Biomaterials Behavior and Characterization of their Behavior By

Biomaterials Behavior and Characterization of their Behavior By: Dr. Murtaza Najabat Ali (Ceng MIMech. E) 1

Important Characteristics The selection of the biomaterial and selection of the appropriate processing techniques for a given application is determined primarily by • Surface • Bulk, and • Degradative properties of the material 2

Important Characteristics contd. . Surface Properties of Biomaterials What constitutes a surface? An interface is the boundary region between two adjacent layers We recognize (S/G), (S/L), and (L/V) as surfaces L G S L S S V L L S L = Liquid G = Gas S = Solid V = Vapor 3

Important Characteristics contd. . Surface Properties of Biomaterials Biomaterial surface Protein adsorption Proteins (red) adsorb differently to the different materials and are depicted as elongated on metal and globular on polymer. Cells (blue) interact with the materials via the adsorbed proteins and conformation of the adsorbed proteins dictates how the cells will respond (adhere, proliferate, differentiate, etc. ). The effect of protein size on interaction with a surface. Notice that the larger protein composed of more amino acids is capable of making more interactions 4

Important Characteristics contd. . Surface Properties of Biomaterials § The surface of a material is defined to be the few atomic layers on the exterior of the object § Surface properties can be different from the bulk properties § Surface properties include both chemical and physical characteristics 5

Important Characteristics contd. . Surface Properties of Biomaterials v Surface Chemical Property § Contact Angle The contact angle is the angle at which a liquid-vapor interface meets the solid-liquid interface • The contact angle is determined by the resultant between Adhesive and Cohesive forces • As the tendency of a drop to spread out over a flat solid surface increases, the contact angle decreases § Wetting A shows a fluid with very little wetting, while C shows a fluid with more wetting. A has a large contact angle, and C has a small contact angle. Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together Thus, the Contact angle provides an inverse measure of Wettability Water bead on a fabric that has been made hydrophobic 6

Important Characteristics contd. . Surface Properties of Biomaterials § Surface Tension • Surface tension is a measurement of the cohesive energy present at an interface • This forms a surface “Film” which makes it more difficult to move an object through the surface § Hydrophilicity • If the liquid is very strongly attracted to the solid surface • The droplet will completely spread out on the solid surface § Hydrophobicity • If the solid surface is hydrophobic, the contact angle will be larger than 900 C 7

Important Characteristics contd. . Surface Properties of Biomaterials v Surface Physical Property § Surface Roughness 8

Important Characteristics contd. . Surface Properties of Biomaterials Example 2. 1: Would a hydrophobic or hydrophilic polymer be a more appropriate choice for a Contact lens application? WHY ? Would a melting temperature (Tm) of the polymer above 37 o. C or below 370 C be more appropriate for this application ? WHY ? Example 2. 2: A 1 ml droplet of distilled water is dropped onto each material “A” and “B” as shown below. Which material is more hydrophilic ? Justify your answer. Example 2. 3: Why do atoms at the surface of a crystalline material generally possess higher energy than those inside of the crystal and what is the term for this heightened energy ? 9

Important Characteristics contd. . Bulk Properties of Biomaterials § Most important parameter § They may play a less significant role in initial biological interaction/response, but they have a large long-term impact § They can be altered to allow the biomaterial to mimic the physicochemical properties of the biological system § Bulk characteristic of biomaterials include • Mechanical Properties • Physical Properties • Chemical composition 10

Important Characteristics contd. . Bulk Properties of Biomaterials § Mechanical Properties Such as, • Strength • Stiffness • Anisotropy • Fatigue properties/strength Mechanical properties of materials are highly affected by their physical and chemical characteristics 11

Important Characteristics contd. . Bulk Properties of Biomaterials § Physical Properties Such as, • Crystallinity • Thermal Transition Points Melting Point (Tm) Glass Transition Point (Tg) § Chemical Composition • It varies when chemicals are added or subtracted, and when the ratio of substances changes • It determines the (bulk) properties of the substance • It also dictates other properties such as chemistry at the surface SEM Micrograph of steel showing grain boundaries 12

