Lec1_PSC475_CourseOverview

PSC 475: Biomaterials Course Overview

January 14, 2013 Lecture 1

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Course Objectives • This course is designed to provide a general understanding

of the multidisciplinary field of biomaterials. Course materials will rely on general concepts learned in polymer and biology/biochemistry courses and will further extend the understanding about the interactions at the interface of material and biological systems.

• Current applications of biomaterials will be evaluated to provide an understanding of material bulk and surface properties, degradation processes, various biological responses to the materials and the clinical context of their use. The advanced study of multiple material systems applications will also provide an expanded understanding of how disciplines are merging to provide solutions for society while reinforcing the concept of self learning.

• X. COURSE OUTLINE (Spring 2012) • Focused Chapters/Sections • Biomaterials Science • A History of Biomaterials • Part I

– Properties of Materials – Classes of Materials Used in Medicine

• Part II – Some Background Concepts – Biomaterials Surfaces: Physics

• Surface (vs. Bulk) Structure and Properties • Surface Energy • Adsorption, Segregation, and Reconstruction at Surfaces

– Biomaterials Surfaces: Chemistry • Reactions at Surfaces

– Surface Modification Methods Applicable to Biomaterials – Surface Characterization

• Ex situ/in situ characterization

– Protein-Surface Interactions – Host Reactions to Biomaterials and Their Evaluation – Biological Testing of Biomaterials – Degradation of Materials in the Biological Environment – Application of Materials in Medicine, Biology, and Artificial Organs

• Tissue Engineering • Drug Delivery

• Practical aspects of Biomaterials – Implants, Devices, and Biomaterials: Issues Unique to this Field

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Intro Learning Objectives • Intro to Biomaterials (1.1-1.5)

• Understand why the study of biomaterials is an important aspect of the educational background of the biomedical engineer;

• Understand what a biomaterial is;

• Be able to differentiate between the different general applications of biomaterials;

• Provide specific examples of classes of products on the basis of application; and,

• Be able to differentiate between the scientific and the regulatory definition of biomaterials.

• Know what government body regulates medical devices;

• Be able to differentiate between the various classes of medical devices from a regulatory perspective and be able to provide examples of each class of device;

• Understand what is meant by biocompatibility testing and why it is a necessary consideration in the manufacture of medical devices;

• Understand what is meant by the phase, “biological response to contact with materials”;

• List the different procedures used to test biocompatibility;

• Become aware of the Society of Biomaterials, of the various professional journals; and relevant websites;

• Differentiate between the concepts of bioinertness and bioactivity; and

• From a biomedical device standpoint, know what is generally required of a biomaterial to be incorporated within a biomedical device;

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Excerpt from THE DANCE OF THE SOLIDS

by John Updike

Midpoint and Other Poems, Alfred A. Knopf, Inc., 1969

Prince Glass, Ceramic's son, though crystal-clear

Is no wise crystalline. The fond Voyeur

And Narcissist alike devoutly peer

Into Disorder, the Disorderer

Being Covalent Bondings that prefer

Prolonged Viscosity and spread loose nets

Photons slip through. The average Polymer

Enjoys a Glassy state, but cools, forgets

To slump, and clouds in closely patterned Minutes

The Polymers, those giant Molecules,

Like Starch and Polyoxymethylene,

Flesh out, as protein serfs and plastic fools,

The Kingdom with Life's Stuff. Our time has seen

The synthesis of Polyisoprene

And many cross-linked Helixes unknown

To Robert Hooke; but each primordial Bean

Knew Cellulose by heart: Nature alone

Of Collagen and Apatite compounded Bone.

http://www.sciam.com/article.cfm?id=john-updike-poem-1969

Definitions • Biomaterial

– “A nonviable material used in a medical device, intended to interact with biological systems” (Williams, 1987)

– “Any substance, synthetic or natural in origin which treats, augments, or replaces any tissue, organ or function of the body” (Greco 1994)

– A systematically and pharmacologically inert substance designed for implantation within or incorporation with living systems (Clemson University Advisory Board for Biomaterials)

– Materials that constitute parts of medical implants, extracorporeal devices, and disposables that have been utilized in medicine, surgery, dentistry, and veterinary medicine as well as in every aspect of patient health care (Dee et al.)

• Biomaterials science – The physical and biological study of materials and their

interaction with the biological environment

B. Ratner, A. Hoffman, F. Schoen, and J. Lemons: Biomaterials Science, 2nd edition (San Diego: Elsevier Academic Press. 2004).

