Effects of metal salts on poly(DL-lactide-co-glycolide)polymer hydrolysis

Yan Zhang, Stephen Zale,* Laura Sawyer, and Howard BernsteinAlkermes, Inc., 64 Sidney Street, Cambridge, Massachusetts 02139

The effects of encapsulated metal salts on poly(DL-lactide-co-glycolide) (PLGA) water uptake and degradation propertieswere investigated in this work. Salts of varying aqueoussolubility characteristics were incorporated into PLGA filmseither as particles or by codissolution in polymer solutions.Polymer films were characterized with respect to the kinet-ics of water uptake, morphology changes, degradation, andweight loss after hydration. It was found that these proper-

ties are strongly influenced by the presence and nature ofencapsulated salts. Effects range from minor changes in wa-ter uptake profile with no significant difference in degrada-tion kinetics to major alterations in water uptake kineticstogether with a several-fold decrease in the polymer degra-dation rate. Possible mechanistic explanations for the ob-served effects are discussed. © 1997 John Wiley & Sons, Inc.

INTRODUCTION

Poly(DL-lactide-co-glycolide) polymers (PLGAs)are among the biodegradable materials which havebeen most successfully used in medical applications1–4

such as sutures, bone implants, and drug delivery sys-tems. For the case of drug delivery systems, therapeu-tic agents have been formulated into PLGA micro-spheres, disks, and rods.2–4 Drug release from thesePLGA matrices is governed by both diffusion of thedrug1,5 and degradation of the polymer.5–7 The majormechanism of PLGA degradation under physiologicconditions is hydrolysis.8 Thus, water penetration intoPLGA devices is a key factor in determining the over-all rate of polymer degradation, and hence, the rate ofrelease of the therapeutic agents from a PLGA deliv-ery system.

Poly(DL-lactide-co-glycolide) polymers have beenextensively characterized with respect to water uptakerates, degradation kinetics, and morphology changesafter incubation in aqueous media.6–8 These studieshave examined the effects of environmental factors(temperature, pH, and medium),6,7 polymer type (mo-lecular weight, lactide/glycolide ratio, etc.),8 or devicefabrication parameters.6,8 Comparably little data areavailable on the effects of drugs9 or excipients10 incor-porated into PLGA matrices on polymer degradationrates.

In the current work, we examine the effects of en-capsulated metal salts on the rate of water uptake, therate of polymer degradation, and the time course ofmorphologic changes in PLGA films. The salts evalu-ated varied with respect to water solubility, the iden-tity of the cation component, and the relative alkalin-ity of the anion component. The results providemechanistic information of use in the design of PLGA-based devices for the controlled release of drugs orother medical applications.

MATERIALS AND METHODS

Materials

Poly(DL-lactide-co-glycolide) (50:50), a random copoly-mer of DL-lactide and glycolide, (MW = 42,000, MW/Mn =2.4) was from Birmingham Polymers (Birmingham, AL).Magnesium carbonate [4MgCO3 z Mg(OH)2 z 5H2O],magnesium hydroxide [Mg(OH)2], zinc carbonate[3Zn(OH)2 z 2ZnCO3], calcium carbonate (CaCO3), so-dium chloride (NaCl), and zinc sulfate [ZnSO4 z 7H2O]were from Spectrum Chemical Manufacturing Corp.(Gardenia, CA). Magnesium acetate [Mg(CH3CO2)2]was from Fischer Chemical and magnesium sulfate[MgSO4 z 7H2O] from Mallinkrodt. Glass beads (1–20mm in diameter) and carbon black were from Poly-sciences.*To whom correspondence should be addressed.

Journal of Biomedical Materials Research, Vol. 34, 531–538 (1997)© 1997 John Wiley & Sons, Inc. CCC 0021-9304/97/040531-08

Methods

Film preparation

Polymer films with and without salts were pro-duced using a film-casting technique. Salts were in-corporated either as particulates or by cosolubilizingthe salts with the polymer in an appropriate cosolvent.Salts with low water solubility examined include mag-nesium carbonate, magnesium hydroxide, zinc car-bonate, and calcium carbonate. The water soluble saltsused in this study include magnesium acetate, sodiumchloride, magnesium sulfate, and zinc sulfate. Poly-mer films contained encapsulated salts in the range of2–30% (w/w).

