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Development of Cutaneous Bioadhesive Ureasil-Polyether Hybrid Films

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The hydrolysis and condensation reactions involved in synthesis of ureasil-polyether films influence the film formation time and the number of chemical groups able to form hydrogen bonds, responsible for the bioadhesion, with the biological
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  Research Article Development of Cutaneous Bioadhesive Ureasil-PolyetherHybrid Films João Augusto Oshiro Junior, Flávia Chiva Carvalho, Christiane Pienna Soares,Marlus Chorilli, and Leila Aparecida Chiavacci Pharmaceutical Sciences School of S˜ao Paulo State University (UNESP), Araraquara-Ja´u Interstate Highway,Km 󰀱, 󰀱󰀴󰀸󰀰󰀱-󰀹󰀰󰀲 Araraquara, SP, Brazil  Correspondence should be addressed to Leila Aparecida Chiavacci; leila@car.unesp.brReceived 󰀸 December 󰀲󰀰󰀱󰀴; Accepted 󰀲󰀶 January 󰀲󰀰󰀱󰀵Academic Editor: Alenka VeselCopyright © 󰀲󰀰󰀱󰀵 Jo˜ao Augusto Oshiro Junior et al. Tis is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the srcinal work isproperly cited.Te hydrolysis and condensation reactions involved in synthesis o ureasil-polyether 󿬁lms in󿬂uence the 󿬁lm ormation time andthe number o chemical groups able to orm hydrogen bonds, responsible or the bioadhesion, with the biological substrate.Te objective o this work was to study the in󿬂uence o the use o an acid catalyst (hydrochloric acid) and a basic catalyst(ammonium 󿬂uoride) in the hydrolysis and condensation reactions on the time ormation and bioadhesion o ureasil-polyether󿬁lms. Te toxicity o the 󿬁lms was evaluated. Te M assay has shown cell viability o human skin keratinocytes higher than󰀷󰀰% o all analyzed materials suggesting low cytotoxicity. Te bioadhesion o the 󿬁lms is strongly dependent on the viscosity and hydrophilic/hydrophobic balance o the polyether chains used to synthetize the hybrid molecules. Te use o acid catalystpromotestheormationolessviscous󿬁lmswithhigherbioadhesion.TehybridsormedbymorehydrophilicPEOchainsaremorebioadherent,sincetheycaninteractmoreefficientlywiththewaterpresentinthestratumcorneumincreasingthebioadhesion.Dueto their low toxicity and high bioadhesion, the ureasil-PEO 󿬁lms obtained by using HCl as catalyst agent are good candidates orapplication to the skin as bioadhesive 󿬁lms. 1. Introduction Several types o polymeric materials have been studied overthe last decade, in order to orm bioadhesive 󿬁lms orcontrolled release o drugs through the skin [󰀱, 󰀲]. Tese devices can improve the bioavailability o drugs that sufferpresystemic hepatic metabolism when administered orally and consequently optimize the pharmacotherapy, since they can maintain the correct concentration in the plasma, thusnot requiring multiple daily administrations [󰀳]. Bioadhesive󿬁lms can be applied to a variety o drugs, are o low costand easy to apply, and allow immediate discontinuation o treatment or some side effect.For polymeric materials to be employed as 󿬁lms orcutaneous applications, they must show bioadhesion, whichisthepropertyoadheringandremainingincontactwiththeskin or an extended period o time [󰀴, 󰀵]. Bioadhesive 󿬁lm ormers can also be used to control drug release and or thispurposethematerialsmusthavetheollowingcharacteristics:(a) sufficient chemical groups capable o orming hydrogenbonds (OH and COOH, e.g.) to the biological substrate;(b) high molecular mass, which is related to the size o the polymer chain; (c) high 