FDA Definition: Biomaterial

• "an instrument, apparatus, implement, machine, contrivance, implant, in vitro reagent, or other similar or related article, including a component part, or accessory which is recognized in the official National Formulary, or the United States Pharmacopoeia, or any supplement to them, intended for use in the diagnosis of disease or other conditions, or in the cure, mitigation, treatment, or prevention of disease, in man or other animals, or intended to affect the structure or any function of the body of man or other animals, and which does not achieve any of it's primary intended purposes through chemical action within or on the body of man or other animals and which is not dependent upon being metabolized for the achievement of any of its primary intended purposes."

Bio-inertness vs. Bioactivity

Bioactive materials play a more aggressive role in the body. While a biocompatible material should affect the equilibrium of the body as little as possible, a bioactive material recruits specific interactions between the material and surrounding tissue.

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Bioactive Materials

• Encourage tissue integration to aid in the fixation of an implant in the body.

• Many total hip implants operations today rely partially on a porous coating of Hydroxyapatite (HA), a normal component of bone, to help permanently stabilize the stem of the implant in the bone. The coating encourages ingrowth from the surrounding tissue that interlocks within the pores much like the pieces of a puzzle lock together. Although many current medical procedures call for inert biocompatible materials, the increasing understanding of tissue interaction promises many more applications for aggressive bioactive materials.

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Relevant Terminology Distinctions

• Biocompatibility refers to any construct that can be brought into direct contact without: – Causing a systemic toxic reaction – Have tumorigenic response

• Bioactive material is biocompatible but plays a more aggressive role in the body – Recruits specific interactions between the material

and the surrounding tissue – Encourage tissue integration

• Bioinert material is biocompatible and does not elicit a significant biological response


Commonly Used Biomaterials

Material Applications

Silicone rubber Catheters, tubing

Dacron Vascular grafts

Poly(methyl methacrylate) Intraocular lenses, bone

cement

Polyurethanes Catheters, pacemaker

leads

Stainless steel Orthopedic devices, stents

Collagen (reprocessed) Cosmetic surgery, wound

dressings

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Requirements of Biomaterials

A biomaterial must be:

• inert or specifically interactive

• biocompatible

• mechanically and chemically stable or

• biodegradable

• processable (for manufacturability)

• non-thrombogenic (if blood-contacting)

• sterilizable

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The Food and Drug Administration

• FDA (www.fda.gov)

• Food-Foodborne Illness, Nutrition, Dietary Supplements…

• Drugs-Prescription, Over-the-Counter, Generic….

• Medical Devices -Pacemakers, Contact Lenses, Hearing Aids…

• Animal Feed and Drugs-Livestock, Pets …

• Cosmetics-Safety, Labeling…..

• Radiation Emitting Products-Cell Phones, Lasers, Microwaves…..

Classification of Medical Devices

• Based on the duration of the device use, invasiveness and risk to the user.

• Class I devices: crutches, bedpans, tongue depressors, adhesive bandages etc. –minimal invasiveness, does not contact the user internally.

• Class II devices: hearing aids, blood pumps, catheters, contact lens, electrodes etc. –higher degree of invasiveness and risk, but relatively short duration.

• Class III devices: cardiac pacemakers, intrauterine devices, intraocular lenses, heart valves, orthopedic implants, etc. -considerably more invasive and can pose immense risk to the user-implantables.

B. Ratner, A. Hoffman, F. Schoen, and J. Lemons: Biomaterials Science, 2nd edition (San Diego: Elsevier Academic Press. 2004). 628.

Biomaterial or Medical Device?

• The FDA is not in the “materials” approval business • It is important to know that the FDA neither approves materials nor

maintains a list of approved materials • Although FDA recognizes that many of the currently available

biomaterials have vast utility in the fabrication of medical devices, the properties and safety of these materials must be carefully assessed with respect to the specific application in question and its degree of patient contact.

• An important principle in the safety assessment of medical devices is that a material that was found to be safe for one intended use in a device might not be safe in a device intended for a different use.

• Accurate characterization is an essential step in selecting a material for a medical device, but ultimately the final assessment must be performed on the finished product, under actual use conditions.

Evolution of Biomaterials

Ramakrishna, S. et al Biomaterials: A Nano Approach (Boca Raton CRC Press 2010) 17


From costumer “advice” to specific “device”

Many steps and specific skills involved

Biomaterialists include physical scientists,

engineers, dentists, biological scientists,

surgeons, and veterinary practitioners in

industry, government, clinical specialties,

and academic settings.

