Loading

Alison's New App is now available on iOS and Android! Download Now

Module 1: Material Characterization

Study Reminders
Support
Text Version

Set your study reminders

We will email you at these times to remind you to study.
  • Monday

    -

    7am

    +

    Tuesday

    -

    7am

    +

    Wednesday

    -

    7am

    +

    Thursday

    -

    7am

    +

    Friday

    -

    7am

    +

    Saturday

    -

    7am

    +

    Sunday

    -

    7am

    +

Video 1: Mechanical CharacterizationGood morning everyone.So, previous class we have learned about what are surface properties and what are the differenttechniques we can use for characterizing those surface properties, today we will be learningabout the bulk propertiesSo, what are the bulk properties?So, other than the outer surface of a material all other section is bulk section only; so,for a material to be used for the implant first that bulk property has to be satisfied.So, that it is has to be mimicking the already existing natural material.Then only we can go for surface analysis and those things for making it more biocompatibleSo, what are the bulk properties?They are elastic behavior whether it can be extended based on the application, stressand strain, tension, compression and shear stress, fatigue and creep.Bulk property is totally different from surface, where it would not have any reaction withthe system, but it, it is majorly for the orthopedic implants where there is a loadbearing applications and all are involved.So, why it is important?Because, if you consider the orthopedic implants once you surgically insert it into the body,it has to be there for lifelong and it has to maintain its function all over that period.To have that capability we have to characterize that mechanical property of the implants.So, consider a intervertebral disc which is inserted into the vertebra of a human.So, for each position, the load applied onto that intervertebral disc is different.As you can see in normal supine position, which is lying down, the load on the discwould be 294 Newton only, but if you are standing and walking and all it increases graduallydue to the force applied on the vertebral and if you are lifting that load would befurther increased So, your material has to have that wide rangeof load bearing capacity.So, that it can be in the host system for prolong period of time because if there isa failure and if you are inserting a implant at the age of 30 around 50 if it fails, thenyou have to go for again surgery which has a higher risk and all.So, bulk property thus plays a major role for a longer patency of the implant and whatare the major obstacles in these bulk properties?One is corrosion and due to the excessive usage of the implant inside the body, thematerial can break down into smaller pieces which will causes further complication inthe biological system ok.So, the fundamental concepts in the mechanical properties are stress; stress is when an forcesapplied onto the surface that is called stress.So, the mathematical term is stress equal to force by area.So, you are applying a force on a specific cross sectional of a material so, that isthe stress.So, based on this stress, the material will go and deformation.So, if there is a dental implant so, while you are eating and all.So, that stress would be applied on the dental implant so, that has to maintain its integrityso, that deformation is called strain.So, strain is change in the length by original length, initial length.So, based on these two factors there is a relation called Hook’s law which introducesa new parameter called Young’s modulus which is a ratio of stress by strain.So, this is a important and commonly used parameter in all of the biomaterials for whiledefining a mechanical property of that material.So, other properties related to these tensile strengths are elastic modulus, yield strength,ultimate tensile strength, ductility, and toughness.So, these properties I will explain while explaining the stress strain curve.So, you know what is stress and what is strain.So, if you have a relation based on this stress and strain, if there is a increase in stressthere would be an increase in strain.So, the initial proportionate linear increase is called in that linear increase in regionis called elastic region.So, in that elastic region what it represents is, if a material if it is in the elasticregion even if that strain, due to the strain if it forms, if it deforms into a differentstructure then it can retain back, it back into its original structure; it is like arubber band whether you extend it.So, if you leave it can come back into the original shape original length.So, in that region is called elastic region.So, there is a particular region were if you extend it further more it will cause breakageof the rubber band.So, that region is called plasticity region, where it cannot come back to its originalregion.So, the initial region is called a elastic region.So, up to this the stress and strain would be very linearly proportional then after it,the yield point is there.So, after this yield point the material cannot go back to its original state ok.So, if you are having an implant, polymer materials used for the vascular grafts, cathetersand all.So, if that material if the force applied on the material is going beyond that yieldpoint, it shape can change into a different one.So, that will cause problems while introducing into the biological system.So, your