The polymer was first dissolved in methylene chlo-ride (5% w/v) at room temperature for up to 24 h.Salts incorporated into polymer films as particles weresieved prior to film casting using a 38-mm U.S. stan-dard testing sieve to control the particle size. The av-erage particle diameters of the sieved salts were foundby light scattering to be <6 mm. The sieved salt wasadded into the polymer solution to a final concentra-tion in the 0–30% (w/w) range. The salt–polymer sus-pension was sonicated for approximately 4 min to dis-perse salt particles. One hundred milliliters of the sus-pension was then cast in a Teflon petri dish (9 × 5 × 1inch). The films were cast in two relatively equal lay-ers to minimize settling of the solid material. Themethylene chloride was allowed to evaporate at roomtemperature for the first 24 h at atmospheric pressure.The films were then transferred to a vacuum oven anddried at 30°C for 6 h, 40°C for 3 days, and then at 50°Cfor 3 days. No further reduction in dry weight wasobserved at the end of this drying cycle. Residualmethylene chloride in film samples were determinedby head-space GC analysis (Shimatzu) using dioxaneas solvent and detection by electron capture. Essen-tially complete removal of the solvent was confirmedby GC (final residual solvent content ø0.5%).

Polymer films incorporating nonionic water in-soluble particles (i.e., carbon black and glass particles)were prepared according to the same procedure.

Polymer films containing magnesium acetate andzinc acetate were prepared by codissolving the salts(5% w/w) with PLGA in acetone (5% w/v). Salts en-capsulated by codissolution were used directly with-out sieving. Polymer/salt solutions were sonicated,cast, and dried as described above.

Water uptake

Film samples (50–80 mg) were immersed in HEPESbuffer (10 mM) HEPES, 130 mM NaCl, 0.1% NaN3,0.1% Pluronic F68, pH 7.3) at a weight/volume ratio

of 2.0 mg film/mL buffer and incubated at 37°C. Du-plicate film samples were incubated and characterizedin parallel.

Samples were periodically recovered for water up-take determination. Surface water on films was re-moved with absorbent paper and the samples wereweighed. The samples were then frozen at −80°C andsubsequently lyophilized for 3–4 days to constant dryweight. Buffer solution was replaced in full at eachtime interval (approximately 4 days) for the filmsamples undergoing incubation for later water uptakedeterminations.

Water uptake was calculated at each time point us-ing the following equation:

% H2O uptake = 100 × (Wwet − Wt)/Wt,

where Wwet and Wt are weights of wet and dried filmsamples determined at time = t. Values obtained forduplicate samples were averaged.

Polymer degradation

Freeze-dried samples were characterized by differ-ential scanning calorimetry (DSC), gel permeationchromatography (GPC), elemental analysis and SEM.Degradation rates were assessed by molecular weightdetermination using GPC. Dried film samples weredissolved in chloroform (5–10 mg/mL) and filteredthrough a 0.22-mm filter. GPC was conducted using aMIXED column (300 × mm; Polymer Labs) eluted withchloroform. Detection was by refractive index. Mo-lecular weights were calculated using polystyrenestandards (Mw from 580–950,000) and the universalcalibration method.

Glass transition temperature (Tg) of freeze-driedfilm samples was determined using a DSC (DSC 7Serial; Perkin Elmer, Norwalk, CT) under nitrogenand using indium as a standard. Each sample wascooled to 0°C before heating to 60°C at 10°C/min. (Tg

was obtained from the second scan.)Polymer weight loss during film hydration was

measured by the changes in dry weight after incuba-tion for specified time periods. Percent weight losswas computed according to the following equation:

% Weight loss = 100 × (Winit − Wt)/Winit,

where Winit is the starting dry weight.

