The Mechanics of Hydrogels: Mechanical Properties, Testing, and Applications (Elsevier Series in Mechanics of Advanced Materials) PDF
Preview The Mechanics of Hydrogels: Mechanical Properties, Testing, and Applications (Elsevier Series in Mechanics of Advanced Materials)
Elsevier Series in Mechanics of Advanced Materials The Mechanics of Hydrogels Mechanical Properties, Testing, and Applications Edited by Hua Li School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore Vadim Silberschmidt Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Loughborough University, Loughborough, United Kingdom WoodheadPublishingisanimprintofElsevier 50HampshireStreet,5thFloor,Cambridge,MA02139,UnitedStates TheBoulevard,LangfordLane,Kidlington,OX51GB,UnitedKingdom Copyright©2022ElsevierLtd.Allrightsreserved. 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Silberschmidt LoughboroughUniversity,UK Series Editors Thomas B€ohlke KarlsruheInstituteofTechnology,Germany David McDowell GeorgiaInstituteofTechnology,USA Chen Zhong NanyangTechnologicalUniversity,Singapore List of contributors Mark Ahearne Department of Mechanical, Manufacturing and Biomedical Engi- neering, School of Engineering, Trinity College Dublin, The University of Dublin, Dublin,Ireland;TrinityCentreforBiomedicalEngineering,TrinityBiomedicalScien- ces Institute,TrinityCollege Dublin, The University of Dublin, Dublin,Ireland Xing Gao Research Centre for Medical Robotics and Minimally Invasive Surgical Devices, Shenzhen Institutes of Advanced Technology, Chinese Academy of Scien- ces, Shenzhen, Guangdong, China K.B. Goh School of Engineering, Monash University Malaysia, Bandar Sunway, Selangor, Malaysia Mohammad R. Islam Department of Ophthalmology, University of Pittsburgh, Pittsburgh, PA, United States J.JinchengLei InternationalCenterforAppliedMechanics,StateKeyLaboratory forStrengthandVibrationofMechanicalStructures,Xi’anJiaotongUniversity,Xi’an, Shaanxi,China Hua Li School of Mechanical & Aerospace Engineering, Nanyang Technological University, Singapore, Republic ofSingapore Ziqian Li International Center for Applied Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an, Shaanxi,China Qimin Liu School of Civil Engineering and Architecture, Wuhan University of Technology,Wuhan, Hubei,PR China Zishun Liu International Center for Applied Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an, Shaanxi,China TongqingLu StateKeyLaboratoryforStrengthandVibrationofMechanicalStruc- tures, International Center for Applied Mechanics, Department of Engineering Mechanics, Xi’anJiaotong University, Xi’an, Shannxi,China Lianhua Ma College of Quality and Technical Supervision, Hebei University, Baoding, PR China Yinggang Miao Joint International Research Laboratory of Impact Dynamics and ItsEngineeringApplications,SchoolofAeronautics,NorthwesternPolytechnicalUni- versity,Xi’an, Shaanxi, China x Listofcontributors Michelle L. Oyen Department of Biomedical Engineering, Washington University in St.Louis, St. Louis, MO, United States Zhi-JunShi CollegeofLifeScienceandTechnology,HuazhongUniversityofSci- ence and Technology, Wuhan, Hubei,China Vadim V. Silberschmidt Wolfson School of Mechanical, Electrical and Manufac- turingEngineering,Loughborough University, Leicester, United Kingdom Emrah Sozumert Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Loughborough University, Leicester, United Kingdom WilliamToh SchoolofMechanicalandAerospaceEngineering,NanyangTechno- logical University, Singapore,Singapore Xingquan Wang Department of Engineering Mechanics, Faculty of Materials and Manufacturing,Beijing University ofTehnology,Beijing, PR China Tao Wu School of Mechanics, Civil Engineering and Architecture, Northwestern PolytechnicalUniversity,Xi’an,Shaanxi,PRChina;MIITKeyLaboratoryofDynam- ics andControl of Complex Systems,Xi’an, Shaanxi,PR China Shuai Xu International Center for Applied Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an, Shaanxi, China Qingsheng Yang Department of Engineering Mechanics, Faculty of Materials and Manufacturing,Beijing University ofTehnology,Beijing, PR China Jianxun Zhang State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, China QiangZhang SchoolofElectricPower,CivilEngineeringandArchitecture,Shanxi University, Taiyuan, Shanxi, China Wenlei Zhang State Key Laboratory for Strength and Vibration of Mechanical Structures, International Center for Applied Mechanics, Department of Engineering Mechanics, Xi’an Jiaotong University, Xi’an, Shannxi, China Wei-Wei Zhao School ofMechanicalandElectronicEngineering, Wuhan Univer- sityof Technology, Wuhan, Hubei,China ShoujingZheng InternationalCenterforAppliedMechanics,StateKeyLaboratory forStrengthandVibrationofMechanicalStructures,Xi’anJiaotongUniversity,Xi’an, Shaanxi, China YifanZhou StateKeyLaboratoryforStrengthandVibrationofMechanicalStruc- tures, International Center for Applied Mechanics, Department of Engineering Mechanics, Xi’an Jiaotong University, Xi’an, Shannxi, China Preface Inrecent years, hydrogels eoneoftheadvanced materials e haveattracted increas- ingly