Synthesis and characterization of biodegradable hydrophobic

Synthesis and Characterization of Biodegradable Hydrophobic–Hydrophilic Hydrogel Networks with a Controlled Swelling Property YELI ZHANG, CHEE-YOUB WON,* CHIH-CHANG CHU Fiber and Polymer Science Program, Department of Textiles and Apparel & Biomedical Engineering Program, Cornell University, Ithaca, New York 14853-4401

Received 8 November 1999; accepted 16 March 2000

ABSTRACT: A biodegradable polymer network hydrogel system with both hydrophilic and hydrophobic components was synthesized and characterized. The hydrophilic and hydrophobic components were dextran and poly(D,L)lactic acid (PDLLA), respectively. These two polymers were chemically modified for incorporating unsaturated groups for subsequent UV crosslinking to generate a hydrogel with a three-dimensional network structure. The effects of the reaction conditions on the synthesis of a dextran derivative of allyl isocyanate (dex-AI) were studied. All newly synthesized materials were characterized by Fourier transform infrared and NMR. The swelling property of the hydrogels was studied in buffer solutions of different pHs. The results of this study showed that a wide-range swelling property was obtained by changes in the dex-AI/PDLLA composition ratio, the type and degree of unsaturated groups incorporated into dextran, and the UV photocrosslinking time. The solvent extraction effect on the swelling property of the hydrogels was also studied. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2392–2404, 2000

Keywords: hydrogel; dextran; poly(lactic acid); allyl isocyanate; biodegradation; swelling ratio; photocrosslinking; UV; polymer network

INTRODUCTION Hydrogels have received significant attention for their use as medical implants, diagnostics, biosensors, bioreactors, bioseparators, and matrices for drug delivery because hydrogels have high water contents like body tissues and are highly biocompatible.1 The properties that make hydrogels biocompatible are (1) low interfacial tension with surrounding biological fluids and tissues to minimize the driving force for protein adsorption and cell adhesion, (2) biological cells and tissues

* Present address: Biopharmaceuticals Department, Hoffmann-La Roche, Inc., 340 Kingsland St., Nutley, New Jersey 07110-1199 Correspondence to: C.-C. Chu Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 2392–2404 (2000) © 2000 John Wiley & Sons, Inc.

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having hydrodynamic properties similar to those of hydrogels in many ways, and (3) minimal mechanical and frictional irritation to the surrounding tissue because of the soft and rubbery nature of hydrogels.2 Furthermore, certain hydrogels undergo phase transitions with environmental changes, including solvent composition, ionic strength, pH, temperature, electric field, and light.3 All these properties make hydrogels attractive carriers for the controlled delivery of drugs.4,5 Biodegradable hydrogels as drug carriers have become increasingly important because they can not only be degraded and eliminated from the body after use but also provide flexibility in the design of delivery systems for large molecular weight drugs, such as pharmaceutically active proteins and peptides.2

HYDROPHOBIC–HYDROPHILIC HYDROGEL NETWORK

It is advantageous to have a hydrogel system that has a wide range of hydrophilicity to hydrophobicity for meeting the needs of a variety of bioactive compounds and for regulating the release of drugs with a wide range of hydrophobicity.4,6 To obtain such a hydrophilic– hydrophobic balanced hydrogel, polymer networks have been suggested as a viable means both to increase the compatibility among polymer components and have the properties that are hybrids of those constituent macromolecules.7 One of the common methods to achieve randomly crosslinked networks is photocrosslinking, which has the advantage of simpler processing and the absence of a potentially toxic crosslinker. Biodegradable hydrogels can be prepared from both synthetic and natural polymers. In this article, we suggest a new chemical means to prepare biodegradable hydrogel networks with both polysaccharide and synthetic aliphatic polyester components. The use of polysaccharides as the hydrophilic component of a bicomponent hydrogel network would be beneficial because they are abundant in nature and inherently biodegradable; they have unique properties, such as selfassembly, specific recognition of other molecules, and the formation of reversible bonds.8 Dextran was chosen as the polysaccharide component in our prior studies of hydrogel networks.9 –12 Dextran consists mainly of (1 3 6) ␣-D-glucoside linkages with about 5–10% (1 3 3) ␣-linked branching.13 It is one of the most abundant and naturally occurring biodegradable polymers and has been experimentally used for the delivery of pharmaceutically active drugs, peptides, and proteins.14 –18 Dextran has chemically active functional groups (i.e., OOH group) that can be used to provide greater flexibility in the formulation of hydrogels,19 and dextran is susceptible to enzymatic digestion in the human body.2 Poly(D,L)lactic acid (PDLLA) was chosen as the synthetic biodegradable component because it is one of the most thoroughly investigated synthetic biodegradable hydrophobic polymers and is biocompatible.20 Because PDLLA is a hydrophobic polymer, it could counterbalance the hydrophilic nature of the dextran component in the bicomponent network system. The extent of this counterbalance can be controlled via the composition ratio of dextran to PDLLA so that not only the hydrolytic biodegradation of the network hydrogels could be tailored to specific clinical needs but also the degree of hydrophobic interaction between the drug and the hydrogel could be de-