material has to be below that yield point whatever the force which is naturallyoccurring for that material, the application, based on the application that should be belowthat yield point.So, the ultimate strength, ultimate strength is the maximum load that material can withstand.So, considering a hip prosthesis; so, if you are introducing into the system, the maximumload where that material can withstand even if it deforms that is the ultimate strength.So, after that if you continue to give the load then that material will break.So, that is called rupture strength.So, this is a typical stress strain curve, but for different types of biomaterials thestress strain curve varies because polymers will have a higher elasticity and everythingwhereas, ceramics has no elasticity at all.So, as you can see the brittle ceramic aluminum oxides it has a initial Young’s modulusthen after that if you give a load it will break, there is no point of elongation oranything for the ceramic.Then for brittle metals like cobalt chromium alloys and titanium for the ductile metalslike titanium alloys and all there would be a little bit elongation then after which itbreaks down.In polymer also there is a thermosetting polymer such as epoxy polymers those kinds of polymersit is very brittle in nature.So, it cannot have that elastic point.So, this epoxy polymers and all used for the adhesive surgical glues and adhesives things.So, they will immediately seal up the wounds.So, that it should not leak anything.So, those materials would be very brittle.So, those things would not have any elasticity and all.Then ductile polymers are all other low density polymers polyethylene then carbohydrate polymersall those materials will have a higher range of elasticity such as PVA, chitosan polyethyleneso, normal plastic polyethylene.So, if you can extend, it can go for the very longer length and all so, that polymers willhave a different Young’s modulus and different plasticity region.So, based on this you have to decide and the application you are going to use you haveto decide the material and characterize whether these materials has the same property as thatof naturally occurring materials.So, like if you are introducing a new vascular grafts and all that vascular grafts has to,has the mechanical property such that it can withstand the force of the blood stream passingthrough that graft and if you are going for a hip prosthesis and all.So, you have to look for how the loads and all applying on the region around the hipprosthesis.So, that it does not break or does not fall above the yield strength ok.So, how we can find out all these parameters Young’s modulus, ultimate strength, fractureand that rupture point and all.So, that can be found using universal testing machine.So, in this machine you can do variety of tests where each of the test will give a differentparameters.So, this is how the instrument will look like.So, instrument the bottom one is called mounted ahead.So, it is a immovable head.So, the bottom so, you can see the two clamps.So, in between those two clamps only we will fix the sample ok.So, the top one is a movable head.So, in that one it will, load cell would be there at the top.So, the load cell will apply the force on the sample.So, and it will tension will be applied onto the sample.So, that will introduce stress and then the change in length will be calculated by thesensors.So, which will, from that we can calculate the ultimate strength, yield strength andrupture point and Young’s modulus and all.So, for different application different clamps would be available as you can see one is fortension.So, tension stress is where you elongate a sample.So, that is the tension stress and compression; compression is where you compress the sample.So, that the material how much the load it can compress into a smaller in on that materialsurface.Then bending where the material would be kept and there would be a bending at a differentpoints and all.So, there is based on the application.So, if you are having screw plates and all in the legs and disc prosthesis; so, therethe bending would be different based on the how many screws and nuts you are introducinginto the plates.So, based on that you can go for three point bending four point bending to exactly characterizehow that will function inside the biological system.Then torsion and all; so, there would be a twisting occurs on the sample.So, this kind of testing will be done for the ligaments and all.So, where the ligaments would has to be turned.So, that it does not break or rupture during that torsion stress.Then in plane shear is mainly for hip prosthesis where there would be a metal contact, therewould be a shear stress applied whenever that ball and cub would be moving over the plane.So, that will be find out using the shear stress in-planar testing.So, all this can be done for different size of samples different sample dimension andall the metals, polymers, ceramics and everything.So, using this instrument you can find out all the mechanical properties and all andcan see in that sample dimension it looks like a dog bone shape.So, it is an important factor that if you are keeping a single lengthy rectangular sampleand all.So, the breakage would occur at the where you are clamping the sample.So, that is not the actual tension stress applied onto that sample.So, they make a dog bone shape so, that the sample at the ends would be thicker.So, that it would clamp and the