Biomaterials 22 (2001) 547}554

Potassium ion release from a glass ionomer cement matrix

J.A. Williams!,*, R.W. Billington!, G.J. Pearson"

!Department of Biomaterials, Eastman Dental Institute, 256 Gray+s Inn Road, London WC1X 8LD, UK"Deptartment of Biomaterials, St. Bartholomew+s and Royal London Medical & Dental School, Queen Mary and Westxeld College,

Mile End Road, London E1 4NS, UK

Received 7 January 2000; accepted 13 June 2000

Abstract

Glass ionomer was mixed with water alone and with water containing two levels of potassium ion, added as potassium chloride.The dimensions of the specimens varied; discs, cylinders and a rectangular bar were used. Release of potassium was measured until itceased. A little potassium was released from cement mixed with water but could be accounted for by traces present in the glass. Releasetook 2 months for the lower level and 1.5 years for the higher level of addition. For some specimen groups not all the potassium addedwas subsequently released. Potassium release tended towards the pattern found for #uoride release naturally occurring from glassionomer cements in being proportional to t1@2 but was less certain at the lower addition. Release was not related to the dimensions ofthe sample as has been shown for release of intrinsic #uoride. ( 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Glass ionomer; Potassium release; Dimensions; Addition to matrix

1. Introduction

The ability of glass ionomer cements to release #uoridehas been a signi"cant factor in their use in dentistry.Many investigations of #uoride release have been carriedout both in vivo and in vitro using a range of glassionomer cements. However, #uoride is not the only ion tobe released. Many of the glasses used contain othermonovalent ions, such as sodium, in addition to divalentions such as calcium and the trivalent ion of aluminium.While the latter ions take part in the crosslinking reac-tion which causes acid}base glass ionomer cements toset, the role of monovalent cations is ill de"ned. Althoughthere may be interactions between them and other com-ponents of the matrix they are comparatively looselybound and come to be distributed throughout the matrix,possibly homogenously, as in the case of #uoride [1].They may be removed from it when the cement is sur-rounded by an aqueous medium [2] although the evid-ence that this follows the same pattern as #uoride releaseis inconclusive [3,4]. Replacement or addition of ions canoccur, either internally from the glass or from externalsources as is the case with #uoride ion [4}6]. It has been

*Corresponding author. Tel.:#44-207-915-1133; fax:#44-207-915-1133.

shown [7] that adding #uoride to a glass ionomer matrixcan alter the di!usion processes. The intrinsic #uoriderelease dependent upon surface area alters initially al-though as time progressed this dependence returned.This indicates that the ion release process may not al-ways be one of simple Fickian di!usion and is easilyperturbed.

Potassium was selected as the cation for this studysince it has the same valency as sodium and a similarionic radius [8] to the #uoride ion present in many glassionomer glasses. Sodium is already present in the com-mercial glass ionomer cement used in this study as it is inthe majority of cements and release of this ion could havebeen measured. However, one study [3] has found thatsodium release from a cement of similar composition tothat used in this study, unlike that of #uoride ion, was notrelated to the square root of time. Adding potassium inknown quantities to this cement and monitoring thesubsequent release would enable, it was hoped, a betterunderstanding of the behaviour of monovalent cations.

The aims of this study were to

f "nd the release pattern for potassium added to thematrix of a commercially available glass ionomer ce-ment,

f "nd whether the release pattern was a!ected by theamount of potassium added in the form of potassium

0142-9612/01/$ - see front matter ( 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 2 1 2 - X

Table 1Dimensions of samples used in the study

Ref. Type Diameter(mm)

Height(mm)

Length(mm)

Area(mm2)

Volume(mm3)

Number ofspecimens

A1 Cylinder 4.0 6.0 100 75 3A4 Disc 14.0 1.0 352 154 3

B1 Cylinder 4.0 6.0 100 75 3B2 Bar 2.0 2.0 24.0 184 88 3B3 Cylinder 6.0 12.0 283 339 3B4 Disc 14.0 1.0 352 154 3B5 Disc 20.0 1.5 722 471 3B6 Disc 22.5 1.0 865 397 3

C2 Bar 2.0 2.0 24.0 184 88 3C3 Cylinder 6.0 12.0 283 339 3C4 Disc 14.0 1.0 352 154 3C5 Disc 20.0 1.5 722 471 3

chloride, and investigate the e!ect of sample dimen-sions on total release.