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signed for meeting the hydrophilic/hydrophobic nature of the incorporated drugs and their release rates. Very recently, we synthesized a series of dexAC/PDLLA hydrogels based on dextran acrylate (dex-AC) and PDLLA diacrylate macromer.9 Because of the sensitivity of the ester linkage (in the dex-AC) to the pH media, the swelling of the dex-AC hydrogel was strongly pH dependent. In this article, we report the synthesis of dextran derivatives of allyl isocyanate (dex-AI) and the effects of the reaction time and temperature, the molar ratio of the reactants on the dex-AI synthesis, and the formation of three-dimensional network hydrogels from dex-AI and PDLLA diacrylate macromer precursors. Because of the relatively hydrolytically resistant urethane linkages in dex-AI compared with the ester linkages in dex-AC, we expected the swelling behavior of the dex-AI-based hydrogels in buffer media to be different from the dex-AC-based hydrogels. These new polymers were characterized by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra. The swelling characteristics of the dex-AI/PDLLA hydrogels in buffer solutions at 37 °C were examined as a function of the dex-AI/PDLLA ratio, pH, type of unsaturated groups in dextran, degree of substitution (DS) by allyl isocyanate, and UV crosslinking time.

MATERIALS AND METHODS Materials Dextran with a molecular weight of 43,000 was purchased from Sigma Chemical (St. Louis, MO) and dried in a vacuum oven for 24 h at 50 °C before use. PDLLA with a molecular weight of 740 (polydispersity ⫽ 1.53) was obtained from Boehringer Ingelheim (Milwaukee, WI) and stored in a freezer before use. Dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dibutyltin dilaurate (DBTDL), allyl isocyanate (AI), 1,3-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBT), 2-aminoethanol, acryloyl chloride, triethylamine, and 2,2-dimethoxy 2-phenyl acetophenone were obtained from Aldrich Chemical (Milwaukee, WI) and used without further purification. The DMSO and THF were anhydrous grade; all the others were laboratory grade. Standard phosphate buffer solutions (PBS) of pH 3, 7, and 10 (0.1M) were purchased from VMR Scientific (West Chester, PA).

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Methods

Synthesis of PDLLA Diacrylate Macromer Detailed procedures for synthesizing the PDLLA diacrylate macromer have been described elsewhere.9 In brief, the synthesis of this macromer involved two steps: the conversion of OCOOH end groups to OOH end groups and the subsequent incorporation of crosslinkable unsaturated units at the chain ends of PDLLA. The original PDLLA polymer (e.g., 4 g) was first dissolved in THF (60 mL) under nitrogen. After the solution was chilled in an ice bath, the calculated amounts of HOBT (0.73 g) and DCC (1.52 g) were carefully added. The reaction was allowed to proceed at the ice-bath temperature with continuous stirring for 30 min. After the solution mixture was brought to room temperature, 2-aminoethanol (0.36 mL) was then added slowly to the flask and the reaction was proceeded for another 30 min. The precipitated byproduct, cyclohexylurea salt, was filtered off. The PDLLA diol product was obtained by the filtrate being poured into a large excess of dry hexane and was purified by dissolution and precipitation several times with THF and hexane, respectively. The PDLLA diol was then dried under vacuum at room temperature for 1 day. For synthesizing the PDLLA diacrylate macromer, dried PDLLA diol (e.g., 3.5 g) was first dissolved in THF (50 mL) under nitrogen. Fixed amounts of triethylamine (2.63 mL) and acryloyl chloride (1.53 mL) were added dropwise to the PDLLA diol/THF solution at 0 °C. The mixture was stirred in an ice bath for 3 h and at room temperature for 18 h. The triethylammonium hydrochloride byproduct was removed by a glass filter. The final product, the PDLLA diacrylate macromer, was precipitated with hexane (10 times the amount of PDLLA diacrylate macromer) and further purified by repeated dissolution and precipitation from THF and hexane several times. The purified macromer was dried at room temperature under vacuum for 1 day. The PDLLA diacrylate macromer used in this study was the same as the PDLLA2⫹ used in our previous study,9 which had a number-average molecular weight of 1096 and a polydispersity of 1.39 from gel permeation chromatography in THF as a solvent.