breakage would occur at the sample middle only.So, that will give the exact value of how that stress strain observation will be occurring.So, usually we consider.So, when a stress is applied the strain will instantaneously produced.So, that is how our idea was so, but it does not actually follow that all over the period.So, consider different parameters such as temperature, frequency and time.So, based on these parameters with the constant stress also it can have a different strain.So, in the stress-strain curve if you can see there is applied stress the strain vary.So, if you increase the strain vary, but if you apply the constant stress also the straincan vary.So, that is a dynamical mechanical, dynamic mechanical testing and all.So, why this is important?So, temperature as I said earlier dental implants and all where you can have a change in temperaturelike while eating ice cream or the hot soup.So, the temperature changes.So, the implant should maintain its integrity.So, to identify whether that affects that mechanical property, the same normal roomtemperature if you are applying a load on to your material that will have a differenteffect when the temperature is very cold.So, those things has to be calculated.Then frequency, Frequency is how much time a material is being used for that specificapplication consider the hip prosthesis where it has to move while walking, how many distance,how much distance you are walking and all based on that the frequency varies.Consider the heart valves and all.So, heart valves has to function lot number of times for a minute it has to beat for 72around 72 beats.So, consider for a minute it is like that then for the day how much beats it has tobe there and for a year and for the prolonged period of time how much time the valves hasto open and close.So, over those periods what happens is the material is in contact with the nearby surfacesand all.So, there would be some corrosions there would be some surface etching and erosion that willreduce the efficiency of that material.So, those factors has to be tested before clinically use that heart valves and thesematerials and all.So, what are these properties change changing based on this frequency and time are.So, time dependent properties are some of the there is a lot of properties.So, these are the widely used properties.Creep or recovery test, relaxation test; so, creep or recovery test is that it is where,in creep or recovery test the stress should be maintained constant whereas, the strainwould vary.So, if you consider a ligament.So, as you can see in the figure, the weight wt has been a load that is the load has beengiven.So, when it is given at that time point t the it will not elongate if it is within thatlinear range of the stress-strain curve.After the particular period of time it tends to elongate ok.So, that is that percentage elongation changing that is what it is changing.So, how much that time can affect that strain on based on a constant stress.So, that is creep or recovery test.Then relaxation test it is exact opposite of the creep or creep test where the strainwould be kept constant the elongation would be kept constant, but the load will vary.So, how much load it can vary based, to maintain that elongation.So, these things would be very useful when you are designing a new material and all;so, that the material should not go to a point where it cannot recover its original stateback ok.So, for those things this time dependent properties will involve and to identify that materialprolonged usage we will study these time dependent properties.Frequency dependent properties; so, frequency dependent properties as I have said earlier,the number of frequency your material is being used like in the heart valves and hip prosthesis.So, wear and tear it is the major problem in orthopedic implants where the materialwould be in contact with the nearby materials and all.So, due to the frequency high number of frequency we are using that material inside the biologicalsystem, the surface will get eroded and that will lose its functionality, lose its efficiency.Fatigue is again the same thing where you are not in fatigue you are below the yieldpoint only.The number of the load we are giving is below the yield point only, but due to the overusage of the implants for that application, the material will lose its efficiency thatis called fatigue it will lose its functionality.So, this usually happens when the material you are designing it should be almost similarto that natural material like the metal implants, it should be similar to the bone already present.If it is above also what happens is that there is a stress yielding effect it can happen.So, stress yielding effect is an effect where if a material can have a load of around 500Newton, if we make that material into a 1000 Newton it can have that much capacity of a1000 Newton material what happens is, it can have that all the load by itself and it willnot pass on to the nearby bones and all.So, what happens due to that is the bones surrounding that material will get, looseits integrity and it would be weaker compared to the normal bones.So, that is, by this material it is shielding the stress of the nearby natural materialsand all thereby reducing its integrity.So, that is one of a major problem in orthopedic implants.So, those things will come when fatigue is also involved.So, it can seal the stress of the load and thereby reducing the integrity of the bone implants.