Biomaterials Scientists

• Study the interactions of natural and synthetic substances and implanted devices with living cells, their components, and complexes such as tissues and organs.

Biomaterials Engineers

• Develop and characterize the materials used to measure, restore and improve physiologic function, and enhance survival and quality of life.

Subjects Integral to Biomaterials Science

• Toxicology

• Biocompatibility

• Inflammation and healing

• Functional Tissue Structure and Pathobiology

• Dependence on Specific Anatomical Sites of Implantation

• Mechanical Requirements and Physical Performance Requirements

• Industrial Involvement

• Risk-Benefit and Corporate Realities

• Ethics and Regulation 21


Toxicology

• Biomaterials should be nontoxic, unless engineered for such requirements, i.e. a “smart” drug delivery system that targets cancer cells with a toxic drug

• Nontoxic requirements are the norm for most applications – biomaterial toxicology has evolved into a sophisticated science

• Substances that leach and migrate out of biomaterials, i.e. heavy metals, low molecular weight polymers

• Biomaterials ideally should not lose mass in the biological environment unless specifically designed to do so – Biodegradable polymers

– Polymer and its degradation byproducts must be nontoxic

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Inflammation and Healing

• Specialized biological mechanisms are triggered when a material interfaces with the body

• Tissue injury stimulates well-defined inflammatory reaction sequence ultimately leading to healing

• Healing – normal (physiological) and abnormal (pathological)

• Foreign body response – reaction sequence initiated when a foreign body is present in the wound site (i.e. an implant)

• Reaction and severity of the reaction differs depending upon anatomical site involved

• Understanding how a foreign body shifts the normal inflammatory reaction sequence is a critical concern for biomaterials scientists

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Functional Tissue Structure and Pathobiology

• Biomaterials are implanted into almost all tissues and organs

• Tissues and organs vary widely in cell composition, morphological organization, vascularization, and innervation

• Implantation into bone, liver, or heart will have specific physiological consequences

• Key principles governing the structure of normal and abnormal cells, tissues, and organs are important to biomaterials research (often see biomaterials scientists collaborating with clinicians, pathologists, etc.)

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Dependence on Specific Anatomical Sites of Implantation

• Consideration of the anatomical site of an implant is essential

Examples

• Intraocular lens may be implanted into the lens capsule or the anterior chamber of the eye

• Hip joints will be implanted in bone across an articulating joint space

• Prosthetic heart valves will be sutured into cardiac muscle, and will contact both soft tissue and blood

• Catheters may be placed in an artery, vein, or urinary tract

• Each of these sites have special requirements for anatomy, physiology, geometry, size, mechanical properties, and bioresponses

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Mechanical Requirements and Physical Performance

• Mechanical performance

• Hip prosthesis must be strong and rigid; heart valve leaflets must be flexible and tough; dialysis membranes must be strong and flexible, but not elastomeric;

• Mechanical durability

• Catheters may have to perform for 3 days; bone plates perform for 6 months or longer; heart valves must flex 60 times/min without tearing for the lifetime of the patient

• Physical properties

• Dialysis membranes must be specifically permeable; cup of the hip joint must have high lubricity; intraocular lens must have high transparency and refractive properties

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Ethical Concerns

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Overview of Biomaterials Applications

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Brief History of Biomaterials

• Gold dental implants by early civilizations • 1000 BC

– gold strands as soft tissue sutures

• 1900-1920 – Pig bladders and other organics used to restore

mobility to deformed hip joints

• 1939 – PE introduced for plastic surgery

• 1947 – PMMA bone cement first used


Brief History of Biomaterials (cont)

• 1953 – Cardiopulmonary bypass devices to pump blood

external to the body

• 1958 – Implantation of the first cardiac pacemaker

• 1968 – First use of intraaortic balloon pump

• 1980 – Implantation of the first cardioverter-defribillator for

arrythmias



Sir Harold Ridley,

intraocular lens

Sir John Charnley,

hip prosthesis Dr. Willem Kolff,

artificial kidney

Dr.Julio Palmaz,

coronary artery

stent


• 4th Century BC - Aristotle called the heart the most important organ in the body

• Galen proposed that veins connected the liver to the heart to circulate “vital spirits” throughout the body via arteries

• 1628 - William Harvey described a relatively modern view of heart function when he wrote “the heart’s one role is the transmission of blood and its propulsion, by means of arteries, the the extremities everywhere”

• Artificial heart device described by Etienne-Jules Marey in 1881.