catalyst. A series of dex-AI with different DS (the number of AI groups per 100 anhydroglucose units) were synthesized and characterized. At room temperature, known weights of dry dextran (e.g., 5 g) were dissolved in anhydrous DMSO (60 mL) under nitrogen. Dibutyltin dilaurate catalyst (1.83 mL) was injected into the solution at room temperature dropwise, and AI (2.73 mL) was subsequently added dropwise. The reaction mixture was stirred at a predetermined temperature (i.e., 25 °C) and time; samples at different reaction times (i.e., 2, 4, 6, and 8 h) were withdrawn directly from the reaction flask with pipettes for analysis. The resulting polymer samples were precipitated in cold excess isopropanol. They were further purified by dissolution and precipitation with DMSO and isopropanol, respectively. The dex-AI products were dried at room temperature under vacuum for several days and stored in a cold dark place before characterization and hydrogel fabrication.

Preparation of the Dex-AI/PDLLA Hydrogels To make hydrogel networks, both precursors (dex-AI and PDLLA diacrylate macromer) were dissolved in a common solvent (DMF), and, after UV-induced free-radical photocrosslinking, these two precursors formed crosslinked three-dimensional networks. Different weight ratios of dex-AI and PDLLA diacrylate macromer were dissolved in fresh DMF to make 50% (w/v) concentration solutions. The dex-AI/PDLLA weight ratios were 100% dex-AI (100/0), 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80, 10/90, and 100% PDLLA diacrylate macromer (0/100). An initiator [2,2-dimethoxy 2-phenyl acetophenone, 5% (w/w) of the precursors’] was added to the solution of the precursors. This homogenous solution was then poured onto a hydrophobic poly(tetrafluoroethylene) plate and irradiated by a portable long-wavelength UV lamp (365 nm, 8 W) at room temperature for 1–3 h. A disk-shaped hydrogel was obtained by the photocrosslinking of the unsaturated functional groups of dex-AI and PDLLA diacrylate macromer. The resulting hydrogels were dried under vacuum at room temperature for several days. Characterization

Synthesis of Dex-AI In the preparation of dex-AI, dextran reacted with AI in the presence of dibutyltin dilaurate

The characterization of the dex-AI and dex-AI/ PDLLA hydrogels was done by an FTIR spectrophotometer and NMR spectroscopy. All PDLLA-


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related characterizations were detailed in our previous study.9 FTIR analyses of dex-AI were performed with a Nicolet Magna 560 FTIR spectrophotometer (Madison, WI). The FTIR samples were prepared either by polymer powder being compressed with 10 times as much KBr powder to make a pellet or by a thin film being cast onto KBr disks from a DMF solution (1% w/v). 1 H NMR spectra of dex-AI were recorded in a 99.8% DMSO-d6 solution with a Varian 300-MHz spectrophotometer (Palo Alto, CA). All of the chemical shifts, ␦, were reported in parts per million (ppm) and referenced internally by the central DMSO peak being set at 2.50 ppm. The DS of dex-AI was determined from the peak ratio of anomeric proton to the three protons on the vinyl group. The details were described in our previous study9 and Katsura and Isogai’s research.21 Swelling Test A known weight of a dry hydrogel (⬃35 mg) was immersed in a vial containing a 15-mL buffer solution of different pHs (3, 7, and 10) at 37 °C. The swollen hydrogels were removed from the buffer solutions at predetermined intervals (i.e., 0.5, 1.5, 3, 5, 7, 11, and 23 h), and their excess surface water was wiped gently with kimwipe until there were no visible water droplets. The swollen hydrogel was then weighed until equilibrium was attained. The swelling ratios of all the dex-AI/PDLLA hydrogels in buffer solutions were calculated by the formula shown next and expressed in terms of the percentage of initial dry weight. The swelling test was stopped after 23 h because the 100% dex-AI hydrogel reached its equilibrium swelling ratio. The swelling profile of a hydrogel was determined in triplicate: Swelling ratio 共%兲 ⫽ 共W t ⫺ W 0兲/W 0 ⫻ 100 where W t is the weight of the hydrogel at time t and W 0 is the initial dry weight of the hydrogel before swelling. Most of the swelling data were obtained from extracted hydrogel specimens. The purpose of the extraction was to remove uncrosslinked polymer precursors from the hydrogel network. The extraction was obtained by the dexAI/PDLLA hydrogels being immersed in excessive acetone for 24 h and then in pure water for another 24 h at room temperature. Because the PDLLA diacrylate macromer precursor dissolved

Scheme 1. Synthesis of dex-AI from dextran and allyl isocyanate.

in acetone but not in water, whereas the dex-AI precursor dissolved in water but not in acetone, the use of acetone and water would extract uncrosslinked PDLLA diacrylate macromer and dexAI from the hydrogel network, respectively.