more attention thanks to their suitability for a wide range of emerging applica- tions. For example, hydrogels are adopted in various biological systems due to their unique properties such as biocompatibility and biostability, biomimetic applications such as soft robots, as well as medical/pharmaceutical applications such as drug delivery systems, articular cartilage, biomaterial scaffolds, corneal replacement, and tissue engineering. However, a common and serious concern regarding hydrogels is their mechanical properties. A large amount of interstitial water within networked structure of cross-linked hydrophilic polymer chains results in the hydrogels’ soft mechanicalbehavior.Thisoftenbecomesanobviouslimitationtotheirvariousappli- cations,especiallyinbiologicalandmedicalfields.Assuch,itiscriticallyimportantto develop the mechanics of hydrogels, in order to obtain a deep understanding of the fundamentalmechanismsofdeformation,damageandfracture,mechanicalcharacter- istics of soft hydrogels, and also to provide a bridge between the mechanical and biologicalperformanceofhydrogelssoastopushthemechanicalapplicationofhydro- gels beyondtheircurrent boundaries. Thisbookispreparedbyagroupofleadingacademicsandexpertsfromdifferent institutionsandcountries,includingthemostactiveresearchersgloballyintheareaof hydrogelmechanics.Theirresearchinterestscoveralmostallrelevanttopicsintheme- chanics of hydrogels, from theoretical modeling and numerical simulation to experi- mental tests and further to various applications, from micro to macro scales. Examples of the research topics include mechanical characterization of hydrogel at differentscales;elasticandinelasticbehaviorsofhydrogels;rheologicalcharacteriza- tionofhydrogels;fatigueandfractureofhydrogels;indentationtestingofhydrogels; phasetransitionsinhydrogels;responsesofsmarthydrogelstovariousenvironmental stimuli;mechanicalpropertiesofcellularlyresponsivehydrogels;multiscalemodeling ofhydrogels;applicationsofhydrogelsincornealreplacementandartificialmuscles; soft robots; and manufacturing of hydrogels with controlled mechanical properties. These topics are covered in this collection of such contributions, with each written by world-leading experts in the relevant research areas. The volume’s contributors not only present their own pioneering work in their research fields, but also provide valuable literature reviews andrecommend thesignificant futurework. Therefore, this book covers the most advanced knowledge on the mechanics of hydrogels, making it informative reading for experts; concurrently, it can serve as a rich reference source for graduate students intending to work in this area. It will be xii Preface alsousefulforscientistsandengineersinthebroadareasofpolymermaterialsscience, mechanicsofmaterials,biomaterialsengineering,biomedicalengineering,biosensors/ actuators,microelectro-mechanicalsystem(MEMS)andbioMEMS,artificialmuscles and soft robotics, microfluidic control, physics, chemistry, biophysics, biochemistry, andbioengineering.Itwillbeespeciallyusefulformedicalpractitionersandbiomed- ical companies as a reference source with benchmark results to compare and verify their experimental data against the mechanical properties of hydrogels. The book also provides key guidance for medical practitioners planning to conduct further studies to extend their work into practical mechanical applications of soft materials anddesignaswellastooptimizevariousapplicationssuchashydrogel-basedsoftro- bots, orperformnumericalstudies,where theknowledge ofmechanical propertiesis crucial. Hua Li School ofMechanical and Aerospace Engineering Nanyang Technological University Republic ofSingapore Vadim V.Silberschmidt WolfsonSchool ofMechanical, Electrical and Manufacturing Engineering LoughboroughUniversity UnitedKingdom 1 Mechanical characterization of hydrogels Mohammad R. Islam1 andMichelle L.Oyen2 1Department of Ophthalmology, University of Pittsburgh,Pittsburgh, PA, United States; 2DepartmentofBiomedicalEngineering,WashingtonUniversityinSt.Louis,St.Louis,MO, United States 1.1 Introduction Hydrogels are polymeric materials consisting of a sparse network of polymer chains embeddedinanaqueousmedium.Hydrogelscanretainlargeamountsofwaterwithin theirintermolecularspaceduetostronghydrophilicityofthepolymerchainsandlarge porosity.As such, hydrogelscan undergo significant swelling inwater, from 10% to 1000 times of their dry weight [1]. The polymer network does not dissolve in water as interchain cross-linking prohibits water flow at scales larger than network pore size,preservingthestructuralintegrityofthematerial.Hydrogelstructureandswelling behaviorlargelydependonpolymercomposition,natureofcross-linking,fabrication routes, and external environment, making gel properties exquisitely tunable over a broadrange.Thisdiversityinhydrogels’chemical,physical,andmechanicalproper- tiesoffersanexcitingavenueofsoftmaterialdesignformultidisciplinaryapplications. Hydrogels are ubiquitous around us. The extracellular matrix (ECM) of most soft tissues inourbodysuchascartilage,cornea,heart