󿬂exibility o the polymer chain,because low 󿬂exibility leads to wrinkling o the 󿬁lm on theskin, decreasing its bioadhesiveness; and (d) suitable suracetension to induce the diffusion o the drug through the layero epithelial tissue [󰀶, 󰀷]. Many polymers have been investigated or use asmucoadhesive systems, but there are ew reports in theliterature about the mechanisms involved in the process o bioadhesion when the biological substrate is skin. A study involving materials based on polyethylene oxide demon-strated that unctional groups such as COOH are strongly correlated with the bioadhesive process on the oral mucosa,sincehighmoistureatthesiteavourstheormationohydro-genbondsbetweenwaterandCOOHgroups,whichincrease Hindawi Publishing CorporationInternational Journal of Polymer ScienceVolume 2015, Article ID 727324, 7 pageshttp://dx.doi.org/10.1155/2015/727324  󰀲 International Journal o Polymer Science WaterLipidsStratum corneumFilmSkinCytoplasmFilmCell membraneHybrid ureasil-PEOHybrid ureasil-PPOH 3 CCH 2 OH 3 CCH 2 OHO Si NH NH NH NHCOOOOOCH 2 CH 3 OCH 2 CH 3 CH 3 CH 2 OH n ⌊⌈⌋⌉  SiCH 3 CCH 2 OH 3 CCH 2 OHO Si NH NH NH NHCCOOOOCH 2 CH 3 OCH 2 CH 3 CH 3 OH n ⌊⌈⌋⌉ SiH 3 C F󰁩󰁧󰁵󰁲󰁥 󰀱: Te structural ormulas o the molecules o ureasil-PEO and ureasil-PPO hybrid materials and the possible sites o bonding withthe biological substrate, highlighted in the blue circles. bioadhesion.However,theoralmucosaisverydifferentromthe skin [󰀸]. Te stratum corneum has a natural moistureactor (NMF) caused by water-soluble compounds that havethe property o absorbing water rom the atmosphere andholding it in the deeper layers o stratum corneum, to keep ithydrated [󰀹]. Tis water in the stratum corneum could alsomake hydrogen bonds with the COOH groups, promotingbioadhesion.Te ureasil-polyether hybrid materials are polymersthat are constituted by poly(ethylene oxide) (PEO) andpoly(propyleneoxide)(PPO)chainsandbysiloxanemoieties.Te structural ormulas o the molecules o ureasil-PEOand ureasil-PPO hybrid materials are shown in Figure 󰀱,revealing the presence o sufficient chemical groups able toorm hydrogen bonds (-OH, -COOH) with the biologicalsubstrate. Te possibility o modiying the molecular weightto control the size and 󿬂exibility o the polymer chainmakestheureasil-polyetherhybridsagoodcandidatetoormbioadhesive 󿬁lms.Previous studies [󰀱󰀰–󰀱󰀲] showed that these ureasil- polyether hybridmaterialsareusually obtainedby hydrolysisand condensation reactions o a unctionalized polyetherbased on PEO or PPO, catalyzed by hydrochloric acid (HCl).However, reports in the literature also show the possibility o using basic catalysis to produce these materials. Te typeo catalysis used can alter the structural organization o the󿬁lm, leading to changes in the bioadhesive orces and in theaesthetic characteristics o the 󿬁lm [󰀱󰀳].Te objective o this work was to control the hydrol-ysis and condensation reactions during the ormation o biocompatible 󿬁lms composed o ureasil-PPO and ureasil-PEO hybrid materials, with the aim o producing strongly bioadhesive systems to act as skin and wound protectors andpotential drug delivery systems or dermal application. Forthis purpose our different polymers (󰀲 hydrophilic and 󰀲hydrophobic) with differing molecular weights were used toassess the in󿬂uence o hydrophilic/hydrophobic balance onthecharacteristicsothebioadhesivematerials.Tein󿬂uenceo the type o catalysis on the visual characteristics andbioadhesive properties o the 󿬁lms was assessed by usinghydrochloricacid(HCl󰀲M)andammonium󿬂uoride(NH 4 F󰀰.