale, while maintaining its integrity. Above the critical concentration (critical gel concentration, CGC) of a polymer, the gel phase ap- 2. Natural and modified natural polymers pears. The CGC is most often inversely related to the molecular weight of the polymer employed. The Historically, natural biopolymer gels have been development of physical junctions in the system is used as food and food processing aids as well as in regarded as one of the prerequisites in determining pharmacy. Thermoreversible gelation has been regelation, which must be sufficiently strong with ported for gelatin (a protein prepared from the partial respect to the entropically driven dissolving forces of hydrolysis of collagen and containing proline, the solvent. The gelation of organic or aqueous glycine, and hydroxy proline as its major amino polymer solutions occurs by various mechanisms that acids) and polysaccharides such as agarose (extracted have been reviewed extensively and summarized from red sea weed; alternating copolymer of 1,4- [4,5]. linked 3,6-anhydro-a-L-galactose and 1,3-linked b-D-

The determination of the boundary between the sol galactose), amylose (a 1,4-linked a-D-glucan linear and gel phases depends on the experimental method. polymer), and amylopectin (a-1,6 glucan with a large A simple test-tube inverting method was employed number of a-1,4-glucan branches), cellulose derivato roughly determine the phase boundary [6]. When a tives, carrageenans (extracted from red sea weed; test tube containing a solution is tilted, it is defined alternating copolymers of 1,4-linked a-D-galactose as a sol phase if the solution deforms by flow, or a and 1,3-linked b-galactose, containing ester sulfate), Ògel phase if there is no flow. The flow is a function and Gellan (from bacteria; a polysaccharide conof time, tilting rate, amount of solution, and the sisting of b-glucose-b-D-glucuronic acid-b-glucosediameter of the test tube. Considering the time– a-L-rhamnose) [10–14]. All of these biopolymers temperature superposition principle in polymer de- form gels in water rather than organic solvents. formation, the test parameters should be fixed before Renaturation to the triple helical conformation in determining the sol–gel boundary. The falling ball gelatin and double helical conformation in polysacmethod is another simple way to determine the charides drives the nucleation and growth of crys-

Fig. 1. Gelation mechanism of polysaccharides in water. Random coils become helices, which subsequently aggregate to form the junction zones of a gel.

tallites during gel formation [15]. Helix formation An interesting reverse thermogelation of a combifollowed by aggregation of the helices results in a nation of chitosan and glycerol phosphate disodium junction point (Fig. 1). At high temperatures, they salt was reported by Chenite et al. [18]. A typical are assumed to have a random coil conformation. On solution was obtained by mixing a chitosan (91% reducing the temperature, they start to form double deacetylation) solution (200 mg in 9 ml HCl solution helices and aggregates that act as knots, i.e. the [0.1 M]) and a glycerophosphate disodium salt physical junctions of the gels. solution (560 mg in 1 ml distilled water). At neutral

Most natural polymers form a gel phase on pH, the formulation was a homogeneous, clear liquid lowering the temperature. However, aqueous solu- at room temperature and became a gel in the vicinity tions of some cellulose derivatives exhibit reverse of 378C. The gelation temperature increased with a thermogelation (gelation at elevated temperatures). decrease in the degree of deacetylation of the Cellulose is not soluble in water, but, by introducing polymer, but was not significantly influenced by the hydrophilic moieties, cellulose derivatives become molecular weight of the chitosan. The primary water soluble. When cellulose derivatives have an gelation force was believed to be a hydrophobic optimum balance of hydrophilic and hydrophobic interaction of neutral chitosan molecules, which can moieties, they undergo a sol-to-gel transition in be enhanced by the structuring action of glycerol on water. The sol–gel transition temperature depends on water at elevated temperatures. The gel was capable the substitution of cellulose at the hydroxy group of maintaining the bioactivity of loaded bone protein [16]. Water becomes a poorer solvent with increasing (BP, an osteogenic mixture of TGFb family memtemperature and polymer–polymer interactions be- bers) and released BP. The viability of various cells, come dominant at higher temperatures, resulting in a including chondrocytes, entrapped in the gel was gel [17]. Methyl cellulose and hydroxypropyl cellu- .80%. When the chondrocytes in the gel were lose are typical examples. implanted in an animal model, remodeling chon- drocytes secreting a matrix characteristic of normal above 328C (LCST). Below the LCST, the enthalpy cartilage was observed after 3 weeks. term, which is mostly contributed by the hydrogen bonding between polymer polar groups and water molecules, leads to dissolution of the polymer. 3. N-Isopropylacrylamide copolymers Above the LCST, the entropy term (hydrophobic interactions) dominates, resulting in precipitation of