2. Method

2.1. Specimen preparation

A water mixed, conventional (acid}base reaction) glassionomer material, OpusFil LR 92/06 (Schottlanders,Letchworth, UK). This material comprised an alumino-#uorosilicate glass blended with polyacrylic and tartaricacids. It was mixed at the powder/water ratio of 7.0/1.0given by the manufacturers instructions, using weighedamounts to provide greater accuracy. Metal moulds wereused to prepare samples of discs, cylinders and a bar. Thedimensions and numbers of specimens are given inTable 1.

Three sets of samples were prepared. The "rst (A)contained no added potassium. The second (B) wasmixed with water containing 100 mg l~1 potassium (po-tassium chloride Analar grade, BDH Ltd., Poole, UK).The third (C) was made by mixing with water containing1000mg l~1 potassium ion. The amount of potassiumadded to specimens in sets B and C could be calculated.

2.2. Immersion procedure

The cement was packed into the moulds within 2minof the start of mixing and clamped between acetate sheetand metal plates for 60min at 373C and high humidity.At 60min from the start of mixing the specimens wereremoved from the moulds. Any loose particles were re-moved and each specimen immediately placed in 10ml ofdeionised water in a lidded plastic container and main-tained at 373C. The deionised water was changed atintervals of 1 h, 1, 7, 15, 21 and 28 days and then at 2}4week intervals until no further release occurred.

Deionised water changed at the same time periods asthose surrounding the specimens acted as blanks to de-termine background levels of potassium. In order toreduce potential contamination during the course of thestudy all water was stored in plastic containers and thatused for the blanks changed before those of the speci-mens.

The pH of the water in which the specimens wereimmersed was measured before and after contact usinga combination pH electrode bu!ered at pH 7.0 and 4.0(Orion, MA, USA).

2.3. Potassium analysis

A Varian A10 atomic absorption spectrometer (VarianLtd., Birchwood Science Park, Cheshire WA3 7BM, UK)was calibrated with standard solutions containing 0.2,0.5, 1.0, 5.0, 10.0mg l~1 potassium ion. The concentra-tion of potassium in the experimental solutions was givendirectly by the instrument computer programme. Thecalibration was checked using a standard solution be-tween every 10}15 samples. The median concentration ineach set of solutions was calculated. Subtraction of thebackground level of potassium then gave an incrementalmeasure of release. The cumulative potassium releasewas calculated.

3. Results

The cumulative potassium release against time isshown in Figs. 1}3 for sets A}C. Incremental results aregiven in Table 2.

3.1. Blanks

After 120 and 875 days the total amount of potassiumfound in the water alone was 1.00 and 1.83 mg l~1,

548 J.A. Williams et al. / Biomaterials 22 (2001) 547}554

Fig. 1. Potassium release for set A (no added potassium).

Fig. 2. Potassium release for set B (mixed with water containing100mg l~1 potassium).

Fig. 3. Potassium release for set C (mixed with water containing1000mg l~1 potassium).

respectively. The incremental amounts found at eachtime interval varied from 0.00mg l~1 to a maximum onone occasion of 0.33mg l~1 with an average of0.08mg l~1. Any variation in results appeared to be ran-domly distributed with time.

3.2. Release from samples in set A (no added potassium)

After subtraction of the blank readings it was foundthat only small amounts of potassium were released fromsets A1 and A4. Essentially no potassium was releasedfrom A1 after the "rst day. The cumulative maximum forA4 was 0.39 after 120 days. Thereafter, there was nosigni"cant di!erence between the amount of potassiumfound in the blank or sample #uids. There was no rela-

tionship between release and time or time t1@2 for A1.Release for A4 was proportional to t1@2 after an initialperiod of less than one day.

3.3. Potassium release from sample in set B(100 mg l~1 K`)

After subtraction of blank readings the potassium re-lease from these groups was also low but when specimensof similar sizes were compared to those in set A it couldbe seen that potassium release had increased. At 64 days,the time of maximum release, B1 had released 0.36mg l~1

(compared with 0.14mg l~1 for A1) and B4 had released0.38mg l~1 (A4 released 0.31mg l~1). At the peak ofrelease the amounts released from B1 to 6 ranged from0.2 to 0.6mg l~1. Only B4 seemed to show a relativelysteady rise over the period of 120 days and only for thisgroup was release was linear with respect to t1@2.