󰀱M) as catalytic agents. 2. Materials and Method 󰀲.󰀱. Preparation of Ureasil Hybrid Materials.  Te ureasil-polyether hybrid materials were synthesized by the well-knownsol-gelprocess[󰀱󰀵,󰀱󰀶].Aprecursorwaspreparedrom a unctionalized polyether, based on poly(ethylene oxide)(NH 2 -PEO-NH 2 )(Mw󰀵󰀰󰀰–󰀱󰀹󰀰󰀰gmol/L)andpoly(propyleneoxide) (NH 2 -PPO-NH 2 ) (Mw 󰀴󰀰󰀰–󰀲󰀰󰀰󰀰gmol/L), addinga modi󿬁ed alkoxide, 󰀳-(isocyanatopropyl)-triethoxysilane(IsorEOS) (molar ratio o the polymer:alkoxide = 󰀱:󰀲).Te sources o these materials were obtained rom Sigma-Aldrich. Tis solution is normally made up in tetrahy-drouran (HF) [󰀱󰀰, 󰀱󰀱], but in this study ethanol was used as solvent, to minimize toxic effects, and the solutionremained under re󿬂ux or 󰀲󰀴h at a temperature o 󰀶󰀰 ∘ C[󰀱󰀷], to promote the ormation o the hybrid precursor(EtO) 3 Si-(CH 2 ) 3 NHC(=O)NHCHCH 3 CH 2 -(polyether)-CH 2 CH 3 CHNH(O=)-NHC(CH 2 ) 3 Si(OEt) 3  [󰀱󰀰].Subsequently the solvent was removed by heating under reduced pressureto orm the hybrid precursor. Tis hybrid precursor wassubjected to hydrolysis and condensation reactions [󰀱󰀸], pro-moted by adding ethanol and water and catalyst. wocatalysts were tested, HCl at a concentration o 󰀲M andNH 4 F at a concentration o 󰀰,󰀱M. During these reactionsthe OH groups were progressively eliminated, leading tothe ormation o the 󿬁lms (ureasil-PPO and ureasil-PEO),in which the inorganic-organic networks were joined by covalent bonds [󰀱󰀹]. Tese reactions are shown in Scheme 󰀱. 󰀲.󰀲. Formation of Bioadhesive Films.  o investigate the in󿬂u-ence o the nature o the catalytic process on the appearanceand ormation o the 󿬁lms, the precursor hydrolysis andcondensation reactions were catalyzed by using two differentagents as described in item preparation o ureasil-polyetherhybrid materials. 󰀱󰀵󰀰  L o the ureasil hybrid solutions wasapplied to the porcine skin with a micropipette. Te ureasil-polyether hybrid solutions were also spread on a e󿬂on-coated acrylic plate with the assistance o a metal spreadingdevice (slot cavity o 󰀰.󰀲󰀵󰀴mm). 󰀲.󰀳. Cytotoxicity Assay.  A material in contact with humantissues should not liberate any component that may be toxic  International Journal o Polymer Science 󰀳 Polyethylene oxide (PEO)Polypropylene oxide (PPO)orO OOOor(Y)(Z)(X)O O O OO OorO O(X): hydrolysis reactions(Y and Z): condensation reactions Ethanol3-(Isocyanatopropyl)-triethoxysilaneHybrid precursor PEOHybrid precursor PPOCatalytic agentHybrid PEOHybrid PPOH 3 CCHCH 2 − (OCH 2 CH 2 ) n − OCH 2 CHCH 3 H 3 CCHCH 2 − (OCH 2 CH 2 ) n − NH 2 NH 2 NH 2 NH 2   CH 3 CH 3 CH 3 CH 3 CH 3  CH 3 O OCH 3  CH 3 H 3 CCH 2 O − Si(CH 2 ) 3 NHCNHCHCH 2 −( OCH 2 CH 2 ) n − OCH 2 CHNHCNH(CH 3 ) Si − OCH 2 CH 3 OCH 2 CH 3 OCH 2 CH 3 OCH 2 CH 3 OCH 2 CH 3 H 3 CCH 2 OH 3 CCH 2 OH 3 CCH 2 OH 3 CCH 2 OH 3 CCH 2 OH 3 CCH 2 O 60 ∘ C 24 hH 3 CCH 2 O − Si ( CH 2 ) 3 N = C = OH 3 CCH 2 O − Si ( CH 2 ) 3 NHCNHCHCH 2 −( OCH 2 CH ) n − NHCNH ( CH 2 ) 3 Si − OCH 2 CH 3 HO − Si ( CH 2 ) 3 NHCNHCHCH 2 −( OCH 2 CH 2 ) n − OCH 2 CHNHCNH ( CH 2 ) 3 Si − OHHO − Si ( CH 2 ) 3 NHCNHCHCH 2 −( OCH 2 CH ) n − NHCCH 2 ( CH 2 ) 3 Si − OHHCl or NH 4 FSi(OR  ) 4  +  H 2 OSiOH  +  OHSiSiOH  +  ROSi(RO ) 3 SiOH  +  ROHROHSiOSiSiOSi + H 2 O 󲌋󲌉󲌊󲌈 + 2 S󰁣󰁨󰁥󰁭󰁥 󰀱: Scheme or obtaining hybrid materials (adapted rom the study by Santilli et al. 