The AbioCor is the first artificial heart to be used in nearly two

decades. Photo source: Abiomed

Imagine the complexity and concern from the materials and

materials/bio interface standpoint. Photo source: Abiomed

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SynCardia Temporary

Total Artificial Heart

www.syncardia.com

Powered by air flow


• 1788 – Charles Kits writes about electrical discharges to the chest for resuscitation

• 1820-188- electrical pulse known to modulate the heart beat

• 1930-1932- portable pacemaker (Fig 7)

• 1959 – First implantable pacemaker developed using two Texas Instrument transistors encased in epoxy to prevent body fluids from deactivating the device

Therapeutic Devices in Orthopedics

Applications in Dentistry

Catheters

Ophthalomolgy

Otto Wichterle (Centrifugal Casting of Soft Contact Lenses)

In Drug Delivery

Future Applications-Tissue Replacement

Where are biomaterials?

Would you have considered these items biomaterials?

Biomaterials Represent a Broad Range of Products

General Applications of Biomaterials

• Storage of fluids, tissues, and other biological products

• Diagnosis

• Monitoring

• Therapy



20 million individuals have an implanted device $300 billion/yr cost associated with prostheses and organ replacement therapies Total global biomaterials market is expected to be worth US$58.1 billion by 2014 (http://www.marketsandmarkets.com),

http://www.marketsandmarkets.com/

Classes of Materials Used in Medicine

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Broad Classification- Types of Biomaterials

– ceramics

– metals

– polymers, synthetic and natural

– composites

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Ceramics

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Definition

• Inorganic compounds that contain metallic and non-metallic elements, for which inter-atomic bonding is ionic or covalent, and which are generally formed at high temperatures.

• Derivation: From the Greek word "keramos" meaning the art and science of making and using solid articles formed by the action of heat on earthy raw materials.

• Most ceramics occur as minerals: – The abundance of elements and geochemical

characteristics of the earth’s crust govern mineral types. – Alumina, Zirconium, Calcium phosphate, Silica,

hydroxyapatite – Composition of Earth’s Crust: [84% = O + Si + Al]


Ceramic Properties

• Advantages - Inert or bioactive in body - High wear resistance

(orthopedic & dental applications)

- High modulus (stiffness) & compressive strength

- Fine esthetic properties for dental applications

• Drawbacks - Brittle (low fracture resistance,

flaw tolerance) - Low tensile strength (fibers are

exception) - Poor fatigue resistance (relates

to flaw tolerance)

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Ceramic Applications

• Femoral heads and cup inserts for ceramic on polyethylene

• Ceramic on ceramic hip replacement bearings

• Knee prostheses • Spinal fusion devices • Orthopedic instrumentation • Dental-crowns • Bridges, implants and caps • Inner ear implants (cochlear

implants) • Drug delivery devices

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Metals

2/19/2013 53

Metals Structure and Properties

• Closely packed crystal structure; the type of bonding in metals and metal alloys render them valuable as load bearing implants as well as internal fixation devices used for orthopedic applications as well as dental implants;

• When processed suitably they contribute high tensile, fatigue and yield strengths; low reactivity and good ductility to the stems of hip implant devices

• Their properties depend on the processing method and purity of the metal, however, and the selection of the material must be made appropriate to its intended use.

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Metals as Biomaterials

• Worldwide Market view – $5 Billion late 80s

– $20 Billion 2000

– Est. $23 Billion 2005

• In 1988 11 million Americans had at least one implant (4.6% of the civilian population, 1990 data)

• Critical view – Metal composition, structure, and properties for current implant

alloys

– Structure property relationships

– Role of biomaterials design, production, and proper utilization of medical devices

Metals Manufacturing

Other Uses of Metals

Medical Tubing

Catheters

Stents

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2/19/2013 58

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The Future of Orthopedic Devices?

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Increasing Numbers of Joint Replacement

2/19/2013 61 http://webs.wichita.edu/depttools/depttoolsmemberfiles/academicaffair

s/CIBOR/NAC%20wooley.pdf

Outcome of Joint Replacements



Cause of Failure



Cause of Failure

64 http://webs.wichita.edu/depttools/depttoolsmemberfiles/academicaffair


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Cause of Failure

http://webs.wichita.edu/depttools/depttoolsmemberfiles/academicaffair


Revision Surgery

66 http://webs.wichita.edu/depttools/depttoolsmemberfiles/academicaffair


How Can We Improve Success Rates?