The cumulative amount released at various times, suchas at 7, 64 and 120 days, did not depend on eitherspecimen surface area or volume and was con"rmed bythe low values of the correlation coe$cients of!0.18 to0.81 (surface area) and!0.62 to 0.52 (volume).

Since specimen dimensions had no in#uence, theamounts released (mg l~1) were pooled to increase thesample size and the mean calculated (Fig. 2). The best "tplot for the pooled mean was that of a power regressionwhere ln release (mg l~1)"!2.06#0.40 ln (day)1@2.

3.4. Potassium release from samples in set C(1000 mg l~1 K`)

For this group potassium release was less erratic andlasted for 1.5 year. After this time release appeared to be

J.A. Williams et al. / Biomaterials 22 (2001) 547}554 549

Table 2Incremental potassium release (mg l~1) (control subtracted)

Time (d) A1 A4 B1 B2 B3 B4 B5 B6 C2 C3 C4 C5

0.04 !0.010 0.060 0.070 0.245 0.020 0.030 0.015 0.035 0.060 0.175 0.120 0.2701.04 0.135 0.165 0.135 0.155 0.085 0.235 0.115 0.070 0.285 0.325 0.150 0.1807 !0.095 0.040 !0.070 !0.110 !0.025 0.095 0.025 !0.005 0.060 0.065 0.140 0.075

15 !0.055 0.045 0.000 !0.105 0.020 0.015 !0.035 0.215 0.050 0.090 0.234 0.13022 0.045 0.080 0.240 !0.020 0.170 0.121 0.040 0.07529 !0.055 !0.059 !0.005 0.045 !0.030 !0.059 !0.045 0.005 0.110 0.055 0.110 0.15036 0.005 0.000 0.000 0.170 0.045 0.020 0.015 0.01553 0.010 0.025 0.020 0.030 0.020 0.025 0.065 0.085 0.025 0.085 0.040 0.04564 0.040 0.020 !0.020 0.035 0.035 0.035 0.07582 0.020 !0.065 !0.105 !0.050 0.115 !0.090 !0.135 0.095 0.095 0.140 0.17094 0.020 !0.060 !0.150 !0.065 0.085 0.050 !0.070

110 0.020 !0.070 !0.010 !0.070 0.060 !0.025 0.015120 0.018 !0.050 0.000 !0.040 0.620 0.270 0.165 0.125 0.155 0.135 0.110180 0.000 0.260 0.540 0.225 0.125231 0.495 0.510 0.445 0.085273 !0.010 0.025 0.170 0.000337 !0.020 0.150 0.270 0.070405 0.192 0.288 0.298 0.093475 0.000 0.135 !0.095 0.020547 0.150 0.105 0.085 0.065875 !0.005 0.005 !0.005 !0.005

Table 3Total amount of potassium added compared to maximum released (blank subtracted)

Specimen Specimen Liquidvolume area(mm3) (mm2) Water 100mg l~1 potassium 1000mg l~1 potassium

Weightadded (lg)

Weightreleased (lg)

Weightadded (lg)

Weightreleased (lg)

% released/added

Weightadded (lg)

Weightreleased (lg)

% released/added

75 100 0 (A1) 1.4 1.9 (B1) 3.6 190116 (B1!A1)

88 184 2.2 (B2) 3.1 140 22 (C2) 18 79339 283 8.5 (B3) 3.4 40 85 (C3) 27 31154 352 0 (A4) 3.9 3.9 (B4) 8.2 210

110 (B4!A4)38 (C4) 25 64

56 (C4!A4)471 722 11.8 (B5) 4.2 35 118 (C5) 16 14397 865 9.9 (B6) 6.4 65

complete with no signi"cant quantities released between1.5 and 2.4 year. For all specimens sizes release tendedtowards the pattern seen for #uoride release of an initialnon-linear phase followed by linear release against t1@2.The cumulative release ranged from 1.5 to 2.8mg l~1,higher than that from specimens mixed with 100mg l~1

potassium by a factor of 4}7.Again there was a low correlation between release and

specimen dimensions. At 7, 120 and 547 days forexample, correlation coe$cients were 0.42}0.76 for sur-face area and 0.76}0.86 for volume. Pooling the resultsfor all groups produced a best "t plot for the mean ofrelease (mg l~1)"0.17#0.09 (day)1@2 with a correlationcoe$cient 0.986. Correlation with time was poorerat 0.96.