Salt content

Ion-release kinetics were determined based on el-emental analysis of freeze-dried film samples. Elemen-tal analyses were conducted by Galbraith Laboratories(Knoxville, TN).

532 ZHANG ET AL.

Polymer morphology

Scanning electron microscopy was used to observequalitative changes in film porosity and to monitormorphology changes of the film surface and interiorover time. Samples were lyophilized as describedabove. The dried samples were sputter-coated withgold 200–300 Å and the samples observed using JEOL-6400 SEM.

RESULTS

Effects of encapsulated salts on water uptake

Water uptake profiles for some PLGA films withand without salts are displayed in Figures 1 and 2. Thecontrol films without incorporated salts showed aninitial water uptake during the first 24 h after hydra-tion of <2% by weight. This was followed by a lagperiod during which there was negligible water up-take. A second phase of water uptake began at ap-proximately 7 days and continued at 2–4% water up-take/day for the remainder of the experiment.

The effects of salts with low water solubility (mag-nesium hydroxide, magnesium carbonate, calciumcarbonate, and zinc carbonate) on water uptake kinet-ics are shown in Figure 1. All films exhibited increasedinitial water uptake over the first 24 h compared to thecontrol film. The magnitude of the water uptake in7–10 hydration days was between 50 and 80% for films

containing 10% w/w magnesium carbonate and mag-nesium hydroxide. These films showed little addi-tional uptake of water for the next 20 days of hydra-tion, followed by an increase in uptake rate that oc-curred between 20 and 30 days. In contrast, zinccarbonate and calcium carbonate containing films didnot exhibit a distinguishable initial uptake phase, andinstead took up water continuously at a rate of 2%/day for 30 days. After 30 days, the rate increased toapproximately 3% per day.

Water uptake profiles for films containing watersoluble salts, encapsulated either as particles (magne-sium sulfate, zinc sulfate, and sodium chloride) or bycosolubilization (magnesium acetate), are comparedwith the control films in Figure 2. As with sparinglysoluble salts, the initial water uptake by soluble salt-containing films was of a greater magnitude than ob-served with the blank control. Water uptake continuedat a gradual rate and no clearly distinguishable lagphase or secondary water uptake was observed.

Unlike salt-containing films, PLGA films containingcarbon black and glass particles displayed water up-take behavior that was essentially identical to that ofthe control films.

Effects of encapsulated salts on polymerfilm morphology

Incorporation of salts into PLGA films had amarked effect on macroscopic and microscopic mor-phologic changes that occur upon hydration under

Figure 1. Time course of water uptake (% w/w) by PLGAfilms containing encapsulated salts of low water solubility.Film samples were incubated in HEPES buffer at 37°C.

Figure 2. Time course of water uptake (% w/w) by PLGAfilms containing encapsulated water soluble salts. Filmsamples were incubated in HEPES buffer at 37°C.

533EFFECTS OF METAL SALTS ON PLGA HYDROLYSIS

physiologic conditions. SEMs of lypophilized filmsamples are shown in Figures 3–5. Prior to hydration,all films displayed a dense structure with few poresscattered throughout the sample.

Control films exhibited a dense, smooth structurefor up to 14 days after hydration [Fig. 3(a,b)]. Between14 and 22 days, large pores became visible on the filmsurface and throughout the sample cross section [Fig.3(c)]. This change in morphology was coincident withthe increase in water uptake rate (see above) and with

a decrease of molecular weight in the film samples(see below).

In contrast to the controls, films loaded with saltswith low water solubility exhibited increased porosityafter hydration times as short as 24 h. For example,SEM analysis of a film loaded with 2% magnesiumcarbonate after hydration for 24 h showed a formationof a porous network (pore diameter ≈1–5 mm), con-centrated at the film surface [Fig. 4(b)]. After 7 days,the film had become uniformly porous throughout thecross section [Fig. 4(c)]. Pores ranged in diameter fromapproximately 1 to 20 mm. No further increase in po-rosity was observed for 30 days. Similar behavior wasobserved with films that contained higher percentagesof magnesium carbonate or other sparingly solublesalts (e.g., magnesium hydroxide or zinc carbonate).