Polymer precipitation in solution on raising the the polymer in water. The shift of C–H stretching temperature often occurs in aqueous systems and band can be observed on the nano scale by an atomic results from the balance of intermolecular forces force microscope (AFM) [25]. This is caused by the between the polymer and the solvent as well as dehydration of the hydrophobic isopropyl groups between polymers. Table 1 shows some examples of during the coil-to-globule transition. However, after polymers showing a low critical solution temperature precipitation, most (83%) of the carbonyl groups of (LCST) in water. NiPAAM still form hydrogen bonds with water

N-Isopropylacrylamide homopolymer (poly- molecules [26]. The LCST of NiPAAM polymers

(NiPAAM); Fig. 2) and its copolymers are most can be controlled by copolymerizing with other often investigated for the structure–property relation- monomers with different hydrophobicity [27]. The ship [19,20], drug delivery [21], tissue engineering more hydrophobic the comonomer, the lower the [2], and enzyme or protein modification [23,24]. resulting LCST. By controlling the polymer topolo-

An aqueous poly(NiPAAM) solution precipitates gy, the kinetics of the coil-to-globule transition can also be controlled. NiPAAM copolymers grafted with oligoNiPAAM [28] or PEG [29] show a fast response

Table 1 to temperature changes. The grafted short chain ofPolymers showing a LCST in water oligoNiPAAM in the former case contributes to rapid

Polymer LCST (8C) dehydration while PEG provides the water channel Poly(N-isopropylacrylamide), PNIPAM |32 for fast rehydration.

Poly(vinyl methyl ether), PVME |40 It was found that an aqueous solution of high-Poly(ethylene glycol), PEG |120 molecular-weight NiPAAM/acrylic acid (2–5 mol%)Poly(propylene glycol), PPG |50 copolymer synthesized in benzene showed reversiblePoly(methacrylic acid), PMAA |75 gelation above a critical concentration (| 4 wt%),Poly(vinyl alcohol), PVA |125

Poly(vinyl methyl oxazolidone), PVMO |65 without noticeable hysteresis around 328C, rather

Poly(vinyl pyrrolidone), PVP |160 than polymer precipitation [30]. The polymers werePoly(silamine) |37 characterized as having a distribution of polymerMethylcellulose, MC |80 composition. Gelation was attributed to polymerHydroxypropylcellulose, HPC |5 chain entanglements and the weak physical associa-Polyphosphazene derivatives 3–100

Poly(N-vinylcaprolactam) |30 tion of polymer precipitates with fewer ionizable

Poly(siloxyethylene glycol) 10–60 groups at lower temperatures while maintaining hydration by more charged and expanded polymer strands. This resulted in an opaque, loose gel that was deformable under shear stress. It was proposed that such properties could be used for the design of a refillable macrocapsule-type biohybrid artificial pancreas. Isolated islets of Langerhans suspended in the polymer solution were effectively entrapped in the gel when the solution temperature was raised from 258C to body temperature, and the gel showed no cytotoxicity. Another advantage of the gel was the significantly higher permeability of insulin secreted Fig. 2. The chemical structure of poly(N-isopropylacrylamide). from entrapped islets, because of the gel’s heteroge-

neous character, rather than a traditional cell-entrap- Pluronic (BASF) or Poloxamer (ICI)) series with ping matrix of alginate [31]. various molecular weights and PEG/PPO block

Chondrocytes immobilized in a thermoreversible ratios was used as a non-ionic surfactant, and the