vessels, andskin tissueareessen- tially fibrous hydrogel composites, consisting of a hydrated proteoglycan gel rein- forced with biopolymer (i.e., collagen or elastin) fibers. Bacterial biofilms are also hydrogels [2] and marine plant tissues (i.e., kelp) are fiber-reinforced polysaccharide gels[3].Gelatin,agar,andalginategelshavebeenusedinfoodindustryfor decades [4]. Beyond these traditional hydrogels, the technological advantage of synthesizing hydrogels as engineering materials was not realized until 1960. Wichterle and Lim [5] first developed a hydrogel for soft contact lenses by polymerizing poly(2- hydroxyethyl methacrylate) (polyHEMA) with cross-linking agents in the presence ofwaterandothersolvents.Sincethisimportantdiscovery,alargevarietyofhydrogels with intriguing chemical, physical, and mechanical properties have been developed. Buwaldaetal.[6]presentedasystematicreviewofthishistoricalevolutionofhydro- gel asmaterial. Hydrogels have found diverse and rapidly increasing applications in recent years. Theyarewidelyusedinconsumerproductssuchascontactlenses,cosmeticimplants, hairgels,anddiapers[7].Hydrogelsloadedwithpharmaceuticallyactivecompounds are used as drug carriers to achieve controlled and targeted drug release into human body [8]. Hydrogels act as debriding agent and provide a moist condition in wound TheMechanicsofHydrogels.https://doi.org/10.1016/B978-0-08-102862-9.00014-2 Copyright©2022ElsevierLtd.Allrightsreserved. 2 TheMechanicsofHydrogels dressingstofacilitatewoundhealing[9].Intissueengineering,hydrogelsarecultured withcellsandgrowthfactorstobeusedasartificialscaffoldsfordamagedtissuerepair and regeneration [10,11]. Hydrogels are used as urinary catheter coatings to prevent bacterial colonization on the surface [12]. Hydrogels are also increasingly used for in vitro experiments to study the role of matrix elasticity on stem cell differentiation [13e15]. Several biomimetic machines and functional devices are also developed fromhydrogelsbyleveragingtheiruniquepropertiesandstimuliresponsivecharacter- istics.Examplesincludehydrogelactuators[16],stretchablehydrogelelectronics[17], hydrogelvalvesformicrofluidics[18],color-tunablehydrogelcolloidalcrystals[19], and artificial muscles [20]. Advancementinhydrogeltechnologyhasalsodrawnconsiderableresearchinterest onhydrogelmechanics.Inseveralimportantapplications,ahydrogelactsasaprimary load-bearing component, which often requires an optimal combination of elasticity, strength, and toughness as a material. For example, hydrogel scaffolds for cartilage replacement must possess both high strength and fracture toughness [21]. Artificial skinsmadefromhydrogelsneedtosustainlargestrainwithoutdamage[22].Hydrogel actuators,suchasroboticarms,oftenfunctionunderrepeatedcyclicloading[23].Me- chanicalpropertiesofhydrogelsarealsoimportantforfunctionalapplications.Incell culturestudies,ithasbeenobservedthatthestiffnessofhydrogelsubstrateaffectscell behavior,includingproliferation,migration,anddifferentiation[13].Mechanicalchar- acterization of hydrogels under in situ loading conditions is therefore an important considerationin design of hydrogelmaterials. Hydrogelsaremultiphasecompositematerialsconsistingofanaqueousmatrixrein- forced by solid polymer network. Fig. 1.1 illustrates schematically the structure of hydrogels at different length scales. At macroscale, they resemble continuum solids withdefinedshapedespitelargewatercontent.Microscopically,hydrogelsarediscrete randomnetworksofflexibleorsemiflexiblepolymerchains.Atmolecularscale,they allow diffusion of solute molecules just like pure liquid. Hydrogel mechanics is also strongly multiscale. When subjected to far field loading, individual polymer chains deform in a manner dependent on bulk polymer properties, connectivity with the neighboring chains, interaction with solvent molecules, and the test environment. The collective deformation of all such chains leads to a nonuniform deformation at the network scale, and thereby a nonlinear stressestrain behavior at the macroscale. While theliquid matrix does notsupportanyload, it introduces anincompressibility constraint that prevents lateral contraction of the polymer network under stretch. Hydrogels demonstrate strong time-dependent relaxation behavior that involves two distinct mechanisms associated with viscoelastic and poroelastic deformation [24]. Viscoelastic relaxation occurs due to topological fluctuations of flexible polymer chains under fixed strain, whereas poroelastic relaxation emerges from the migration of liquid over time. Poroelasticity of hydrogels is strongly dependent on material length-scale, whereasviscoelasticityislargelylength-scale independent [25]. Multiscalestructureandmechanicsofhydrogelsnecessitateevaluationoftheirme- chanicalpropertiesatdifferentlength-andtime-scalesaswell.Alargevarietyofme- chanicaltestingmethodsareusedforhydrogelsdependingonthemateriallength-scale and mechanical property of interest (Fig. 1.1b). Elastic and fracture properties (i.e.,
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