Films that contained water-soluble magnesium saltsalso exhibited the formation of internal and surfacepores and voids well before pore formation occurredin control films. For example, in magnesium acetatecontaining films (Fig. 5), pores ranging in diameterfrom approximately 1 to 50 mm were visible insamples that had been hydrated for 5 days. However,the film containing 10% sodium chloride showed simi-lar internal and surface porosities to that of controlfilm during hydration (data not shown).

As with the blank polymer films, the observed mi-croscopic changes in morphology of salt containingfilms (i.e., increased porosity) were accompanied bymacroscopic swelling of the samples.

Film solubility

The presence or absence of sparingly soluble mag-nesium salts had a marked effect on the solubilitycharacteristics of polymer films after hydration andlyophilization. It was observed that magnesium hy-droxide and magnesium carbonate containing poly-mer films that had been hydrated for 2–40 days andthen freeze-dried were only partially soluble in chlo-roform. During that time period, the magnesiumcontent, determined by elemental analysis, graduallydecreased (see below). All other freeze-dried filmsamples were soluble in chloroform throughout thecourse of the experiment. Samples of magnesium car-bonate– and magnesium hydroxide–containing filmsrecovered after hydration times >40 days were com-pletely soluble in chloroform.

Effects of encapsulated salts on polymerdegradation rate

The effect of encapsulated salts on polymer degra-dation was assessed by DSC, weight loss, and molecu-

Figure 3. SEM micrographs (cross-sectional view) of ablank PLGA film, before and after hydration in HEPESbuffer at 37°C. (a) t = 0 days; (b) t = 14 days; (c) t = 22 days.

534 ZHANG ET AL.

lar weight determination. Prior to hydration, all filmsexhibited Tgs in the range of 40°–45°C; close to that ofthe starting polymer. After approximately 10 days hy-dration, Tg values for the control films decreased rap-idly, signaling the onset of polymer degradation. Infilms that contained sparingly soluble salts, decreasesin Tg took place at later times and were more gradual(Fig. 6). In the case of zinc carbonate containing films,the Tg remained above 40°C for 24 days of hydration.There was no appreciable Tg decrease in calcium car-

bonate, magnesium carbonate, and magnesium hy-droxide films after 31 days of hydration.

Tg profiles for water soluble salt-containing filmsare compared with controls in Figure 7. Magnesiumacetate–containing films displayed an increased lagphase prior to the onset of the Tg decrease; the Tg

remained above 40°C for 20–30 days. Films that con-tained magnesium sulfate, zinc sulfate, and sodiumchloride exhibited Tg versus time profiles identical tothat of the control film.

Molecular weight determination (GPC) was basedon chloroform-soluble fractions of the film samples.The degradation curves of films containing salts oflow and high water solubility are displayed in Figures8 and 9, respectively. The molecular weight of the con-trol film (Mw) decreased from an initial value of 47,000to 2,200 D over 31 days. In contrast, rates of decreasein molecular weight were significantly slower in thefilms that contained sparingly soluble salts. Respectivedegradation rates for magnesium hydroxide–, magne-sium carbonate–, calcium carbonate–, and zinc carbon-ate–containing films were 3.0, 2.4, 2.4, and 1.7 timesslower than that of the control film (Fig. 8).

Figure 4. SEM micrographs (cross-sectional view) of aPLGA film containing 10% magnesium carbonate, beforeand after hydration in HEPES buffer at 37°C. (a) t = 0 days;(b) t = 1 day; (c) t = 7 days.

Figure 5. SEM micrographs (cross-sectional view) of aPLGA film containing 5% magnesium acetate, before andafter hydration in HEPES buffer at 37°C. (a) t = 0 days; (b) t= 3 days.