Video 2: Properties of Hydrogels and Scaffolds
So, those are the basic properties and characterization we can do for the bulk properties of the material.Consider hydrogels and scaffolds.So, those has a different properties rather than these polymers flims and metal implantsand all.So, you might have already studied what are hydrogels and scaffolds.So, hydrogels are gels which can have a water intake higher amount of water intake and scaffoldsare the polymeric scaffolds which can be used for cell, tissue regeneration process andall.So, what are the properties important for these hydrogels and scaffolds some basic propertieswhich defines them?So, you can do all the surface characterization and bulk characterization for these hydrogelsand scaffolds also other than those properties these properties also involved porosity, swelling,degradation, kinetics and the thermal behavior.So, porosity; so, porosity for a scaffold porosity is a important factor because porositywill help in cell attachment in depth of that scaffold and the number of pores will helpin transfer of nutrients inside and outside the scaffolds.So, to identify the porosity of the material, tissue engineering scaffold normal techniqueslike SEM, TEM mercury porosimeter and the gas pycnometry can be used.So, in TEM and SEM you can visually see the porous structure.So, as you can see the SEM image the porous scaffold is seen, but this in SEM image youcan see at the surface only.So, you cannot say, it confirm that inside also the porous structure would be there maybe the inside it would be a compact bulk structure and the surface would be a porous structure.In TEM you can actually say that the material is completely porous because due to the transmissionelectrons you can see what is inside the material and all.So, if a material is completely porous inside you can find out using that thing other thanthese sophisticated techniques, you can use mercury porosimeter and gas pycnometry whichuses a Archimedes principle of liquid displacement where you have a how the experiment for thisfinding the porosity goes is that, you will have a liquid in mercury porosimeter you willinsert the porous structures inside.So, the porous and you apply a vacuum also.So, the air would be sucked out of the system and all the pores inside will be filled bymercury.So, those by that, then you take out the scaffold and weigh the amount of scaffold before keepinginto that mercury and after that and based on the volume you can actually calculate theporosity.So, mercury porosimetry and gas pycnometer it involves gas and commonly used anotherliquid is that ethanol.So, by ethanol because it can it will not involve in swelling of the materials and all,some of most of the materials would be hydrophilic.So, it can if you are using water and all it can absorb water and it can swell and allit is not actually the porous volume.So, the volume would be based on the polymer scaffold also.So, using ethanol will have a better that better than the water experiment.So, porosity formula is that the final weight minus initial weight by volume and densityof the ethanol used.So, how much of volume and ethanol density of the ethanol you are using.So, that will give you the porosity of the scaffold ok.So, swelling study is predominantly used for hydrogels because the uptake of water is majorlyinfluences the kinetics of the molecules loading and releasing from the gels.So, a material which can swell faster can mean that the water intake is a higher so,that the nutrients or any other drugs or molecules which you are incorporating into the hydrogelcan be released faster.So, if a swelling is lower then the release would be slower.So, that defines how the kinetics mostly in drug delivery application and all, how thesekinetics would be factored would be affected by this swelling study.So, for swelling analysis similar to that what you do is you weigh the samples beforethe keeping it into water, then you keep it in water then you again weigh the sample.So, that will the difference in that weight by the initial weight, final weight is thepercentage degree of swelling.So, usually for hydrogels for the application wise you can actually use this degree of swellingto understand that the rate of drug release and all.So, if it is a uncrosslinked polymer hydrogels and all the swelling would be faster and ifit is a crosslinked the hydrogel the swelling would be slower.So, to understand the kinetics of the release profile you can use this swelling you canstudy this swelling.So, degradation; so, degradation is an important parameter for both the hydrogels and scaffoldsbecause nowadays most of the implants are coming as a biodegradable implants, wherewhen it is implanted into the body instead of having an another surgery to remove thoseimplants, these things can degrade by itself such as the sutures, resorbable screws andall where you implant it for the metal plates and the sutures for internal sutures and all.So, and over the period, initially it should function like a normal suture where it shouldit should have all the mechanical property and all the physical property of the likethe normal suture there, then after over the period of time when the wound heals that shouldbe degraded by the biological system in the and it should be washed away from the system.So, this would be tested with the biological fluids and enzyme.So, based on the application you are using.So, the biodegradation is calculated by initial weight minus final weight after it is degradedby the final weight will give you the biodegradation.So, this will help you understand how long a material can degrade.So, based on that you can design them; so, if you want your material to be degraded withina week or within a month, then you have to complete degradation should be there withinthe month the final value will be around a 0 or to so, that it should be completely replacedby the natural tissue ok.So, those are the normal physicochemical properties for the scaffolds and hydrogels, coming intothermal behavior.So, thermal behaviors like I said for the implants for dental implants and for normalpolymeric materials also you have to understand the effect of temperature on the material.So, that can be found out using a DSC and TGA.So, this is a commonly used, these two are commonly used techniques other than this thereare the thermal characterization techniques also.So, the DSC it involves what it actually finds out is that, the material melting point, crystallizationpoint, then the glass transition temperature, then the phase change of the polymers.So, those things can be found out using a differential scanning calorimeter.So, the instrumentation involves as you can see in the picture there are two; one is areference and another this is sample, in the