RESULTS AND DISCUSSION Synthesis of Dex-AI The synthesis of dex-AI is illustrated in Scheme 1. Figure 1 shows the FTIR spectra of the original dextran, dex-AI, and dex-AI hydrogel. Compared with the spectrum of the dextran (A), the FTIR spectrum of dex-AI (B) showed the characteristic double-bond absorption bands at 3006 cm⫺1 for the COH stretching vibration and 1645 cm⫺1 for the CAC stretching vibration. The intensity of these peaks decreased after hydrogel formation (Spectra B and C in Fig. 1) because of the consumption of double bonds during the UV-induced crosslinking reactions. The characteristic FTIR peaks from the urethane linkage occurred at 1720 and 1540 cm⫺1, which indicated the presence of both carbonyl and NOH groups of the urethane linkage in dex-AI. A small absorption band at 1650 cm⫺1 in the dextran spectrum (A) was due to the presence of a trace amount of adsorbed water that was difficult to remove because of the hydro-

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Figure 1. FTIR spectra of (A) dextran, (B) dex-AI (dex-AI-3 with a DS of 5.03) and (C) dextran hydrogel. Peak 1 ⫽ CH of vinyl stretch; Peak 2 ⫽ CAC stretch; Peak 3 ⫽ amide II.

philic hydroxyl groups in the dextran.22 This absorption band was masked by a strong carbonyl stretch band around 1720 cm⫺1 in the dex-AI and hydrogel spectra.

Figure 2.

1

Figure 2 shows the 1H NMR spectra of (A) dextran and (B) dex-AI. All peak assignments were determined by comparison with the spectra of the corresponding original polymer and the use

H NMR spectra of (A) dextran and (B) dex-AI (dex-AI-3 with a DS of 5.03).


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Table I. Effect of the Reaction Conditions on the Degree of Substitution of Allyl Isocyanatein in Dextran Molar Ratio of the Reactants Reaction Temperature (°C)

Samples

DBTDL/Dextran

Allyl Isocyanate/ Dextran

Dex-AI-1

0.05

0.25

25

Dex-AI-2

0.05

0.5

25

Dex-AI-3

0.1

1

25

Dex-AI-4

0.2

2

25

Dex-AI-5 Dex-AI-6

0.08 0.1

1 1

45 45

Dex-AI-7

0.1

1

60

of standard literature values.23 The newly appeared signals in dex-AI (Spectrum B) were from the protons of AI. The 1H NMR peak at ␦ 5.86 ppm was from the two protons of the vinyl end group (ACH2), and the peak at ␦ 7.22 ppm was due to the proton of OCHACH2. The proton on OOOCOONHO appeared at ␦ 7.38 ppm. The signal at ␦ 3.61 ppm was assigned to the protons of OOOCOONHOCH2O on the AI. Furthermore, because of the presence of an AI pendant group, extra signals observed in the region of ␦ 4.90 –5.20 ppm were due to the slightly different chemical environment of the anomeric protons in dex-AI in comparison with the original dextran. The intensity of these three new signals became more evident in highly substituted dextran. Because of the different reactivities of hydroxyl groups in the dextran anhydroglucose unit (C2 ⬎ C4 ⬎ C3), the peak intensities of these three signals were different. An increase in the DS of AI in dextran also resulted in an increase in the peak intensity of the unsaturated protons (␦ at 5.86 and 7.22 ppm) in the allyl portion of the AI.

Reaction Time (h)

DS

2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 6 2 4 6 8 2 4 6 8

0.95 1.72 2.31 3.13 1.81 2.06 2.78 3.49 2.74 3.44 4.53 5.03 3.86 4.87 6.92 9.04 8.36 7.81 9.54 11.27 12.94 9.25 10.93 12.38 14.14

Several reaction parameters, such as the molar ratio of the reactants, the reaction temperature, and the time, were examined for the synthesis of dex-AI and are summarized in Table I. Dex-AI with DSs ranging from 0.95 to 14.14 could be successfully synthesized by changes in the reaction parameters.

Effect of the DBTDL Catalyst on the Synthesis of Dex-AI The effect of the DBDTL catalyst on the synthesis of dex-AI is shown in Table I. A comparison between dex-AI-5 and dex-AI-6 at a 6-h reaction period clearly shows that an increase in the DBTDL/dextran molar ratio from 0.08 to 0.1 at a 1/1 molar ratio of dex-AI/AI and a 45 °C reaction temperature led to an increase in the incorporation of the AI group into dextran from 8.36 to 11.27 per 100 anhydroglucose residue units. The DBTDL catalyst played an important role in the synthesis of dex-AI. It is well-known that organotin compounds are extremely effective for

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the isocyanate– hydroxyl reaction.24,25 Luo and Tan24 suggested that the functions of the DBTDL catalyst were generally to increase the reaction rates and to reduce side reactions. The mechanism of DBTDL-catalyzed reactions of isocyanate with hydroxyl was assumed to proceed via the formation of a ternary complex with both isocyanate and hydroxyl. Because DBTDL preferred to activate hydroxyl rather than water due to the higher nucleophilicity of hydroxyl oxygen, the side reaction of AI could be reduced.