535EFFECTS OF METAL SALTS ON PLGA HYDROLYSIS

Polymer degradation also progressed more slowlyin magnesium sulfate, zinc sulfate, sodium chloride,and magnesium acetate films than the control film.This effect, however, was of a lesser magnitude than inthe low soluble salt-containing films. As displayed inFigure 9, these films showed degradation rates in Mw

approximately 1.5 times slower than the control film.Weight loss measurements conducted on polymer

films correlated with degradation rates observed byDSC and GPC. For example, after 46 days, the blankfilm had undergone 68% weight loss. In contrast, thefilms that contained magnesium carbonate, magne-sium hydroxide and calcium carbonate showed only20–30% weight loss. The corresponding value for zinccarbonate containing films was 55%. The net weightloss of polymer itself was close to the total weight lossof films owing to the low solubility of these salts. Wa-ter-soluble salt-containing films displayed weight lossprofiles similar to the control film.

Salt release from polymer films

The release of salts from hydrated polymer filmsduring incubation was assessed by elemental analysis.Release rates correlated with the water solubility ofthe salts tested:

Magnesium acetate ≈ Magnesium sulfate >Magnesium carbonate ≈ Magnesium hydroxide @

Zinc carbonate

The magnitude of salt release ranged from <10% in

zinc carbonate–containing films after 22 days of hy-dration to 75% release in magnesium acetate films af-ter only 3 days and >95% release by day 28. Salt re-lease from magnesium carbonate containing films wasat an intermediate rate, with 64% released by 35 daysof hydration.

DISCUSSION

Because polypeptide drugs are essentially insolublein polymers such as PLGA, their release from polyes-ter-based dosage forms is dependent on diffusivetransport through water-filled pores within the de-vice. Mechanisms of pore formation are mainly basedon the initial morphology of the device, drug dissolu-tion, and polymer erosion. Consequently, factors thataffect rates of water uptake, pore formation, or degra-dation and erosion in bioerodible polymers can be ex-pected to be critical in determining kinetics of drugrelease from systems based on these polymers. Suchfactors include intrinsic properties of the polymer(e.g., molecular weight, polydispersity, lactide con-tent, etc.) together with macroscopic properties of thedevice. We have found that in addition to these prop-erties, the presence of encapsulated salts can have con-siderable effects on kinetics of protein drug releasefrom PLGA-based microspheres.

In this work, we have explored the effects of encap-sulated salts on the kinetics of water uptake, morpho-logic changes, polymer degradation, and weight loss

Figure 6. Time course of glass transition temperature (Tg)changes during hydration of PLGA films containing encap-sulated salts of low water solubility. Tg was determined byDSC analysis of freeze-dried film samples.

Figure 7. Time course of glass transition temperature (Tg)changes during hydration of PLGA films containing encap-sulated water-soluble salts. Tg was determined by DSCanalysis of freeze-dried film samples.

536 ZHANG ET AL.

in hydrated PLGA films. The salts evaluated variedwith respect to water solubility, the identity of thecation component, and the relative alkalinity of theanion component.

Upon hydration, blank polymer films exhibit threedistinct phases of degradation,7 beginning with an ini-tial water uptake of <3% by weight, followed by aplateau phase of little or no uptake (1–7 days) andthen a final uptake of water at a rate of 4–5% per day.The first phase consists of the initial hydration, orswelling of the polymer. This brings about the onset ofhydrolysis of ester linkages, which occurs throughoutthe second, plateau, phase and is accompanied by agradual decrease in the polymer molecular weight.Hydrolytic events occurring during the early portionof the plateau phase do not result in appreciableweight loss or erosion, since the products are them-selves insoluble in water. The third phase ensues oncedegradation has progressed to the point that water-soluble hydrolysis products are formed. At this time,erosion begins to occur, which results in pore forma-tion and accelerated uptake of water into the polymermatrix. It has been proposed8,11 that the polymer deg-radation rate increases due to an autocatalytic processwhereby protons that are evolved during ester hydro-lysis accelerate the reaction rate by acid catalysis.