sample pan we will keep the our material andin the reference pan it would be a empty pan or you can have a reference comparable materialsand all.So, what happens is you will apply a constant temperature on to the both the sample andreference at the same time, at the same temperature.So, what happens is that if your material exert heat, like releases heat by exothermicreaction or endothermic reaction it absorbs heat, there would be a change in the heatflow.So, that would be detected.So, based on that you can actually plot this DSC thermogram where you can find out themelting point, crystallization point and degradation point of the materials.So, as you can see the first when it, polymeric materials if it is amorphous polymers it willgo a glass transition change.So, it can go from brittle nature into a rubbery nature.So, above glass transition temperatures the molecule will be arranged randomly.So, that will have a rubbery nature that is the temperature called glass transition temperature.So, glass transition temperature after that it will molecules will arrange in orderlyfashion where it is you can find out the crystallization temperature.So, after that molecule tends to melt.So, that is the melting temperature then if there is a chemical reaction due to the temperature.So, that is the exothermic behavior you are seeing then finally, the degradation occurswhere the material is completely degraded due to that temperature.So, these temperatures and all these temperatures are very essential when you are using a thermoresponsivegels and other temperature involving implants and devices.For it can also be used for characterization new biomaterial whether you are having a crosslinkedor the mixtures of two materials and all there would be a difference in, shifting in thetemperature in the glass transition temperature and change in melting temperature and allit will occurs.Because higher the crosslinking the melting temperature will vary.So, based on that we can identify that your crosslinked has occurred or your materialproperties has changed due to the incorporation of a new molecule into your samples and all.So, that can be found out using this differential scanning calorimeter.Thermogravimetry analysis is a similar to DSC only, the mechanism is similar to DSConly, but here that it involves as the name suggests the gravimetry which is a weighingof the material.So, the instrumentation is almost similar nowadays the advanced instruments has bothDSC and the TGA is available.So, where in the DSC where you can, change you can observe the change in temperaturedue to the material when the heat is supplied, here due to the heat is supplied you can actuallymeasure the minute change in the weight.So, you will use a sample of around 20 mg or 5 mg only for each of them.So, that can be that minute change in temperature can be observed using this thermogravimetricanalysis.So, this mainly helps us to identify how the degradation occurs for the material and all.So, typical thermogram will looks like that right corner you can see that; so, around100 degree Celsius due to the increase in.So, in DSC and TGA there would be a constant increase in temperature they will be supplying.So, initial increase in temperature there would be a volatile compounds like a moisture,water adsorbed or any solvents present it would get evaporated from that system.So, that would be a you can see there is a decrease in the weight percentage that isthe initial stage.Then the second stage would be the decrease in the polymer, decrease in the weight dueto the polymer degradation.So, that is in the second stage; then the final stage where it is completely turnedout into an ash, which is a degraded polymers everything has been completely turned intocarbon ok.So, the interpretation for this TGA analysis, it if it is there is your material is notaffected by any temperature and all the line would be a very straight line.Then in the curve 2 we can see there is a decreasing, sudden decrease and it becamestabilized that usually denotes that your material some moisture or some solvents arethere.So, which is getting evaporated not a degradation of a compound or molecule present on the scaffolds,that is the curve 2.Then in curve 3 its a single stage decomposition.So, if your material is made up of a single polymer or single material metals or somethingceramics and all.So, there would be a single degradation of that particular molecule only.So, that can be observed due to the single loss of the weight.If your material is having a multiple polymers like inter penetrating networks where youhave a couple of polymers or alloys where you use a silver titanium alloys and all.So, those will have a, each material will have a different degradation temperature.So, the lower degradation temperature first step would be the those materials will getdegradated, then simultaneously based on the degradation temperature it varies.In step 5 if it is not a stepwise if it is just gradually decreasing it means that youare actually heating it very fast so, that your instrument cannot measure the exact weightof that material as scaffolds present.There can be some increase in weight also that happens when your material is reactiveto the atmosphere present inside the system.So, usually it would be inert atmosphere such as we can use such as nitrogen or argon andall, but if you are using a normal atmospheric condition there would be oxidation occurswhich can lead to the increase in the weight of the material.So, the final one is again after decomposition, it can those decomposition products can interactwith the atmospheric compounds and reactive species.So, that it can lead to a increase in that weight.So, this can also happen in magnetic samples also there would be an increase in weightalso.So, by characterizing these thermal properties you will know that your material is pure orif your whatever you are incorporating into your material can have a any thermal effecton to the final material.So, those things can be analyzed using these techniques and all.So, understanding this both the bulk properties and the surface properties will have a clearidea of how exactly you want to use that material for your applications and all.So, after finishing these two characterizations, you have to go for your biological characterization,where you can check for protein absorption, cell attachment and if you are working ona anti bacterial adhesion, or biofilm formation, to avoid biofilm formation.So, those things and all come under the biological characterization how effectively it can improveor decrease the attachment of the biological system.So, before going that you have to understand these parameters and these characterizationtechniques has to be done for all the materials you are trying to do.