Effect of AI on the Synthesis of Dex-AI An increase in the AI-to-dextran ratio resulted in an increase in the DS. For example, an increase in this ratio from 0.25 (dex-AI-1) to 0.5 (dex-AI-2) at the same reaction time, temperature, and catalyst-to-dextran ratio (0.05) resulted in a large increase in the DS. However, this effect became smaller at a longer reaction time. Compared with the dex-AC synthesis described in our previous study,9 the dex-AI reaction was milder, and a wider range of DSs could be obtained by changes in the ratios of the reactants, the reaction time, and the temperature. In our previous dex-AC synthesis,9 only a low DS (e.g., ca. 3.70 unsaturated groups per 100 anhydroglucose units) was obtained; a higher DS was difficult to obtain because the reaction became harder to control at higher temperatures or reactant ratios.

Effect of the Reaction Time and Temperature on the Synthesis of Dex-AI The amounts of AI grafted onto the dextran backbone increased gradually with an increase in reaction time. As is evident in Table I, an increase in the DS with an increase in the reaction time (from 2 to 8 h) was observed. For example, at 25 °C, the DS of dex-AI-3 increased from 2.74 to 5.03 as the reaction time increased from 2 h to 8 h. For examining the reaction temperature effect on the synthesis of dex-AI, we used three different temperatures: 25, 45, and 60 °C. Table I shows that the reaction temperature had a strong effect on the completion of the incorporation of AI onto dextran. For example, the DS increased from 5.03 at 25 °C (dex-AI-3) to 12.94 at 45 °C (dex-AI-6) and then to 14.14 at 60 °C (dex-AI-7) in an 8-h reaction period. Therefore, by changes in the reaction parameters, such as the amounts of DBTDL and AI, the reaction time, and the temperature, dex-AI with a

wide range of DSs could be obtained. This versatility in the design of the dextran hydrogel precursor allows us to synthesize hydrogels with a wide range of swelling properties that are consequently more versatile in their drug delivery applications.

Preparation of the Dex-AI/PDLLA Hydrogels Scheme 2 shows the synthesis of dex-AI/PDLLA hydrogels from their precursors: dex-AI and PDLLA diacrylate macromer. By changes in the amounts of dex-AI to PDLLA diacrylate macromer, hydrogels with different dex-AI/PDLLA ratios (w/w)—100/0, 90/10, 80/20, 70/30, 60/40, 50/ 50, 40/60, 30/70, 20/80, 10/90, and 0/100 —were made. Figure 3 shows the FTIR spectra of some of these hydrogels with different dex-AI/PDLLA ratios. The data clearly demonstrate the successive integration of PDLLA into dex-AI. The peaks in the region of 2880 –3000 cm⫺1 (CH3 and CH stretch) increased gradually as the PDLLA component in the hydrogel increased. In addition, the carbonyl absorption bands at 1720 and 1760 cm⫺1 were from dex-AI pendant group (OOOCOONHO) and PDLLA backbone (OCOOCH(CH3)OOO), respectively. As the PDLLA moiety increased, the peak intensity at 1760 cm⫺1 increased, whereas the peak intensity at 1720 cm⫺1 decreased. The NOH band around 1536 cm⫺1 decreased gradually as the weight ratio of dex-AI to PDLLA diacrylate macromer decreased. This is because each dex-AI macromolecule has three potential sites (OOH) per anhydroglucose unit for incorporating NOH containing isocyanate, whereas each PDLLA diacrylate macromer has only one NOH-containing amide linkage located at one of the two chain ends. Thus, the total concentration of NOH inside a hydrogel would decrease as the composition ratio of dex-AI to PDLLA diacrylate macromer decreases.

Swelling Test A swelling study of the dex-AI/PDLLA hydrogels was performed in pH 3, 7, and 10 buffer solutions at 37 °C. The effects of the dex-AI/PDLLA ratio, the type of unsaturated groups introduced into dextran, the DS of AI in dex-AI, the UV crosslinking time, and the water and acetone extraction treatment on the swelling property were investigated.


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Scheme 2. One possible structure of dex-AI/PDLLA hydrogels from the photocrosslinking of dex-AI and PDLLA diacrylate macromer.

