Biomaterials Notes

Biomaterials

A biomaterial is a substance engineered to interact with biological systems for a medical purpose, such as mechanical and tissue mitral valves.

Biomaterial Inertness

In most cases, biomaterials need to be inert when in contact with the body. Examples include:

• Artificial ear

• Cochlear implant

• Nasal implants

• Dental materials

• Mandibular mesh

• Artificial skin

• Pacemaker

• Pectus implant

• Birth control implant

• Vascular grafts

• Artificial liver

• Spinal fixation

• Cartilage replacement

• Artificial leg

• Ankle implant

• Hydrocephalus shunt

• Ocular lens, contact lens

• Orbital floor

• Artificial chin

• Blood substitutes

• Shoulder prosthesis

• Artificial heart, Heart valves

• Breast prosthesis

• Artificial kidney

• Glucose biosensor

• Dialysis shunts, catheters

• Adsorbable pins

• Temporary tendons

• Hip implant

• Finger joint

• Testicular prosthesis

Characterization of Biomaterials

Mechanical Properties

• Deformation of the material when subject to a force.

• Dog-bone shaped specimen is commonly used for testing.

• Stress $s$ (N/m^2 or Pascals) = force/cross-sectional area.

• Strain $e$ (%) = deformation as percentage of original length.

• $E = \frac{stress}{strain}$ -> Young’s modulus.

Biomaterial Characteristics

Mechanical Properties and Testing

• Stress $s$

• Strain $e$

• Yield Strength: Point at which material deforms permanently. If not a specific point, an offset point is used.

• Design based on Yield Strength, while Ultimate Tensile Strength is used for quality control.

Deformation

• Elastic deformation: Deformation disappears when the strain is not applied.

• Plastic deformation: Permanent deformation.

Stress-Strain Curve

A stress-strain curve visually represents the mechanical properties of a material. Brittle materials reach failure with only a small amount of deformation, while ductile materials stretch or compress a great deal before failure. Toughness is represented by the area under the stress-strain curve and is equal to the integral from 0 to failure of $\sigma d\epsilon$.

Tensile vs. Compressive Test

• Some materials mostly undergo compression during use.

• Compression testing is similar to tensile testing.

• Materials undergo elastic and plastic deformations in compression as well.

Medical Use of Materials

Material Classes and Uses

Metals

Pros

• Strong, tough, and ductile.

Cons

• Corrosion.

Novel Applications

• Shape memory alloys.

Properties

• Contribute high tensile, high fatigue, and high yield strengths when processed suitably.

• Low reactivity metals are preferred for biomedical applications.

• Properties depend on processing method and purity of the metal.

Applications

• Load-bearing implants and internal fixation devices.

• Bone and joint replacement.

• Dental implants.

• Maxillo- and cranio/facial reconstruction.

• Cardiovascular devices (e.g., titanium for pacemaker cases, defibrillators, heart valve structures, intravascular stents).

• External prostheses.

• Surgical instruments.

Physical Properties of Metals

• Luster (shininess).

• Good conductors of heat and electricity.

• High density.

• High melting point.

• Ductile (can be drawn into thin wires).

• Malleable (can be hammered into thin sheets).

Chemical Properties of Metals

• Easily lose electrons.

• Surface reactive.

• Loss of mass (some corrode easily).

• Corrosion causes a gradual wearing away and change in mechanical properties.

Microstructure of Metals

• Basic atomic architecture is a crystal structure.

• Different elements have different crystalline architectures and can combine with different partners.

• Crystalline architecture determines potential alloys.

Plastic Deformations

• Slip

• Twinning

Fatigue

• Stages of Fatigue Failure

  • No harm

  • Small cracks

  • "Clam shell" effect (shiny area)

  • Fracture

• Fatigue Limit: The maximum stress that a metal will withstand without failure for a specified large number of cycles.

• Number of cycles depends on application and is decided at the normative level.

• Often more important than tensile, compressive, or yield strength.

Strengthening by Grain Size Reduction

• Finer and more homogenous grain size results in more homogeneous packing of the crystal and impedes dislocation-type motion (prevents slip).

• Grain-size reduction usually improves toughness.

• Grain size can be controlled by slowing the rate of solidification and by plastic deformation after solidification.

Alloys

• A metal comprised of two or more elements, at least one of which is metallic.

• Generally, metals do not like to mix. When they do, they form in one of two ways:

  • Substitution

  • Interstitial

Specific Alloys

• Dental Alloys: Gold-Silver, Silver-Copper, Silver-Tin

• Other Alloys: Co-Cr alloys, Co-Cr-Ni alloys, Co-Cr-Mo alloys

• Ni-Ti alloys such as Nitinol (Ti-48Ni-2Co) are superelastic wires.

Titanium

• 2.2 million pounds of Ti implanted every year.

• Used in hip joints, bone screws, knee joints, bone plates, dental implants, surgical devices, and pacemaker cases.

• Due to its total resistance to attack by body fluids, high strength, and low Young’s modulus.

Metal Implant Reliability

Depends largely on the corrosion, wear, and fatigue resistance of the materials.

Metals Passivation

Common to metals like Titanium, Chromium, and Aluminum, to prevent corrosion. Formation of an oxidization layer (e.g., for Chromium is $Cr<em>2O</em>3$) when exposed to oxygen. The layer makes the metal impervious to water and air. The material then becomes passive when implanted in the body.

Knee Replacement Therapy

Primary Problem

• Damaged cartilage leads to various forms of arthritis.

• Osteoarthritis affects 20.7 million Americans.

Symptoms

• Hard, bony swelling of the joints.

• Gritty feeling.

• Immobility.