3.5. Comparing the amount of potassium addedto that released

The amounts of potassium ion added to each specimencould be calculated knowing the individual weights(Table 3). For the smaller specimens it appeared that alladded potassium had been released. For larger specimensless potassium was released than was added.

3.6. Acidity of the immersion yuid

The pH of the water in which the specimens wereimmersed became slightly more alkaline during contactwith glass ionomers which were immersed within 24 hfrom preparation. During this time pH increased from

550 J.A. Williams et al. / Biomaterials 22 (2001) 547}554

pH of 5.2}6.0 to 6.0}6.7. Thereafter little variation wasseen with the pH remaining at 5.2}6.0. Adding potassiumto the water did not change pH signi"cantly.

4. Discussion

Analysis of the blank liquids showed the presence ofsmall amounts of potassium in the blanks occurringrandomly throughout the study. Potassium may havebeen present in the distilled water used but since this wasused for cement production and for immersing the speci-mens any e!ects would be subsequently minimised bysubtraction of the blanks. Some experimental error mayoccur during the potassium measurement in spite offrequent checking with standards.

Specimens without added potassium (set A) showedsmall quantities of potassium were released from thecement, mainly during the "rst 24 h. This could be ac-counted for by traces of potassium in the glass. Analysisshowed it to contain less than 0.05% potassium but onlysome 3% of the glass need be converted into soluble,leachable, potassium to produce the amount of potassi-um measured.

The use of mixing water containing 100mg l~1 potassi-um caused slightly increased levels of release. Only one ofthe six groups showed the dependence on t1@2 showntypically by #uoride release from glass ionomer cementsand indicative of a di!usion-controlled mechanism [9].However, the use of the pooled mean found release tofollow a logarithmic dependence with a slightly highercorrelation with t1@2 compared with time. Cements withcopious amounts of #uoride within the glass itself mayhave a reservoir for additional supplies to the matrix,a process denied to the material in the current study. Thisresult may indicate that at this level of addition ionrelease is beginning to establish the pattern of depend-ence on t1@2 and that a certain minimum level of a par-ticular ion is required to produce the type of releasedependent on di!usion. It may also explain why therewas a variation in the type of release obtained for indi-vidual groups.

Increasing the concentration in the mixing liquid from100 to 1000mg l~1 produced a release pattern tendingtowards simple Fickian di!usion. The use of the pooledmean showed release to be dependent on t1@2 with a cor-relation coe$cient of 0.99, higher than that for releaseversus t. This may be due to a higher concentrationgradient and it is possible that even higher additionswould also produce release related to t1@2. Specimens ofthe same size as those in set B4 made from the samecement released 184mg l~1 #uoride after 10 days [7]. Toplace the equivalent amount of potassium in the cementmatrix would require a solution containing 100 g l~1 ofpotassium ion (2.56 mol l~1). While beyond the limit ofsolubility of many potassium salts it would be possible to

use potassium chloride (soluble up to a concentration of4.6mol l~1), in the unlikely event of solutions of thisconcentration producing no disruption of the glassionomer and/or the setting mechanism.

Cements with high additions of cations are known butthese have been produced by immersing cements ina liquid rather than by additions via the mixing process.The mechanism by which this occurs is not known andthe location of the potassium may possibly be somewhatdi!erent if added by immersion rather than addition,although it would be expected that in both cases potassi-um would reside within the cement matrix.

The patterns of release for both sodium and potassiumions in a number of studies, where cations have beenadded either by admixture or immersion or where thecation is one which is present in the glass, were comparedwith the present study. Wilson et al. [2] measured so-dium release from a cement containing 1.3% sodium, thision being a component of the glass. Two specimen di-mensions were used, equal to those of B5, C5 and B3, C3used in the present study. It was found that re-lease"k#at1@2#bt, a power regression, related totime t where k, a, b are constants which di!ered for thetwo sizes of specimens.