Effect of Extraction on the Swelling Behavior of the Hydrogels Figure 4 shows the effect of extraction on the swelling ratios of the dex-AI/PDLLA hydrogels. The dex-AI/PDLLA hydrogels obtained after 3-h UV irradiation were used for the extraction study, and dex-AI-5 (DS ⫽ 8.36) was chosen as the dexAI component. The swelling ratios of the hydrogels that were extracted with both acetone and water were different from those that were not extracted; all of the other parameters remained unchanged. In general, the process of extraction increased the swelling ratio, and the magnitude of the increase depended on the composition ratio of dex-AI to PDLLA. The effect of extraction was

the largest in the pure dex-AI hydrogel and became smaller as the PDLLA component increased. The observed higher swelling ratios from the extracted hydrogels were attributed to the removal of free (uncrosslinked) PDLLA diacrylate macromer, dex-AI, or both. This removal would provide additional porous volume for water uptake, that is, a higher swelling ratio for the dexAI-dominant hydrogels. The effect of extraction on the swelling was not obvious for those dex-AI/ PDLLA hydrogels with a PDLLA component greater than 70%; this is because the hydrophobic attraction among PDLLA segments became a dominating factor to control the swelling behavior of the dex-AI/PDLLA hydrogels.

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Figure 3. FTIR spectra of dex-AI/PDLLA hydrogels with different dex-AI (dex-AI-3 with a DS of 5.03) -to-PDLLA ratios: (A) 100/0, (B) 80/20, (C) 50/50, (D) 20/80, and (E) 0/100. Peak 1 ⫽ hydroxyl; Peak 2 ⫽ CH3 stretch; Peak 3 ⫽ CH stretch; Peak 4 ⫽ carbonyl of dex-AI stretch; Peak 5 ⫽ carbonyl of PDLLA stretch.

Except for Figure 7 (discussed later), all other swelling results were obtained from the extracted hydrogel specimens.

Effect of the Dex-AI/PDLLA Ratio on the Swelling Behavior of the Hydrogels Figure 5 shows the swelling ratios of the dex-AI/ PDLLA hydrogels with different dex-AI-toPDLLA composition ratios (100/0, 80/20, 60/40,

Figure 4. Effect of the extraction on the swelling ratios of dex-AI/PDLLA hydrogels after 23 h in a pH 7 buffer solution (dex-AI-5 with a DS of 8.36 and UV for 3 h).

40/60, 20/80, and 0/100) in pH 3, 7, and 10 buffer solutions after a 23-h immersion. The dex-AI component of the hydrogel system had a DS of 8.36. All hydrogels were crosslinked under UV for 3 h and tested after extraction with both water and acetone. Except for the 100% dex-AI hydrogel, none of the dex-AI/PDLLA hydrogels reached an equilibrium swelling ratio at the end of the study period (23 h). Although the magnitude of the swelling profiles varied with different pHs of the medium, the

Figure 5. Effect of the dex-AI-to-PDLLA composition ratio on the swelling ratios of dex-AI/PDLLA hydrogels after 23 h in pH 3, 7, and 10 buffer solutions (dex-AI-5 with a DS of 8.36 and UV for 3 h, after extraction).


overall swelling behaviors in the same buffer solution were generally similar; that is, the swelling ratio decreased as the PDLLA component increased. The swelling ratio of the dex-AI/PDLLA hydrogels depended on the hydrophilicity of the dex-AI component and the hydrophobicity of the PDLLA component. As the PDLLA component increased, the hydrophobicity of the hydrogel increased because of the hydrophobic attraction among PDLLA segments, which led to a more compact structure and a suppressed swelling ratio, as observed. The composition dependence of the swelling behavior of the dex-AI/PDLLA hydrogels in buffer solutions was generally consistent with other studies.1,26 –29 For example, Inoue et al.1 studied the swelling behavior of a hydrophobically modified hydrogel by grafting methyl methacrylate to the backbone of poly(acrylic acid). The swelling ratios of Inoue et al.’s hydrogels decreased as the graft level of methyl methacrylate (the hydrophobic part) increased. The consistently higher swelling ratios of the dex-AI/PDLLA hydrogels in an alkaline pH medium compared with those of acidic and neutral pH media could be partially attributed to the higher rate of hydrolytic degradation of ester groups in the PDLLA backbone in an alkaline medium. This alkaline-accelerated hydrolysis of aliphatic polyesters was also reported by Chu30,31 in his study of the effect of pH on the hydrolytic degradation of polyglycolide and poly(glycolideco-lactide). Similar findings were also reported by Karmalkar et al.32 in their study of the ester hydrolysis rate of p-nitro pendent groups from 2-hydroxyethyl methacrylate. This faster hydrolysis of ester groups in the PDLLA segment would result in a more open hydrogel network structure in an alkaline medium, which would lead to a higher swelling ratio than that in acidic and neutral media. Our preliminary hydrolytic degradation study of the dex-AI/PDLLA hydrogels through the monitoring of their weight changes as a function of time in a pH 7.4 buffer showed that hydrolytic degradation did occur; that is, the weights of the dex-AI/PDLLA hydrogels decreased as the incubation time increased. Therefore, the parameters that influenced the swelling ratio of the dex-AI/PDLLA hydrogels in different pH media might include hydrophilic and hydrophobic properties of the constituent polymers and the pH-dependent hydrolysis rate of ester linkages located in the PDLLA backbones.