󰀲󰀰󰀰󰀹 [󰀱󰀰]). or have an adverse effect on healing. Te cytotoxicity testeddescribed in this work ollows the International StandardsOrganization (ISO/EN 󰀱󰀰󰀹󰀹󰀳) [󰀲󰀰] guidelines or testingbiomedical materials. Te cell viability was assessed by theMassay.Humanskinkeratinocytes(󰀲 × 󰀱󰀰 5 )wereculturedin DMEM (Dulbecco’s Medium Eagle Modi󿬁ed) containing󰀱󰀰% FBS (etal bovine serum) in 󰀹󰀶-well microplates andexposed to a conditioned medium. Te conditioned mediawere obtained by immersion o the ureasil-polyether hybrid󿬁lms in DMEM or 󰀲󰀴h. Tereafer, the plates were cen-triuged,thesupernatantswerereplacedwithMdissolvedin phosphate buffered saline (PBS), and the cells were incu-bated in the dark or 󰀴h. Finally, the medium was aspiratedand the M reduction product, ormazan, was dissolvedin isopropyl alcohol and determined spectrophotometrically at 󰀵󰀴󰀰nm. Te cell concentration was measured against acalibration curve made or the cell line tested with the Mstaining method and the results were expressed as a percent-ageothecontrol(untreatedcells).Alldatawereexpressedasthe mean ± SD o three independent experiments. 󰀲.󰀴. Adhesion Strength Analysis.  Te tensile strength test wasused to assess the bioadhesion o the ureasil-polyether 󿬁lms.Te measured resistance to the removal o samples rombiological tissue was taken as the bioadhesive strength [󰀲󰀱].In this test a A.X Plus exture Analyser equipped with aMucoadhesive est Ring and a 󰀱󰀰mm diameter cylindricalprobewasemployed.Tebiologicalsubstratewasporcineear  󰀴 International Journal o Polymer Scienceskin, dissected rom pigs at a slaughterhouse. o maintaina uniorm thickness o substrate a dermatome (NouvagCM󰀳󰀰󰀰, Goldach, EUA) was used, set at 󰀵󰀰󰀰  m thickness.Te skin was then wrapped in a packaging 󿬁lm coated onaluminum oil and rozen at − 󰀲 ∘ C or a maximum o 󰀳󰀰 days.Beore the test, the hair was removed with scissors.Te test was perormed placing the biological substratestretched over the top o an acrylic holder and 󿬁xed with anacrylic ring, which was astened with 󰀴 screws. Tereafer,󰀱󰀵󰀰  L o the ureasil hybrid solutions was applied to theporcine skin with a micropipette. As soon as the solutionswere applied, the cylindrical probe was then lowered at aspeed o 󰀱mm/sec until reaching the skin, where it remainedin contact with the solution under a downward orce o 󰀰.󰀵Nor 󰀳󰀰󰀰 seconds. Ten, the probe was removed at a speed o 󰀱mm/sec and the tensile orce negative to remove it romthe polymeric 󿬁lm ormed on the pig skin was measured.Accordingtopreviousstudies[󰀲󰀲],󰀳󰀰󰀰secondswereenoughor the 󿬁lms to become dried, so when the probe waswithdrawn on the skin surace, the 󿬁lm was already ormed,and so the orce o detachment measured was between theskin and the 󿬁lm on dry state. Te values o bioadhesionobtained were compared to commercial polymers tested by Souza et al. [󰀱󰀴]. 󰀲.󰀵. Rheological Measurements.  Rheological measurementswere made in the oscillatory shear mode with the Carri MedAR 󰀲󰀰󰀰󰀰 EX Rheometer, employing cone-plate geometry (󰀴󰀰mm diameter). Te tension was 󰀴󰀰Pa, the requency was󿬁xed at 󰀰,󰀵Hz, and the test continued or 󰀵min. 󰀲.󰀶. Statistical Analyses.  All results obtained in this study arepresentedasmeansandstandarddeviations(SD).Teresultswere compared by ANOVA and post hoc ukey test. Tesigni󿬁cance level ( 󽠵 ) adopted was 󰀰.󰀰󰀵. Statistical analyseswereperormedwiththeprogramInstatorWindows(GraphPads sofware, San Diego, USA). 3. Results and Discussions 󰀳.󰀱. Cytotoxicity Assay.  