The time course of water uptake and polymer deg-radation in PLGA films can be modulated by incorpo-rating salts either as particles or by codissolution inpolymer solutions prior to film casting. The effectsrange from simple changes in the water uptake profileaccompanied by no appreciable change in degradation

kinetics [as observed with sodium chloride, magne-sium sulfate, and zinc sulfate (Figs. 2 and 7)] to majoralterations in the water uptake kinetics together withseveral-fold decreases in degradation rate [as ob-served with magnesium carbonate and magnesiumhydroxide (Figs. 1, 6, and 8)].

A number of forces and processes may contribute tothe observed effects. These include 1) porosity in-duced by the presence of salt particles, 2) osmoticforces due to the ionic nature of encapsulated salts, 3)neutralization by basic salts of protons evolved duringhydrolysis of ester linkages, and 4) chemical interac-tions between ions and functional groups on the poly-mer.

The increased water uptake by all salt-containingfilms cannot be explained solely by the presence ofencapsulated particles, since films that contained inertparticles (e.g., carbon black, glass beads) exhibited wa-ter uptake profiles and degradation kinetics that wereidentical to control films. SEM did not show signifi-cant differences in initial porosity of the different salt-containing films. Thus, we speculate that the observeddifferences in water uptake behavior were not a resultof the initial porosity of the film resulting from incor-poration of salt particles. An additional driving forcetoward more rapid ingress of water is the hydration ordissolution of salt particles. The influx of water intothe polymer film sample appears to bring about theformation of a porous structure well before any ap-preciable polymer degradation has occurred.

Contrary to expectations, the increased uptake of

Figure 8. Time course of polymer degradation during hy-dration of PLGA films containing encapsulated salts of lowwater solubility. Molecular weight (Mw) was determined byGPC analysis of freeze-dried film samples.

Figure 9. Time course of polymer degradation during hy-dration of PLGA films containing encapsulated watersoluble salts. Molecular weight (Mw) was determined byGPC analysis of freeze-dried film samples.

537EFFECTS OF METAL SALTS ON PLGA HYDROLYSIS

water by encapsulated salt-containing films did notbring about an increase in the rate of polymer degra-dation as assessed by GPC or DSC. Degradation rateswere either the same or slower than rates observed inblank films.

In film samples that contain sparingly soluble saltssuch as magnesium hydroxide, magnesium carbonate,zinc carbonate, and calcium carbonate, polymer deg-radation rates were reduced 1.7–3.0 fold. The reduceddegradation rates may be a consequence of the alka-line properties of hydroxide and carbonate salts,which would be expected to disrupt autocatalytic deg-radation brought about by carboxylic acid groups gen-erated during polyester hydrolysis. As a result, de-spite higher initial water uptake levels compared tocontrol films, polymer degradation progressed moreslowly. This effect was least pronounced in zinc car-bonate containing films, presumably due to the lowerwater solubility of this salt compared to magnesiumhydroxide, magnesium carbonate, or calcium carbon-ate. Interactions of a similar nature have been ob-served between encapsulated amine salts (e.g., genta-micin sulfate) and polylactic acid blends.9

Some of the observed effects of salts on water up-take, degradation and changes in morphology, andsolubility upon hydration may also stem from chemi-cal interactions between ions and polymer functional-ities such as carboxylic acid or ester groups. Evidencesupporting this possible mechanism includes the de-crease in solubility after freeze-drying of hydratedfilms that contained magnesium hydroxide and mag-nesium carbonate. Complexion Ca2+ and Zn2+ ions topolyesters by ion–dipole bond has been described12,13;such interactions might bring about changes in reac-tivity toward hydrolysis as well as in solubility. Thechange in solubility could be reversed by incubation ofthe film sample in an aqueous EDTA solution, prior tolyophilization (data not shown). This observation isconsistent with the possible formation of inter- or in-tramolecular crosslinks via coordination with Mg2+

ions. FTIR spectral analysis of freeze-dried polymerfilm samples was not indicative of such alterations inthese functionalities. In view of the relatively low con-centration of chain ends and metal ions in the system,however, this finding cannot be considered proof thatsuch interactions do not occur.