1) Ideal bioengineering profile

2) Ideal biomechanical profile

3) Ideal biocompatibility profile

4) Complete osteointegration

5) Wear-free bearing surface

4 out of 5 are materials related!!

3 out of 5 are materials/bio interface related!! 67

Engineering Design

Engineering/Materials

Property

Materials Property

Materials Property

Materials Property

So what’s the answer? How do we improve the success of joint

replacements?

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Composites

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Composites

• The word “composite” means consisting of two or more distinct parts

• Individual strengths and weaknesses of polymers, ceramics, and metals benefit different applications

• The porosity and hardness of ceramics support tissue integration into the tissue/implants interface, but these properties could hardly suit a ligament replacement

• A composite material incorporates the desired characteristics of different materials to meet the stringent demands of living tissue

• Most composite designs combine strength and flexibility by reinforcing a relatively flexible material with a harder, stronger one

• In some cases, one or more of these materials may be degradable in order to encourage tissue integration

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Back to Joint Replacements

• Bone is highly porous? How do we better mimic it’s structure and mechanical properties?



Polymers

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Polymers

• Flexible structure of polymers has enabled this group of materials to be useful in many applications

• Even DNA has found this structure useful, storing genetic information in thousands upon thousands of repeating sequences of polymers;

• In many materials, processing conditions can induce the polymer chains to pack in various way to produce a wide variety of mechanical properties

• These parameters are easily varied in order to suit current biomedical applications

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Polymeric Biomaterials are used in a Broad Range of Products

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Medical Plastic Market

• Plastic usage in the healthcare field encompasses several distinct markets-including disposable or single use biomaterials.

• Predominant are applications for medical devices and related products and packaging.

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Medical Plastics Market

• Non-disposables comprise slightly over 50% of total volume.

• Commodity thermoplastics currently dominate the market with a little under 50% of total volume, having a consumption level of 956 million pounds in 1999.

• Almost 80% of polymers used in the medical industry are represented by PVC, polypropylene and polystyrene.

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Types of Polymers

• Thermosets

• Thermoplastics

• Elastomers – Classification based on mechanical properties

• Hydrogels- Classification based on chemical properties

• Polyelectrolytes-Classification based on chemical properties

• Natural-Classification based on origin

• Biodegradable-Classification based on biostability

Structure Hierarchy in Materials & Biology

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How many different types of biomaterials are in use today?

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Fact: The FDA regulates 100,000 different products that represent at least 1,700 Different Types of Biomedical Devices

What Do Biomaterials All Share in Common?

Answer: Most were not originally engineered for biomaterials applications!

In general, Biomaterials are defined by their application, NOT their chemical make-up

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Example: Intraocular lenses

Notes Adapted from: Mayes, A. 3.051J Materials for Biomedical Applications, Spring

2006. (MIT OpenCourseWare), http://ocw.mit.edu (Accessed 14 Jan, 2011).

http://ocw.mit.edu/

81 Image from MIT OCW

http://ocw.mit.edu

What governs materials choice? Historically → Today

1. Bulk properties: matched to those of natural organs – Mechanical (i.e. modulus)

– Chemical (i.e. degradation)

– Optical (i.e. whiteness, clarity)

2. Ability to process

3. Federal Regulations: – Medical Device Amendment

of 1976 (all new biomaterials must undergo premarket approval for safety/efficacy

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Today → Future

• Rational design of biomaterials based on better understanding of natural materials and the material/biological organism interface (Biomimicry)

• Adoption of the materials engineering paradigm

What is structure?

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Material Science/Engineering Paradigm

…….the arrangement of matter!!

Both synthetic materials & biological systems have many length scales of structural importance

Structural Hierarchies

Synthetic Materials

Chemical primary structure

Higher order structure

Microstructure

Composites

Parts

Devices

84

Living Organisms

Molecules (H2O, peptides, salts)

Organelles (lysosomes, nucleus, mitochodria)

Cells

Tissues

Organs

Individuals

The realm of

biomaterials

engineering

10-10 m

10-3 m

Biomaterials engineering spans 8 orders of magnitude in structure!!!!