Important Characteristics contd. . Degradative Properties of Biomaterials Factors that affect biomaterials’ degradation in vivo § The size and shape of the implant § Its location in the body § Chemical, physical and mechanical (both bulk and surface) properties Although the temperature and p. H of body environment/fluids are relatively mild, BUT ! • Degradation can be undesired or desired • The biocompatibility of degradation by-products is as important as the biocompatibility of the intact material 13

Mechanical Properties of Biomaterials Mechanical testing is crucial to determine if a biomaterial will be suitable for a certain application, given the loading parameters of the tissue of interest § Types of Testing • • Destructive testing is changing the dimensions or physical and structural integrity of the specimen. (the specimen is essentially destroyed during the test) e. g. , Tensile, Compression, and Shear Non-Destructive testing does not affect the structural integrity of the sample (a measurement that does not effect the specimen in any way) e. g. , weighing, measurements etc. § Modes of Mechanical Testing Main mechanical properties of materials include: • • • Tensile/ Compressive Properties Shear/Torsion Properties Bending Properties Time-Dependent Properties Hardness 14

Mechanical Properties of Biomaterials § Tensile/Compressive Properties • In mechanical testing, force can be applied as tensile, compressive, or shear • The testing frame is designed to subject a sample to uniaxial loading (i. e. one end of the specimen is attached to a moveable platform) at a controlled amplitude and rate A mechanical testing frame used for determining the mechanical properties of various materials 15

Mechanical Properties of Biomaterials § Tensile/Compressive Properties • The shape of the sample is dictated by ASTM standards, and for tensile testing, a rod or film is often shaped into a “Dog Bone” geometry, as this geometry mitigates artifacts • Basic Components of the Mechanical testing apparatus are; v Grips/ Actuator v Load cell v Extensometer v Processor (Computer) 16

Mechanical Properties of Biomaterials § Tensile/Compressive Properties • A typical plot produced from a mechanical testing equipment Stress - Strain Curve 17

Mechanical Properties of Biomaterials § Tensile/Compressive Properties Stress - The intensity of the internally-distributed forces or components of forces that resist a change. Commonly measured in units dealing with force per unit area, such as pounds per square inch (PSI or lb/in 2) or Megapascals (MPa). The three basic types of stress are tension, compression, and shear. The first two, tension and compression, are called direct stresses. Strain - is the parameter used to quantify the deformation of an object l is the gauge length at a given load and l 0 is the original gauge length with zero load 18

Mechanical Properties of Biomaterials § Tensile/Compressive Properties It can be seen that the stress and strain of Plot-1 in a graph, are proportional to each other at all values. This relationship is known as HOOKE’S LAW and can be written as: A composite diagram showing a range of stressstrain responses for various materials Elastic or Young’s Modulus Stress-strain curve of a ductile material. The area under the curve reflects the energy stored by the material prior to failure. The slope of the plot in the linear Elastic region is known as the Modulus and reflects the material’s Stiffness Where K is material’s stiffness, F is the force applied on the body and δ is the displacement produced by the force along the same degree of freedom 19

Mechanical Properties of Biomaterials § Tensile/Compressive Properties In contrast to elastic deformation, Plastic Deformation is permanent, and the sample never returns completely to its original shape The beginning of plastic deformation is the point where the stress-strain relationship no longer follows HOOKE’S LAW, hence the curve changes from a linear region to a non-linear region The stress corresponding to the end of the elastic region of the curve is known as the Yield Strength and the strain at this value is the Yield Point Strain After yielding, there is an increase in stress required to continue plastic deformation until a maximum is reached (i. e. the Ultimate Tensile Strength or just Tensile Strength). 20

Mechanical Properties of Biomaterials § Tensile/Compressive Properties Necking - is the reduction of a cross-sectional area of a sample in a localized area by uniaxial tension 21