NiPAAM/acrylic acid copolymer gel demonstrated aqueous solutions of some Poloxamers exhibited better phenotype expression with a round shape than phase transitions from sol to gel (low temperature that cultured in a two-dimensional matrix (culture sol–gel boundary) and from gel to sol (high temdish) [32]. perature gel–sol boundary) as the temperature in-

Poly(NiPAAM)/poly(ethylene glycol) copolymers creased monotonically when the polymer concenwith various architectures have been investigated, tration was above a critical value. The physicosuch as poly(ethylene glycol)-b-poly(NiPAAM)-b- chemical characteristics and applications of Poloxpoly(ethylene glycol) triblock copolymers and poly- amers were reviewed extensively by Alexandridis (NiPAAM)-g-PEG copolymers, which form ther- and Hatton [35]. Continuously heating the gel phase moreversible micelles [3]. More recently, diblock above the high temperature boundary produces an and star-shaped block copolymers AB, A(B) , opaque solution. The polymer exists in the gel form2

A(B) , and A(B) , where A is the central hydro- only between two critical transition temperatures,48 philic star-shaped PEG block (molecular weight that vary with polymer composition and concen-

(MW) per arm 2000–2460) and B is the tem- tration. perature-responsive NiPAAM oligomer block (MW The gelation mechanism of Poloxamer aqueous 1900–2400), have been synthesized. These were solutions remains a controversial issue. In the early reported to form a somewhat viscoelastic gel upon 1980s, an intrinsic change in the micellar properties, heating (gelation temperature 26–338C) when the such as aggregation number and micellar symmetry, typical polymer concentration was .20 wt%, and was thought to cause aqueous Poloxamer solutions to the resulting gels showed no syneresis [34]. This form a gel. This was based on the observation of a process was reversible without hysteresis. Based on decrease in the critical micelle concentration with differential scanning calorimetry (DSC) and dynamic increasing temperature [36]. A little later, the dehymechanical analysis, the gelation mechanism was dration of poly(propylene oxide) (PPO) was proobserved to be micellar aggregation for the AB posed as a cause of the gelation of aqueous Polox- 13diblock copolymer. It was found to be a strong amer solutions based on a C-NMR study [37]. The associative network formation for the other polymer change in the chemical shift and peak broadening of architectures via hydrophobic interaction of col- the PPO methyl group at the transition temperature lapsed NiPAAM oligomer blocks. The polymer was interpreted as the dehydration of PPO from the architecture influenced the resulting gel strengths and existing micelles. This resulted in increasing friction

A(B) showed the highest gel strength of 860 Pa between the polymer chains followed by viscosity of4 yield stress. the solution, resulting in the gel phase.Vadnere et al.

claimed that the entropy change caused by locally ordered water around the core PPO drove the sol-to- 4. PEG/PPO block copolymers and related gel transition [38]. This is a traditional view of derivatives hydrophobic interactions. They suggested a two-state model for (O–C–C–O) groups, assuming polar

The commercial poly(ethylene oxide-b-propylene (gauche) and nonpolar (anti) states. With increasing oxide-b-ethylene oxide) (PEO–PPO–PEO, Fig. 3; temperature, the population of the nonpolar antiform increases, driving gel formation. Zhou and Chu observed the dehydration of poly(ethylene oxide) (PEO) with increasing temperature. This dehydration was thought to drive the gelation of aqueous Polox-

amer solutions [39]. Attwood et al. observed that theFig. 3. The chemical structure of poly(ethylene oxide-co-prohydrodynamic radii of Poloxamer micelles werepylene oxide-co-polyethylene oxide) (PEO–PPO–PEO) (Poloxconstant, while the aggregation number and volumeamer or Pluronic).

fraction of the micelles increased with increasing (acrylic acid)-g-Poloxamer by coupling monoaminetemperature [40]. Recently, mechanistic studies on terminated Poloxamer to poly(acrylic acid) (PAA) the phase transition and characterization of the using dicyclohexyl carbodiimide [53]. This polymer solution and gel states of Poloxamers were reported showed bioadhesive as well as thermosensitive gelusing various instrumental techniques, such as ultra- ling properties. Bromberg developed Poloxamer-gsound velocity, dynamic and static light scattering, PAA via a C–C bond [54–59]. Simple radical small angle neutron scattering (SANS), rheometry, polymerization of acrylic acid in the presence of dielectric constant measurement, and mi- Poloxamer gave the graft copolymer by chain transcrocalorimetry [41–4]. fer reactions. The transition temperature of the