Structural Hierarchies

85

Biomaterials engineering spans 8 orders of magnitude in structure!!!!



http://ocw.mit.edu/

Length Scales of Structure

1. Primary chemical structure (atomic & molecular: 0.1 – 1 nm)

Length scale of bonding – strongly dictates biomaterials performance Primary

• Ionic: e- donor, e- acceptor ceramics, glasses (inorganic)

• Covalent: e- sharing glasses, polymers

• Metallic: e- “gas” around lattice of + nuclei metals

Secondary/Intermolecular

• Electrostatic

• H-bonding

• Van der Waals (dipole-dipole, dipole-induced dipole, London dispersion)

• Hydrophobic interactions (entropy-driven clustering of nonpolar moieties in H2O)

• Physical entanglement (high MW polymers)

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Primary Chemical Structure Length Scales: Examples

87 Notes Adapted from: Mayes, A. 3.051J Materials for Biomedical Applications, Spring


http://ocw.mit.edu/

Primary Chemical Structure Length Scales: Examples



http://ocw.mit.edu/


2. Higher Order Structure (1 – 100 nm) Crystals: 3D periodic arrays of atoms of molecules

metals, ceramics,

polymers (semicrystalline)

Crystallinity decreases solubility and bioerosion

(biodegradable polymers & bioresorbable ceramics)

Networks: exhibit short range order & characteristic lengths

inorganic glasses, gels

Self-assemblies: aggregates of amphiphilic molecules

micelles, lyotropic liquid crytals

block copolymers



http://ocw.mit.edu/

Higher Order Structure Length Scales: Examples



http://ocw.mit.edu/




http://ocw.mit.edu/




http://ocw.mit.edu/


3. Microstructure (1 µm +) Crystal “grains”: crystallites of varying orientation

Example: Stainless steels Fe-Ni-Cr

Depletes at grain boundaries causing corrosion

Spherulites: radially oriented crystallites interspersed w/ amorphous phase

semicrystalline polymers, glass-ceramics

93

Used for fracture fixation

plates, etc., &

angioplasty stents



http://ocw.mit.edu/

3. Microstructure (1 µm +), cont. Precipitates: secondary phases present as inclusions

metals, ceramics, polymers

Example: Carbides in Co-Cr alloys

Properties:

1) Hardness,

2) Corrosion resistance at grain boundaries)

94

derived from precipitates

Microstructure Length Scales



http://ocw.mit.edu/


3. Microstructure (1 µm +), cont. Porosity: often desirable in biomaterials applications



http://ocw.mit.edu/

3. Microstructure (1 µm +), cont. Porosity: often desirable in biomaterials applications

96




http://ocw.mit.edu/

Take Home Message w/r to Structure

• “Biocompatibility” is strongly determined by primary chemical structure!!

• Higher order structure & microstructure strongly dictate kinetic processes & mechanical response!!



http://ocw.mit.edu/

Mechanical Properties of Materials

98


• A solid material subjected to a tensile force extends in the direction of traction by an amount proportional the applied load (Hooke’s law)

• Most solids behave in an elastic manner if the applied load is not too great




modulus s)(Young' tensile where

E

E

• Since all geometric influences haves been removed (Force/unit area), E and G represent inherent properties of the material

• E and G are macroscopic manifestations of the strengths of the interatomic bonds

modulusshear where

G

G



105


• Area under the tensile curve is proportional to the work required to deform a specimen until it fails

• Area under the entire curve is the product of stress and strain, units of work (energy) per unit volume of material



Creep and Viscous Flow

• Assumption that there is an instantaneous strain response to an applied stress is invalid for most materials

• Figure A – elastic response to ligament is instantaneous, but continues to elongate under a constant load

• Creep – continuous, time-dependent extension under load

• Stress relaxation – continuous, time-dependent decrease in load under fixed elongation

• Both creep and stress relaxation result from viscous flow in the material


• Mechanical analog of viscous flow is the dash pot model or the Dash pot and spring model

• In creep test, instantaneous strain is produced when the weight is first applied (equivalent to stretching the spring to equilibrium length)

• Additional time-dependent strain is modeled by movement of the dash pot


• Fatigue – process by which structures fail as a result of cyclic stresses that may be much less than the ultimate tensile stress

• Susceptibility of materials to fatigue is determined by testing a group of identical specimens in cyclic tension or bending (Figure 11A) at different maximum stresses

• Maximum applied stress plotted vs. number of cycles to failure

• Stress that provides a low probability of failure after 106 – 108 cycles often adopted as the fatigue strength or endurance limit

Lec1_PSC475_CourseOverview

Documents

study of biomaterials

society of biomaterials

history of biomaterials

surfaces biomaterials

biomaterials course

course materials

surface properties

physics surface