Mechanical Properties of Biomaterials § Tensile/Compressive Properties An important property associated with plastic deformation is the Ductility of a material. Ductility reflects the ability of a material to deform plastically before breaking. Those materials that have low ductility will fracture with very little plastic deformation and are considered Brittle. 22

Mechanical Properties of Biomaterials § Tensile/Compressive Properties Compression Testing ----- is often performed on biomaterials, particularly of they are to be subjected to compressive forces while is use, such as for orthopedic implants. § Typical compressive specimens are cylindrical with a length at least twice that of their diameter § Same way of calculating stress-strain as for tensile test, but the signs of the values is taken negative § The calculated strain is also negative in this case 23

Mechanical Properties of Biomaterials § Tensile/Compressive Properties Compression Testing – Procedure During a typical compression test, data are collected regarding the applied load, resultant deformation, and condition of the specimen. For brittle materials, the compressive strength is relatively easy to obtain, showing marked failure. Ductile materials do not exhibit the sudden fractures that brittle materials present. They tend to buckle and "barrel out". Bulging of a Sample under Compressive Loads 24

Mechanical Properties of Biomaterials § Shear/Torsion Properties Shear testing produces forces that are parallel to the top and bottom faces of the sample. Direct shear occurs when parallel forces are applied in the opposite direction. Shear stress can be calculated by, And shear strain (ϒ) Shear Modulus The shear modulus “G” is concerned with the deformation of a solid when it experiences a force parallel to one of its surfaces while its opposite face experiences an opposing force (such as friction). In the case of an object that's shaped like a rectangular prism, it will deform into a parallelepiped 25

Mechanical Properties of Biomaterials § Shear/Torsion Properties Torsional shearing forces occur when the forces applied lie in parallel but opposite directions. Twisting motion One end of the specimen is placed in a fixture that applies torsional load and the other end is connected to a Similar stress-strain curve Tropometer, which measures the rotation or torsion A torsional force is applied from one end of the sample and Tropometer measures the rotation from the other end 26

Mechanical Properties of Biomaterials Poisson’s Ratio § When a sample of material is stretched in one direction it tends to get thinner in the other two directions § Poisson's ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force § The definition of Poisson's ratio contains a minus sign so that normal materials have a positive ratio 27

Mechanical Properties of Biomaterials Example 2. 4: A cylindrical chicken bone specimen is tested in compression. The original outer diameter of the bone is 7. 4 mm, and the original length of the bone specimen is 45 mm. At one point during the compressive testing, the bone specimen is compressed to a length of 42. 75 mm. One researcher hypothesizes that the Poisson’s ratio of the sample at this point is 0. 3. What would the outer diameter of the bone specimen need to be at this point for the Poisson’s ratio of 0. 3 to be correct ? 28

Mechanical Properties of Biomaterials Example 2. 5: Consider the following test specimens that were subjected to tensile testing. Label each series with the type of deformation that occurs and justify your answer. Match each series to the corresponding stress-strain curve. For which series is the mechanical deformation subject to Hooke’s Law through out the testing illustrated? 29

Mechanical Properties of Biomaterials § Bending/ Flexure Properties Bending forces occur when load is applied to a beam or rod that involves compression forces on one side of a beam and tensile forces on the other side. § Flexure is the bending of a material specimen under load § Deflection of a beam is the displacement of a point on a neutral surface of a beam from its original position under the action of applied loads. The deflection in Figure below, is an indication of the overall stiffness of the material. § The Modulus of Rupture (also known as Flexural strength) are the main parameters determined from this type of testing. A load-versus-deflection curve or stressstrain curve can be plotted based on the data. 30

Mechanical Properties of Biomaterials § Bending/ Flexure Properties For specimens having a rectangular cross section, modulus of rupture is calculated as, Where Ff is the load at fracture, L is the distance between the supports and b and d are specimen dimensions. Whereas, for samples having a circular cross section, the equation becomes, Where R is the radius of the sample 31