Solution

Total Knee Replacement (TKR): Nearly 250,000 Americans receive knee implants each year.

Results

• Stops or greatly reduces joint pain

• Improves the strength of the leg

• Increases the quality of life and comfort

Current TKR Design - Assembly

Four Primary Components

1. Femoral Component

2. Tibial Component

3. Plastic Insert

4. Patellar Component

Materials

• Femoral Component: Cobalt-chromium-molybdenum & Ti-6Al-4V ELI Titanium Alloy

• Patellar Component: Polyethylene & Cobalt-chromium-molybdenum (Ti Alloy)

• Tibial Component: Cobalt-chromium-molybdenum (cast) & Ti-6Al-4V ELI Titanium Alloy

• Plastic Insert: Polyethylene

Problems

1. Polyethylene "The Weak Link"

   • Articulation wear produces particulates leading to osteolysis and bone resorption at the implant interface, causing loosening and eventual malfunction of the implant.

2. Metal-Bone Interface

   • Stress-shielding leads to bone degeneration

• Average lifespan of 10-20 years.

Stress Shielding

One complication that can occur from the use of metals in orthopedic applications. The high strength of the metal in the implant induces it to assume more than its share of responsibility for the load in that region. This decreases the load born by the surrounding tissue and therefore shields it from experiencing stress. Lack of stress causes bone density to decrease, causing complications in the implant/tissue interface.

Osseointegration

Discovery

• 1952 - Per Ingvar Branemark discovered the titanium screw and introduced the concept of Osseointegration.

• All existing designs are based on Branemark Titanium Screw

A fixture is osseointegrated if it provides a stable and apparently immobile support of the prosthesis under functional loads, without pain, inflammation, or loosening.

Titanium Properties

• Easily available.

• Lightweight, corrosion resistant, easily milled into different shapes while maintaining its strength.

• Forms a layer of titanium oxide, which is a stable and reactive interface that becomes coated with plasma proteins.

• Ti-6Al-4V was alloyed to create a biocompatible material with added strength.

Ceramics and Glasses

• Do not conduct heat and electricity

• Brittle materials

• Common ceramics used are metal oxides, sapphire, and silica

Pros

• Corrosion resistance

Cons

• Brittleness

Bioceramics

The class of ceramics used for repair and replacement of diseased and damaged parts of the musculoskeletal system. Ceramics are refractory polycrystalline compounds; usually inorganic, highly inert, hard and brittle, high compressive strength, generally good electric and thermal insulators, with good aesthetic appearance.

Ceramic Toughening Strategies

1. Changing components: adding reinforcing second phase

2. Changing surface structure: coating; chemical toughening; thermal tempering

3. Changing manufacturing processes: powder preparation, green forming, densification

Types of Bioceramics

• Bioinert

• Bioactive

• Bioresorbable

Bioinert

• Maintain their physical and mechanical properties while in the host.

• Resist corrosion and wear.

• Have reasonable fracture toughness.

• Typically used as structural-support implants such as bone plates, bone screws, and femoral heads.

Bioactive

• Direct and strong chemical bond with tissue.

• Fixation of implants in the skeletal system.

• Low mechanical strength and fracture toughness.

• Examples: Glass ceramic, dense nonporous glasses

Bioresorbable (Biodegradable)

• Chemically broken down by the body and degrade.

• The resorbed material is replaced by endogenous tissue.

• Chemicals produced as the ceramic is resorbed must be able to be processed through the normal metabolic pathways of the body without evoking any deleterious effect.

• Synthesized from chemical (synthetic ceramic) or natural sources (natural ceramic).

Polymers

Examples

• Nylon

• Silicone rubber

• Polyester

• Polyethylene (PE)

• Polymethylmethacrylate (PMMA)

• Polyvinylchloride (PVC)

Definition

Derived from Greek roots “poly” meaning many and “meros” meaning parts, also referred to as a "macromolecule." Differences are determined by intermolecular and intramolecular forces, and functional groups. Practically everything else in the world is polymeric (excluding metals and inorganic compounds).

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.

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

• Major nondisposable markets include testing/diagnostic equipment, surgical instruments and related equipment, prostheses/implants, dental/ophthalmic devices.

• Disposable products include syringes, kits, labware, tubing, blood bags, utensils, gloves, trays, catheters, thermometers, etc.

Molecular Arrangement of Polymers

Most polymers are large linear macro-molecules. This chain is called the backbone. Atoms in the chain will have small chains of atoms attached, called pendant groups. Pendant chains normally have just a few atoms, but the backbone chain usually has hundreds of thousands of atoms.

Structure of Polymers

Polymers can contain only carbon and hydrogen (hydrocarbons) such as Polypropylene, polybutylene, polystyrene, and polymethylpentene. Other elements, such as Oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous, and sulfur, can also be involved.

• Polyvinyl chloride (PVC) contains chlorine.

• Nylon contains nitrogen.

• Teflon contains fluorine.

• Polyester and polycarbonates contain oxygen.

Polymer Chain Structure and Properties

Each carbon-to-carbon bond allows full rotation in both molecules, so that in reality the chains are seldom extended to their full contour length but are present in many different shapes, or conformations, leading to low crystallization and high elasticity.

Drug Delivery

Polymers allow for developing highly flexible drug delivery systems that can:

• Target specific organs

• Modulate drug release

• Dissolve and be metabolized

Hydrogels

• Biphasic

• Porous polymeric network (solid)

• Water (liquid)

• Good for applications where water or humidity is important (e.g., contact lenses)

3D Printing

Some polymeric biomaterials can be 3D printed to reduce production cost and increase flexibility, enabling additive manufacturing of multi-scale porous soft tissue implants that encourage vascularization and tissue ingrowth.