A study by Hadley et al. [3] which investigated sodiumrelease and re-release from cements made from four dif-ferent glasses concluded that neither release, nor re-re-lease following immersion, was dependent on either timeor t1@2. The concentration of sodium #uoride used for theimmersion process was 2 g l~1, equivalent to 1095mg l~1

sodium. One of the glasses, MP4, was also used byWilliams et al. [4] to investigate release from cementswith potassium added either by admixture or immersion.Data from this experiment are later referred to as Will-iams (mixed) and Williams (immersed) in Fig. 4. Thesolution used contained 1028mg l~1 potassium. Follow-ing admixed addition the release was found to be linearlydependent on t1@2 whereas when immersed the sub-sequent release was time dependent.

Using still higher concentrations of potassium,5000mg l~1, Billington et al. [10] concluded that potassi-um release from cements after immersion in this solutionhad an initial non-linear release phase lasting for some 10days after which release was time dependent. Data from thisexperiment are designated Billington (immersed) in Fig. 4.

The potassium release studies are compared (Fig. 4)plotting release as ng mm~2, to attempt to minimizee!ects of the various sample sizes, against t1@2. Four setsof results including set C from the present study wereused. Since the correlation coe$cients for sets A andB were low these were not included. The y-axis is logar-ithmic to allow for the wide range of amounts released.Four types of equation were tried to best "t the releasepatterns, namely:

Trend release " a#bt1@2 or a#bt.

J.A. Williams et al. / Biomaterials 22 (2001) 547}554 551

Fig. 4. Comparisons of potassium release after addition by admixtureand immersion for four separate studies.

Table 4Relationships between potassium release and time

Series Best "t equation versus t1@2 (day)1@2 Best "t equation versus time (day)

Type a b Correlationcoe$cient

Type a b Correlationcoe$cient

C Trend 0.439 0.287 0.974 Trend 1.247 0.014 0.955Williams (mixed) Trend 0.484 0.393 0.993 Trend 1.526 0.019 0.937Williams (immersed) log reg 62.90 76.15 0.948 log reg 62.90 38.08 0.823Billington (immersed) Trend 386.2 95.35 0.945 log reg 73.15 309.94 0.840

Exponential ln release " a#bt1@2 or a#bt.

Logarithmic regression release "a#b ln t1@2

or a#b ln t.

Power regression ln release"a#b ln t1@2 or a#b ln t.

The values of a, b and the correlation coe$cients withboth t1@2 and time are given in Table 4.

For the two sets of results where the potassium wasadded by admixture the trend plot "tted well with cor-relation coe$cients of 0.97 and 0.99 for t1@2. Set C com-pared very closely with the Williams (mixed) results interms of the amounts released and the pattern of release.Although two di!erent potassium-free glasses were used,

both were admixed with 1000mg l~1 potassium. Thiswould imply that in this instance the type of glass haslittle e!ect on ion release from the matrix.

There were two studies where potassium was added byimmersion. That where 1000 mg l~1 potassium solutionwas used, Williams (immersed) best "tted a log regressionequation dependent on t1@2 while that using 5000mg l~1,Billington (immersed) had a trend plot dependent on t1@2.Again two di!erent glasses were used but unlike theaddition by mixing, the pattern di!ers for each cement.The method of addition may a!ect the chemical interac-tion as well as the location and amounts of ion. Potassi-um added by immersion must of neccessity be added toan already set cement whereas that added during themixing process is present when setting occurs. After im-mersion in potassium #uoride solution potassium hasbeen shown to reside very much deeper within thematrix, at a depth greater than 13lm, than #uorideadded by the same method [11] which is concentratedwithin 3 lm of the surface. Potassium in this case is notevenly distributed in the matrix as it should be whenadded by mixing. The location of an ion within thecement may a!ect its subsequent di!usion character-istics.

An equation of de Moor et al. [12] describing therelease of #uoride from glass ionomer cement could notbe applied to the present study since it relies upon releasebeing dependent on t1@2 and for the initial release to occurover several days.