A bioadhesive 󿬁lm-orming systemor cutaneous application has to adhere to the skin andnot to show cytotoxicity. Cytotoxicity is important since theprecursorisuseddirectlyontheskinandmustnotharmit,sothe catalyst used to promote 󿬁lm ormation cannot increaseits toxicity.Tereore the 󿬁rst step in this study was to assess thecytotoxicity employing the M reduction test, which iswidely used to test the cytotoxicity o biomaterials. Figures 󰀲and 󰀳 show the cell viability, calculated as described inSection 󰀳.󰀲. Te data in Figures 󰀲 and 󰀳 con󿬁rm that all ureasil-polyether hybrid materials showed a cell viability greater than 󰀷󰀵% over a period o 󰀲󰀴 hours. Tere was nostatistical difference between the values o cell viability o ureasil-PPOandureasil-PEOhybridmaterialscatalyzedwithNH 4 F and HCl. Tereore these materials can be consideredsaetouse,andureasil-PPOhybridmaterialsprovedtobetheless cytotoxic. Tese results suggest that, although the HCl 100806040200    C  e    l    l  v   i  a    b   i    l   i   t  y    (   %    )   P   P   O      4     0     0    H   C    l   P   P   O      4     0     0    N   H    4    F   P   P   O      2     0     0     0    H   C    l   P   P   O      2     0     0     0    N   H    4    F   C    −      (    D   M   E   M    )   C     +     (   p  e  r  o  x   i    d  e    ) F󰁩󰁧󰁵󰁲󰁥󰀲:Cytotoxicityassaywithhumanskinkeratinocytescellsorureasil-PPO. Results are expressed as the mean  ±  SD o triplicates( 􍠵 = 3 ). 100806040200    C  e    l    l  v   i  a    b   i    l   i   t  y    (   %    )   P   E   O      5     0     0    H   C    l   P   E   O      1     9     0     0    H   C    l   P   E   O      1     9     0     0    N   H    4    F   C    −     (   D   M   E   M    )   C     +     (   p  e  r  o  x   i    d  e    )   P   E   O      5     0     0    N   H    4    F Samples F󰁩󰁧󰁵󰁲󰁥󰀳:Cytotoxicityassaywithhumanskinkeratinocytescellsorureasil-PEO. Results are expressed as the mean  ±  SD o triplicates( 􍠵 = 3 ). catalyst leads to a pH more acidic than NH 4 F, both can beused or the ormation o 󿬁lms on skin because they causedno cytotoxicity to human skin keratinocytes. 󰀳.󰀲. Formation of Bioadhesive Films.  Te catalysts in󿬂uenceboth, the speed and the mechanism o 󿬁lm ormation.Differentmechanismscanchangethespeedodryingprocessinvolved in the 󿬁lm ormation resulting in structural modi󿬁-cations that can be responsible to irregular 󿬁lm resulting or-mations[󰀱󰀳].Teappearanceothe󿬁lmsproducedwhenHCland NH 4 F are used as catalysts is shown in Figures 󰀴 and 󰀵,  International Journal o Polymer Science 󰀵 F󰁩󰁧󰁵󰁲󰁥 󰀴: Te visual appearance o 󿬁lms produced with HCl ascatalyst agent.F󰁩󰁧󰁵󰁲󰁥 󰀵: Te visual appearance o 󿬁lms produced with NH 4 F ascatalyst agent. respectively. It can be seen in the photos that the catalystin󿬂uences homogeneity o the 󿬁lms.Te catalyst NH 4 F led to the ormation o an irregular󿬁lm, whereas HCl avored the ormation o homogeneousand transparent 󿬁lms.Tiscanbeexplainedbythekineticsothereactionspro-motedbythecatalyst.Whenthehydrolyticandcondensationstep is promoted by basic catalysis, it leads to nucleophilicsubstitution, where Si-OH is the nucleophilic moiety andthe H 2 O is removed. Tus, the Si-OH group determinesthe reaction kinetics and results in the ormation o morebranched chains at the beginning o the process [󰀲󰀳, 󰀲󰀴]. Te presence o more branched chains leads to a decreasein the rate o hydrolysis and condensation, causing slow andirregular ormation 󿬁lms, whereas in hydrolysis and conden-sation promoted by acid catalysis, the initial reaction o theprocess leads to the ormation o longer and less branchedchains [󰀲󰀳, 󰀲󰀴]. Te acid catalysis reaction is preceded by  rapid protonation o the substituents OR or OH attached tothe Si, increasing the speed o reaction and resulting in atendency to produce a more linear network, which resultsin more homogeneous 󿬁lms (󰀱󰀵mm wide and 󰀰.