Mechanical Properties of Biomaterials Example 2. 6: The following is a stress-strain curve resulting from a three-point bending test on an Alumina (ceramic) test sample: 1) Calculate the modulus of elasticity of the Alumina sample. 2) Using this information, what is the modulus of rupture? 3) The sample is a cylindrical specimen with a radius of 1 cm and a distance of 10 cm between the lower supports. What force was necessary to cause this fracture? 4) Given the following diagram of the three point bending test of a rectangular sample, which portion of the sample is in compression and which portion is in tension? 32

Mechanical Properties of Biomaterials § Time-Dependent Properties • Traditional mechanical tests do not provide a full picture of the stress-strain behavior of materials, since they measure response to loading over a relatively short time. • However, some materials experience changes at the molecular level that result in alterations in mechanical properties when loaded for longer times § Creep • Creep can be defined as plastic deformation of a sample under constant load over time. It can be observed in all materials, but it is only a concern at temperatures greater than Absolute Melting temperature for metals and even higher temperatures for ceramics. In contrast, creep can occur at or around room temperature for some polymer systems. • Creep tests are performed by exerting a constant (usually tensile) load on the specimen while maintaining the system at a fixed temperature. • The Resulting Strain is recorded as a function of time. 33

Mechanical Properties of Biomaterials § Time-Dependent Properties § Creep curve can be divided into three distinct regions: Primary Creep ------- After the initial elastic deformation, the first stage is characterized by an increase in strain with time, while the creep rate (the slope of the curve) decreases. Secondary Creep ------- As the load continues, an equilibrium is established within the material substructure and a minimum creep rate is attained. In this area there is a linear relationship between creep strain and time. Secondary creep lasts for the longest period. Tertiary Creep ------- This region leads to failure. In this stage, gross defects appear inside the material, such as grain boundary separation, cracks, or voids. Here, elongation proceeds rapidly until material failure. 34

Mechanical Properties of Biomaterials § Time-Dependent Properties § Stress relaxation • While creep involves plastic deformation of a sample under constant load over time • Stress Relaxation is the decrease in stress seen over time under constant strain, and is most often found in polymeric materials • Stress relaxation experiments are conducted in a similar manner to creep experiments. The specimen is loaded with sufficient stress to produce a small strain. • The changes in stress needed to maintain a constant strain are monitored as a function of time as the system is maintained at a constant temperature 35

Mechanical Properties of Biomaterials § Fracture If either time-dependent or time-independent deformation is continued, eventually the material will fracture • Ductile Fracture If the material undergoes plastic deformation before breaking, it experiences ductile fracture. It is characterized by plastic deformation in the area of the crack. Ductile fracture is the preferred mode of failure, since there is warning available in the form of change in shape of the specimen during plastic deformation • Brittle Fracture If there is little plastic deformation, the material demonstrates Brittle Fracture. This type of fracture proceeds quickly with little warning and can lead to catastrophic failure of the device. 36

Mechanical Properties of Biomaterials § Fatigue and Fatigue Failure • Repeated loading can lead to failure at stresses significantly less than the tensile or yield strengths as determined by the static testing methods. • This type of failure, known as Fatigue Failure, occurs suddenly after the material has been subjected to many cycles of altering stress or strain. • Fatigue fracture is brittle, with little plastic deformation around the facture, even in ductile materials. • Fatigue failure occurs in three main stages: 1. Crack Initiation ----- a small crack is created at an area of high stress 2. Crack propagation ----- the crack increases in size with each successive loading cycle 3. Final failure ----- occurs rapidly after the crack has reached a certain size Fatigue testing vascular stents involves placing stents into mock arteries and subjecting them to 400 million cycles of internal pressure pulsation (10 years of human heartbeats), forcing them to radially expand contract in each cycle under simulated physiological environment 37