In sets B and C increasing the amount of potassiumalso extended the time over which release occurred. ForB release had been essentially completed in 64 dayswhereas C continued for 18 months. It may be coincid-ence that the ratio of the concentration increase (10.0)was similar to the ratio of release times (8.5). The study byWilliams et al. [4] using MP4 glass mixed with watercontaining 1000mg l~1 potassium also found potassiumrelease to occur for at least 500 days. It may be concludedon the basis of these two studies that potassium is re-leased for prolonged periods. This is in contrast to an-other cation, sodium, which appears to have shorterrelease times [3]. It is possible that this is a function ofionic size, sodium having a smaller ionic radius

552 J.A. Williams et al. / Biomaterials 22 (2001) 547}554

(0.095nm) than potassium (0.133nm). It is possible thatin a cement where approximately 12.5% water is presentthe ions are hydrated to a certain, but unknown, extent.In solution sodium would have a larger hydrated radius(0.24nm) than potassium (0.17 nm) [13] and a largerStokes radius, 0.236 and 0.121 nm, respectively [14]. Flu-oride ion has a Stokes radius of 0.168 nm. The e!ect ofhydration is to reduce the speed at which ions move insolution with potassium having a greater velocity thansodium. This however seems to disagree with the ob-served times taken for sodium and potassium release and,therefore, hydation may either not occur to the sameextent as it would in solution, or, the hydration is modi-"ed by the presence of other constituents. Chief amongthese would be the polyacrylic acid whose carboxylicacid groups would also be expected to attract spheres ofhydration.

Fluoride release from cements using a #uoride-con-taining glass has also been shown to be related to speci-men surface area [7]. This relationship did not occurwhen #uoride was added to the matrix of the samecement. Possibly #uoride addition a!ects the concentra-tion gradients within the cement matrix. Fluoride levelsmight decrease as the distance from a glass particle in-creases. Alternatively #uoride ion in the glass surfacemight be dissolved on contact with the polymeric acidsand be distributed throughout the matrix. Increasing#uoride ion in the matrix may then reduce di!usion fromthe glass and once those higher levels were depleted byleaching there may be a lag before they could be replaced.In the present study with potassium added to the matrixonly (group B or C) potassium release was not related tospecimen surface area or volume. Although only threespecimens were used for each set of di!erent dimensionthis was su$cient to show the relationship for #uoride[7]. It may be concluded that release of an monovalention added to the cement matrix whether positively ornegatively charged is not dependent upon sample dimen-sions. The questions then are whether release from thematrix di!ers from that of the hydrogel/glass particlesand whether overall release is a combination of bothmechanisms.

Since potassium was added via the mixing liquid, a cal-culation could be made of the amount of ion added to thecement. Table 4 compares the ratio of added ion toreleased ion at 64 days (the maximum for 100 mg l~1)and at 18 months for 1000mg l~1. Although B1, B2 andB4 appeared to release more than was added, if theamounts released by A1 and A4 are subtracted the re-lease reduces to approximately 100%. It may be con-cluded that all the potassium added to these smallersamples had been released in 64 days. For specimens ofhigher weight (B3, 5, 6) and in set C the ratio of ad-ded/released potassium was substantially less than100%. Incomplete potassium release is not unusual.A study using a glass free of both potassium and #uoride

[4] found that adding 40lg of potassium resulted in therelease of only 31lg. Possibly potassium displaces so-dium in the matrix or reacts with the hydrogel or resides,as suggested earlier, at a greater depth in the matrix. Itseems that some potassium may be permanently retainedsince release was complete after 550 days and this may bedue to the larger ionic radius of potassium.

The processes by which cations are transferred fromwithin a glass ionomer cement to a surrounding aqueousmedium may be more complex than that of #uoridepossibly due to a di!erent position within the matrix andthe positive ionic charge. Since the majority of ion releasestudies have investigated #uoride release from commer-cially available glass ionomer cements it is also possiblethat the simple Fickian di!usion used to describe #uoriderelease covers only one specialised region of release andthat a range of equations may be needed to cover alleventualities.

5. Conclusions

1. A certain minimum, but low, level of potassium of theorder of 10~3}10~4% was required before releasebecame dependent on square root of time.

2. The time taken for potassium release to be completedincreased with the level of added potassium.

3. When potassium chloride was added to the matrix ofa cement the subsequent potassium release was notrelated to specimen surface area or volume.

4. Not all the potassium added was subsequently re-leased. Larger specimen sizes appeared to retain somepotassium.

5. Potassium release appears to di!er from sodium release.

References

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[8] Stark JG, Wallace HG. Chemistry data book, 2nd ed in SI. UK:John Murray Ltd., 1982. p. 28.

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[10] Billington RW, Williams JA, Pearson GJ. Glass ionomer as a re-chargeable reservoir for physiologically active ions. Fourth WorldBiomaterials Congress, Berlin, 1992, abstract no. 213.

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