2011-MA Triggered Release from Polymer Capsules

Published:July06,2011r2011AmericanChemicalSociety5539dx.doi.org/10.1021/ma201014n|Macromolecules2011,44,5539–5553PERSPECTIVEpubs.acs.org/MacromoleculesTriggeredReleasefromPolymerCapsulesAaronP.Esser-Kahn,†,‡,^SusanA.Odom,†,‡,^NancyR.Sottos,†,§ScottR.White,†,||andJeffreyS.Moore*,†,‡†BeckmanInstituteforAdvancedScienceandTechnology,‡DepartmentofChemistry,§DepartmentofMaterialsScienceandEngineering,and)DepartmentofAerospaceEngineering,UniversityofIllinoisatUrbana
Champaign,Urbana,Illinois61801,UnitedStatesI.INTRODUCTIONThecontrolledreleaseofcontentsfrompolymericcapsulesisofconsiderableinterestinapplicationssuchasself-healingmaterials,nutrientpreservation,fragrancerelease,anddrugdelivery.Theutilityofcapsulesasvehiclesforcargostoragestemsfromtheirabilitytodeliverbeneficialagents(e.g.,fertilizer)“justintime”toaffecttheoutcomeoflargersystems(e.g.,cropgrowth).Triggeringisastimuli-dependentphenomenon,andthedevelopmentofappropriateinitiatorsplaysakeyroleinthereleaseofcapsulecontentstoprovidethedesiredoutcome.Avarietyofchemicalandphysicalmethodshavebeendevelopedtoreleasecapsulecontents.Capsulesystemsareparticularlyappeal-ingfordeliveryofsmallmoleculesandparticles.Usingcapsules,adrug,nutrient,orhealingagentisdeliveredwithoutrequiringmodification.Incontrast,theuseoftriggerablesmallmoleculesrequireschemicalmodification,potentiallyleadingtomoretime-intensivepreparationsandgreateruseoftriggeringgroups.Manymechanismscaninitiatechangesinacapsuleshellwallthatresultsinthereleaseofcapsulecontents(Figure1).Indrugdelivery,light-activatedmechanismsarerelevantfortargetedreleaseinbiologicaltissues(Figure1,photo).Tissuesshownegligibleabsorptioninthe800
1200nmregion,1providingawindowforlaserirradiationofnear-IR-sensitivecapsules.UV-andvisiblelight-sensitivecapsulesarealsousedinthecosmetic2andagriculturalindustries3wheresolarirradiationtriggersrelease.Biologicaltriggersareusedfordrugtherapyandvitamindelivery(Figure1,biological).4SlightchangesinpH5orthepresenceofcertainchemicals(e.g.,insulin)6causeappropriatelydesignedcapsulestoreleasetheircontentsinapatient,requiringonlyinvivostimuli(Figure1,chemical).Thermallyinducedreleaseisusefulinapplicationswheresubtlechangesintempera-tureoccur(Figure1,thermal).Forexample,inagriculturalapplications,anincreaseinsoiltemperaturecaninitiatedeliveryofnutrients.3,7Deodorantandantiperspirantmaterialscanbereleaseduponreachingtargetedtemperatures,allowingfordeliveryonlywhereapersonperspires.8,9Magneticallyinducedreleaseisusefulfordrugdelivery,activatingcapsulesonlyintissuessubjectedtooscillatingmagneticfields(Figure1,magnetic).10Electricfieldreleaseisusefulindeliveringanti-corrosivematerialsonlywhenametallicsurfaceiscompromised(Figure1,electrical)11andmaybeusefulinbatterymaterialsthatexperienceahighervoltagethanstandardoperatingpotentials.InthisPerspective,wefocusourdiscussionontheuseofchemicalchangeswithincapsuleshellwallsasatriggeringmethod.First,wepresentabackgroundoncapsulefabricationmethods(sectionII)inordertofamiliarizethereaderwiththechemicalconstraintseachtechniqueplacesontriggerincorpora-tionandrelease(sectionIII).InsectionIV,wediscusstriggersresponsibleforactivatingchemicalreactionsincludingchemical,biological,light,thermal,magnetic,andelectricalstimuli.Asthisisstillanascentfieldofresearch,someofthestimuliwediscusshavenotbeenfullydemonstratedfortriggeredreleaseviachemicalchangesintheshellwall.Forcompleteness,wealsodiscussphysicalphenomenaresponsibleforcapsulerelease.Finally,insectionVweconcludebyhighlightingpersistenttechnicalchallenges.Wesuggestpossiblesolutionsanddiscussareaswherenewtriggeringstrategiesaredesperatelyneeded.Anumberofreviewshavefocusedonencapulatingandreleas-ingdeliverablematerials,bothonthenano-andmicroscale.11
20AsthisPerspectivefocusesontriggeringapproaches,weprovidereferencetothesereviewsforthosewhoareinterestedintheresearchareaofmicroencapsulationasawhole.II.METHODSFORPREPARINGMICROCAPSULESMicrocapsulesarepreparedbyseveralmethodsincludingpancoating,spraydrying,centrifugalextrusion,andemulsion-basedmethods.Severalreviewarticleshavefocusedoncapsulepreparation;21herewepresentmethodsthatutilizeemulsions.Received:May3,2011Revised:June13,2011ABSTRACT:Stimuli-responsivecapsulesareofinterestindrugdelivery,fragrancerelease,foodpreservation,andself-healingmaterials.Manymeth-odsareusedtotriggerthereleaseofencapsulatedcontents.Herewehighlightmechanismsforthecontrolledreleaseofencapsulatedcargothatutilizechemicalreactionsoccurringinsolidpolymericshellwalls.Triggeringmechanismsresponsibleforcovalentbondcleavagethatresultinthereleaseofcapsulecontentsincludechemical,biological,light,thermal,magnetic,andelectricalstimuli.Wepresentmethodsforencapsulationandrelease,triggeringmethods,andmechanismsandconcludewithouropinionsoninterestingobstaclesforchemicallyinducedactivationwithrelevanceforcontrolledrelease.5540dx.doi.org/10.1021/ma201014n|Macromolecules2011,44,5539–5553MacromoleculesPERSPECTIVEEachmethodhasadvantagesforspecificapplications,whichdependonavarietyofcapsulecharacteristics.Theseincludeshellwa