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Figure 6. Swelling ratios of 100% dex-AC (DS 3.70) and 100% dex-AI (dex-AI-3 with a DS of 5.03 and dexAI-5 with a DS of 8.36) hydrogels after a 23-h immersion in pH 3, 7, and 10 buffer solutions (UV for 3 h, after extraction).

Effect of the Type of Unsaturated Group in Dextran on the Swelling Behavior Comparing the chemical structure of the 100% dex-AI hydrogel synthesized in this study with that of the 100% dex-AC hydrogel reported in our previous study,9 we would expect the 100% dexAI hydrogel to exhibit relatively less pH-dependent swelling behavior than the 100% dex-AC hydrogel because of different unsaturated groups being incorporated. As shown in Figure 6, the swelling ratio of the 100% dex-AI hydrogel differed from the 100% dex-AC hydrogels over the pH range studied, particularly at pH 10. The 100% dex-AC hydrogel exhibited a significantly higher swelling ratio than the 100% dex-AI hydrogel. In addition to the difference in the DS factor, this observed different swelling behavior between the 100% dex-AC and 100% dex-AI hydrogels might be attributed to their different chemical constituents in the crosslinkers. The 100% dex-AC hydrogel had an ester linkage on the crosslinker, whereas the 100% dex-AI hydrogel had a urethane linkage. The hydrolysis rate of the urethane linkage is known to be slower than that of the ester linkage. Therefore, the 100% dex-AC hydrogel would be more sensitive to alkaline-catalyzed hydrolysis of ester-containing crosslinkers and thus would exhibit a higher swelling ratio in an alkaline medium, as observed. Because ester groups were hydrolyzed so quickly in a pH 10 buffer, the 100% dex-AC hydrogel was totally dissolved after a 3-day immersion, whereas the 100% dex-AI hydrogel was relatively stable in all three pH media up to a 1-month study period.

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The crosslinker-dependent swelling behavior of hydrogels was also reported by Dijk-Wolthuis.17 He found that both hydroxyethyl methacrylate-derivatized dextran (dex-HEMA) and HEMA-oligolactatederivatized dextran (dex-lactateHEMA) hydrogels showed a progressive swelling with time, followed by a dissolution phase; the time required for the dissolution of dex-HEMA, however, was longer than the time for dex-lactateHEMA. Dijk-Wolthuis suggested that the hydrolysis of esters in the crosslinking group (HEMA and lactateHEMA) would lead to an increase in the molecular weight between the adjacent crosslinking points and result in an increased swelling with time and finally a complete dissolution of the hydrogel. Because of the existence of lactate esters in the dex-lactateHEMA, the dissolution of the dex-lactateHEMA network was faster than the dissolution of the dex-HEMA network. By comparing the swelling ratio of the 100% dex-AI hydrogel with a lower crosslinking density (DS ⫽ 5.03) with the ratio of the same hydrogel with a higher crosslinking density (DS ⫽ 8.36) in Figure 6, we clearly observe the effect of the DS on the pH-dependent swelling ratio. The swelling ratio of the hydrogel with a lower crosslinking density increased significantly in an alkaline medium, whereas the highly crosslinked hydrogel did not exhibit a pronounced change in the swelling ratio with pH. Akala et al.27 also reported that the effect of pH on the swelling ratio of hydrogels would be more pronounced in hydrogels with a low DS, that is, a lower crosslinked hydrogel. The details of the effect of the DS of AI on the swelling of the dex-AI/PDLLA hydrogels are given next.

Effect of the DS of AI in Dex-AI on the Swelling Ratio of the Hydrogels Figure 7 shows the swelling ratio of the dex-AI/ PDLLA hydrogels with different DSs of dex-AI in a pH 7 buffer solution at the end of a 23-h immersion. The data demonstrate that a higher swelling ratio was achieved in those hydrogels with a lower DS of AI on dextran, particularly at a higher dex-AI-to-PDLLA ratio (e.g., the 80/20 and 90/10 samples). For example, the pure dex-AI hydrogel with a DS of 5.03 had a swelling ratio more than double that of the same hydrogel with a DS of 8.36. However, there was virtually no DS effect on the swelling ratio when the composition ratio of dex-AI to PDLLA was smaller than 80/20. This suggested that DS was not the dominant factor for controlling the swelling ratio of those dex-AI/ PDLLA hydrogels with higher amounts of PDLLA (i.e., dex-AI/PDLLA composition ratio ⬍ 80/20).