Taken together, triblock copolymers form micelles resulting Poloxamer-g-PAA (M |400,0) aqueousn which equilibrate with Poloxamer unimers at low solution (1% w/v) was 358C when the Poloxamer temperature above the critical micelle concentration F127/PAA weight ratio was 0.5:1.0. The transition (CMC), about 1 mg/ml [45]. As the temperature temperature was controlled between 10 and 308C increases, the equilibrium shifts from unimers to over the concentration range 0.2–2.5 wt%. Comspherical micelles, reducing the number of unas- pared with Poloxamer (CGC .20 wt%), this graft sociated unimers in solution, leading to an increase copolymer showed a gel phase at much lower in the micelle volume fraction (f ). When f . concentrations. This trend implies that the high-m 0.53, the system becomes a gel by micelle packing molecular-weight Poloxamer-g-PAA easily forms

(hard-sphere crystallization) [46,47]. The hard-sphere physical crosslinking junctions in water; however, interaction radius increases steadily with tempera- the sol–gel transition temperature and onset of ture, while the micelle volume fraction increases micellization were observed at similar temperatures, abruptly in a certain temperature range, which is indicating a similar mechanism of gelation with an dependent upon the concentration. When the volume aqueous Poloxamers solution (Fig. 4). The Poloxfraction of the micelles is .0.53, the system amer-g-PAA system contains pH-sensitive functional becomes a gel. groups. When compared with PAA, the carboxylic

The transition from gel to sol at high temperatures groups of Poloxamer-g-PAA are less ionized at pH is relatively poorly understood and could be related 3–12. The ionization of PAA causes expansion of the to the shrinkage of the PEO corona of the micelles polymer and increases the gel modulus over the pH due to temperature effects on PEO solubility and the range 5.4–12.0. The sol-to-gel transition temperature interaction of PEO chains with the PPO hard core decreases with ionization in this range of pH. The [48]. A recent SANS study proposed the transition of addition of NaCl affects the gel modulus. Increasing the micelle structure from spherical to cylindrical, NaCl from 0 to 10% w/v decreased the gel modulus thus releasing micelle-packing constraints, as the decreased significantly (DG9| 80 Pa), while the solcause of the high temperature gel–sol transition to-gel transition temperature remained almost un- [43,49]. changed (DT ,58C). As polymer ionization in-

The phase transition behavior was studied with creases with pH, the environment of the PPO segaltered triblock structures, where poly(butylene ments become more polar, driving the PPO to oxide) (PBO) was used in place of PPO in the aggregate at lower temperatures. middle block or with PEO–PBO diblock copolymers This unique sol-to-gel transition has made the [50,51]. During synthesis via anionic living poly- system attractive as an injectable drug delivery merization, ethylene oxide–butylene oxide block matrix in an in situ gel-forming drug depot. Most copolymers avoided chain transfer reactions, which applications are based on Poloxamer PF-127 and are inherent in propylene oxide polymerization [52]. include delivery of protein/peptide drugs, such as The polymer solutions showed only a gel-to-sol insulin, urease, interleukin-2, epidermal growth factransition with increasing temperature (high tempera- tor (EGF), bone morphogenic protein (BMP), fiture gel–sol boundary). broblastic growth factor (FGF), and endothelial cell

Polymers coupled with Poloxamers also exhibit growth factor (ECGF). Most release profiles show gelation in water. Hoffman et al. developed poly- sustained release kinetics over several hours. Polox-

Fig. 4. Gelation mechanism of PAA-g-Poloxamer.

amer hydrogels show a zero-order release profile for LD for subcutaneous injection for a mouse is 5.550 interleukin-2 and urease over 8 h [60,61]. Tride- g/kg [84,85].