It is likely that the effects observed in this studyarise from a combination of some or all of the mecha-nisms discussed above. The relative contributions ofthese processes are no doubt dependent on the natureof the encapsulated salt. Modulation of water uptake,porous structure formation, and kinetics of polymerdegradation and erosion can be expected to affect therelease kinetics of polypeptide drugs from polymer-

based delivery systems. Coencapsulation of salts, ei-ther as particles or via codissolution in the polymerphase, together with the drug substance to be deliv-ered, can supplement existing parameters (polymertype and Mw, drug loading, device geometry, etc.) formanipulating release kinetics from drug delivery sys-tems.

References

1. L. Duun and R. M. Otterenbrite, eds., Polymeric Drugand Drug Delivery Systems, ACS Symposium Series 469,1990, pp. 11–23.

2. L. M. Sanders, G. I. McRae, K. M. Vitale, and B. A. Kell,‘‘Controlled delivery of an LHRH analogue from bio-degradable injectable microspheres,’’ J. Control. Rel., 2,187–195 (1985).

3. E. J. Frazza and E. E. Schmitt, ‘‘A new absorbable su-ture,’’ J. Biomed. Mater. Res. Symp., 1, 43–58 (1971).

4. F. G. Hutchinson and B. J. A. Furr, ‘‘Biodegradablepolymer systems for the sustained release of polypep-tides,’’ J. Control. Rel., 13, 279–294 (1990).

5. C. G. Pitt, M. M. Gratzl, A. R. Jeffcoat, R. Zweidinger,and A. Schidler, ‘‘Sustained drug delivery systems: II.Factors affecting release rates from poly(e-caprolac-tone) and related biodegradable polyesters,’’ J. Pharm.Sci., 68, 1534–1538 (1979).

6. M. D. Coffin and J. W. McGinity, ‘‘Biodegradable pseu-dolatexes: The chemical stability of poly (D,L-lactide)and poly (e-caprolactone) nanoparticles in aqueous me-dia,’’ Pharm. Res., 9, 200–205 (1992).

7. S. S. Shah, Y. Cha, and C. G. Pitt, ‘‘Poly(glycolic acid-co-DL-lactic acid): Diffusion or degradation controlleddrug delivery?’’ J. Control. Rel., 18, 261–270 (1992).

8. M. Vert, S. Li, and H. Garreau, ‘‘More about the deg-radation of LA/GA-derived matrices in aqueous me-dia,’’ J. Control. Rel., 16, 15–26 (1991).

9. J. Mauduit, N. Bukh, and M. Vert, ‘‘Gentamicin poly-lactic acid blends aimed at sustained release local anti-biotic therapy administered per-operatively. III. Thecase of gentamicin sulfate in films prepared from highand low molecular weight poly-DL-lactic acids,’’ J. Con-trol. Rel., 25, 43–49 (1993).

10. B. G. Amsden, Y. Cheng, and F. A. Goosen, ‘‘A mecha-nistic study of the release of osmotic agents from poly-meric monoliths,’’ J. Control. Rel., 30, 45–46 (1994).

11. E. A. Schmitt, D. R. Flanagan, and R. J. Linhardt, ‘‘Im-portance of distinct water environments in the hydro-lysis of poly(DL-lactide-co-glycolide),’’ Macromolecules,27, 743–748 (1994).

12. R. N. Reusch and H. L. Sadoff, ‘‘Putative structure andfunctions of a poly-b-hydroxybutyrate calcium poly-phosphate channel in bacterial plasma membranes,’’Proc. Natl. Acad. Sci. USA, 85, 4176–4180 (1988).

13. T. Araki, S. Hayase, and M. Iida, ‘‘Selective synthesis ofstructurally isomeric poly(b-ester) and poly(d-ester)from b-(2-acetoxyethyl)-b-propiolactone,’’ J. Polym. Sci.Polym. Lett. Ed., 16, 519–523 (1978).

Received June 12, 1995Accepted May 3, 1996

538 ZHANG ET AL.