󰀵mm thick)[󰀲󰀳, 󰀲󰀴]. Tus the catalyst NH 4 F did not orm the 󿬁lm withthe expected homogeneity within an acceptable time.Te HF solvent was replaced by ethanol in the hybridprecursor synthesis step (see Section 󰀲.󰀱) because ethanol isbiologically and environmentally more appropriate, besidesbeing more cost effective. However, a protic solvent such asethanol, different rom HF, can undergo hydrogen bondingwith the catalysts, thus changing the catalytic activity. In thiscase, with NH 4 F, the presence o ethanol decreases the speedo reaction while it increases the speed o reactions catalyzedby HCl. Tese actors also in󿬂uence the visual aspect o the󿬁lms, as seen in Figures 󰀴 and 󰀵. 󰀳.󰀳. Adhesion Strength Analysis and Rheological Measure-ments.  Tus modiying the reactions o hydrolysis and con-densation by changing the type o the catalyst used canlead to a change in the structure o the hybrid polymericchains, which can affect the bioadhesion o the 󿬁lms on theskin. able 󰀱 shows the values o work o bioadhesion orce(Wad) or hybrid materials ureasil-PPO and ureasil-PEOwith various molecular weights produced with the catalystsHCl and NH 4 F. It was observed that the molecular weighto the polymer chain is not a actor that determines theadhesivestrengthopolymersothesamechemicaltype.Temeasured adhesion orce did not differ statistically betweenthe ureasil-PPO hybrid materials (󰀴󰀰󰀰 and 󰀲󰀰󰀰󰀰gmol −1 ),or between the ureasil-PEO hybrid materials o 󰀵󰀰󰀰 and󰀱󰀹󰀰󰀰gmol −1 . However, when the strength o bioadhesionwas compared between ureasil-PEO and ureasil-PPO hybridmaterials with similar molecular weight, it was observed thatthe value o Wad was signi󿬁cantly greater or ureasil-PEO,irrespective o the catalyst used. Tis result can be explainedby the act that ureasil-PEO hybrids are more hydrophilicthan ureasil-PPO hybrids, since they can interact moreefficiently with the water present in the stratum corneumincreasing the bioadhesion [󰀹]. Evidently, the bioadhesionorceishigherormaterialswithamorehydrophilicbehavior[󰀲󰀵].It can also be observed in able 󰀱 that replacement o theHClcatalystbytheNH 4 Fcatalystdecreasedconsiderablytheadhesion strength.Tismaybeduetotheactthatbioadhesionalsodependson the viscosity o the material. o investigate the in󿬂uenceo the catalyst on the viscosity o the 󿬁lms, rheological testswere perormed. Figure 󰀶 shows that the viscous modulus( G  ) was greater than the elastic modulus ( G  ) throughoutthe interval time that is considered ideal or 󿬁lm ormation(between󰀱󰀲󰀰and󰀳󰀰󰀰s),provingthattherewasnogelor󿬁lmormationinsamplesinwhichthecatalystNH 4 Fwasused.InFigure 󰀷upto󰀱󰀹󰀲sitcanbeobservedthattheelasticmodulus G  is similar to the viscous modulus  G  ; this time is reerredtoasgeltimeortimeo󿬁lmormation.Teseresultscon󿬁rmthein󿬂uenceothecatalystonthetimepro󿬁leotheviscosity o materials.Temechanisminvolvedinthehydrolysisandcondensa-tion reactions in the presence o NH 4 F led to the ormationo more branched chains, which resulted in changes in the viscosity o the polymer, resulting in lower bioadhesion [󰀲󰀶].However,comparingthesematerialswiththeresultsobtainedby Souza et al. [󰀱󰀴] (able 󰀲), it can be concluded that all materials based on ureasil-PEO used in this study showedsigni󿬁cantly greater or similar values o adhesive strengththan commercial polymers.
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