capeptide melanotan-I (MT-I) and mitomycin C Because of the dissociation of packed micelles in were released from Poloxamer 407 hydrogel over 4 an excess of water, the gel integrity of Poloxamers to 6 h [62,63]. The weight percent of gel dissolved does not persist for more than a few days. In vitro was well correlated with the MT-I release profile. experiments showed that 25 wt% of Poloxamer 407 The higher the polymer concentration, the slower the gel was completely dissolved in the release medium release rate observed. The release rate was manipu- in 4 h. For 35 wt% Poloxamer 407, the gel was 50% lated by mixing excipients. dissolved in 4 h [67]. Therefore, Poloxamer formula-

Poloxamers have been suggested for use as an tions are only useful for a short period after adminisocular drug delivery carrier of pilocarpine, but an tration. animal study of PF-127 showed marked destruction of the retina [64]. The addition of poly(ethylene glycol) (PEG) or poly(vinyl pyrrolidone) (PVP) 5. PEG/PLGA block copolymers accelerated pilocarpine release, while the addition of methylcellulose slowed the release rate [65]. Table 2 A novel concept, which combines thermogelation, further summarizes the recent applications of Polox- biodegradability, and no toxicity, has been proposed amers for drug delivery [6–83]. for an injectable gel system with better safety and

Some low-molecular-weight Poloxamers are longer gel duration [86]. Poly(ethylene glycol-b-L- classified as inactive ingredients for currently mar- lactic acid-b-ethylene glycol) (PEG–PLLA–PEG) keted drug products [84,85]. For example, Polox- was synthesized by ring-opening polymerization of amer 188, PEO–PPO–PEO (3500–1570–3500), is L-lactide onto monomethoxy poly(ethylene glycol) used as an intravenous injection formulation and an (MW 5000), which produced PEG–PLLA diblock oral formulation, and Poloxamer 407, PEO–PPO– copolymers, followed by coupling of the resulting PEO (4300–3770–4300), as an ophthalmic solution. diblock copolymers with hexamethylene diisocyanate However, an animal toxicity study of Poloxamers to produce triblock copolymers with a PLLA central showed that rats receiving 7.5 wt% of Poloxamer in block (MW 2000–5000). The copolymers only their diet exhibited a decrease in growth rate. Acute exhibited a single sol-to-gel transition with decreasanimal toxicity data for Poloxamer 188 show that the ing temperature in water, like a gelatin solution. As

Table 2 Applications of Poloxamers for drug delivery

Drug Comments Ref.

Pilocarpine Poloxamer (PF-127)/PEG, PVP, PVA, MC, HPMC [65] Insulin Poloxamer (PF-127)/fatty acid; potential application [6] for buccal delivery

Pilocarpine Carbopol/poloxamer; ophthalmic drug delivery [67] Insulin Poloxamer PF-127; subcutaneous injection [68] S-Nitroso-N-acetyl Poloxamer; local delivery of NO donor to prevent penicillamine initial hyperplasia [69] C-myb antisense Poloxamer (PF-127); prevents initial hyperplasia [70] oligonucleotide Tyrphostin-47 Poloxamer (PF-127); inhibits smooth muscle cell proliferation [71] Urease Poloxamer (PF-127); intraperitoneal injection in the rat [72] Interleukin-2 Poloxamer (PF-127); intraperitoneal injection in mice [73] EGF Poloxamer (PF-127) [74] Gene delivery Poloxamer (PF-127)/fusogenic peptide/haemagglutinin/ [75] chloroquine

Ibuprofen Poloxamer (PF-127)/liposomal gel [76] BMP Poloxamer (PF-127) [7] Vancomycin Poloxamer (PF-127); antibiotic delivery [78] Lidocaine/ Poloxamer (PF-127); epidural injection for prolonged [79] ibuprofen systemic absorption Sulfadiazine Poloxamer 188; antibacterial agent for wound healing [80] BFGF, ECGF Poloxmer (PF-127) [81] Lidocaine Poloxamer (PF-127); rat experiments for local anesthetics [82] Methotrexate Poloxamer (PF-127); topical administration [83]

(Parte 1 de 3)