Figure 7. Effect of the degree of substitution in the dex-AI component on the swelling ratios of dex-AI/ PDLLA hydrogels after 23 h in a pH 7 buffer solution (UV for 3 h, before extraction).

Hovgaard and Brondsted33 reported that the degree of swelling of dextran hydrogels was a function of crosslinking density: as crosslinking density increased, swelling became restricted. The increase in the DS of dex-AI in our study would lead to an increase in the crosslinking density of the dex-AI/PDLLA hydrogel, so its swelling ratio would decrease as we observed. However, our results also suggest that the swelling ratio was not sensitive to the change in the degree of crosslinking (via the DS) in dex-AI as long as a large amount of PDLLA (⬎20%) was incorporated into the dex-AI/PDLLA hydrogel. It may be attributed to the hydrophobic attraction among hydrophobic PDLLA segments that dominated the swelling behavior as the PDLLA component increased.

Effect of the UV Crosslinking Time on the Swelling Behavior of the Hydrogels Figure 8 shows the swelling ratios of dex-AI/ PDLLA hydrogels crosslinked by UV for 1 and 3 h and after 23 h in a pH 7 buffer solution. These hydrogels had the same DS (8.36) of AI in dex-AI, and all were tested after extraction with both water and acetone. As expected, a shorter UV crosslinking time always led to a higher swelling ratio, regardless of the composition ratio of dex-AI to PDLLA. This is because a shorter UV irradiation duration would lead to a lower degree of crosslinking. Similar findings were also reported by Allcock and Ambrosio34 on the synthesis of poly(organophosphazene) ionic hydrogels by gamma irradiation. They observed that the degree of swelling of the poly(organophosphazene)


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Figure 8. Effect of the UV crosslinking time on the swelling ratios of dex-AI/PDLLA hydrogels after 23 h in a pH 7 buffer solution (dex-AI-5 with a DS of 8.36, after extraction).

ionic hydrogel increased with decreasing radiation dose. The effect of the UV crosslinking time was the largest in the 100% dex-AI hydrogel. The incorporation of the PDLLA component, however, significantly reduced the difference in the swelling ratios because of the different UV crosslinking time. For example, in a pH 7 buffer solution, the 1-h UV crosslinked 100/0 dex-AI/PDLLA hydrogel had a swelling ratio of 544%, whereas the same hydrogel with 3-h UV crosslinking had a swelling ratio of only 252%, a difference of 292%. An incorporation of a small amount of PDLLA (10%) into the dex-AI hydrogel was sufficient to reduce the swelling ratio difference between the 1-h and 3-h UV crosslinking times to less than 30%. This finding may result from the reduction of the total number of available unsaturated groups for crosslinking as the dex-AI composition in the hydrogels decreased. Because each anhydroglucose unit in a dextran macromolecule has three potential sites for unsaturated group attachment and there are hundreds of thousands of such units per dextran macromolecule, whereas each PDLLA macromolecule has only two chain ends for unsaturated group attachment, the total number of unsaturated groups available would decrease significantly with a decrease in the weight ratio of dex-AI to PDLLA diacrylate macromer. Thus, the UV time required for the completion of the crosslinking reaction decreased in those dex-AI/ PDLLA hydrogels with high PDLLA-to-dex-AI composition ratios (i.e., an insignificant effect of the UV crosslinking time).

New biodegradable hydrogels have been made from hydrophilic dex-AI and hydrophobic PDLLA diacrylate macromer components over a wide range of composition ratios. Dex-AI was synthesized and characterized. Several reaction parameters were examined to study their effects on the DS of AI. The advantages of synthesizing dex-AI were that a wider DS could be achieved for more versatile applications and that there is no need to isolate byproducts from the reaction mixture, which is very important for the purification of the final products. The swelling property of the dex-AI/PDLLA hydrogels with different dex-AI-to-PDLLA ratios was studied. Through changes in the composition of the hydrogels, the type and DS of unsaturated groups in dextran, the UV crosslinking time, and the pH of the swelling medium, a wide range of swelling ratios was obtained. The swelling property of these hydrogels could also be affected by water and acetone extraction. This newly synthesized hydrogel system may be useful as a vehicle for the controlled delivery of drugs with a wide range of characteristics, such as molecular weight, hydrophilicity to hydrophobicity, and pH sensitivity. The authors would like to express their gratitude to Boehringer Ingelheim Company for its kind offer of PDLLA materials and the College of Human Ecology at Cornell University for its financial support.

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