Biomaterials (Week 1-2) Flashcards
Biomaterial definition
“is used to make devices to replace/repair a function of the body in a safe, reliable, economic, and physiologically acceptable manner” “is any substance (other than a drug), natural or synthetic, that treats, augments, or replaces any tissue, organ, and body function”.
Biomaterials and medical devices comprised of them are commonly used as:
prosthesis in cardiovascular, orthopedic, dental ophthalmological, and reconstructive surgery interventions: surgical sutures, bio-adhesives, controlled drug release devices and particles
Biomaterials science addresses both:
therapeutics and diagnosis.
The path to create new biomaterials
1) Identification of a problem or a need. 2) Research on biomaterials (chemistry, physics of materials science biology) 3) Engineering to develop a medical device 4) Preclinical and clinical testing 5) Regulatory approval 6) Commercialization and clinical application
The success of a biomaterial or implant is highly dependent on three major factors:
1) the properties (mechanical, chemical and tribological) of the biomaterial in question 2) biocompatibility of the implant and 3) the health condition of the recipient and the competency of the surgeon.
Biocompatibility
is the ability of a material to perform with an appropriate host response in a specific application
Desirable biocompatibility
Noncarcinogenic, nonpyrogenic, nontoxic, nonallergenic, blood compatible, non-inflammatory.
Desirable attributes of Biomaterials (4)
1) biocompatibility
2) Sterilizability (Not destroyed by typical sterilizing techniques such as autoclaving, dry heat, radiation, ethylene oxide)
3) Physical characteristics (Strength, elasticity, durability)
4) Manufacturability (Machinable, moldable, extrudable)
Requirements for an implant by examining the characteristics that a bone plate must satisfy for stabilizing a fractured femur after an accident:
- Acceptance of the plate to the tissue surface (biocompatibility). 2. Pharmacological acceptability (non-toxic, non-allergenic, non- immunogenic, non-carcinogenic, etc.). 3. Chemical inert and stable (no time-dependent degradation). 4. Adequate mechanical strength. 5. Adequate fatigue life. 6. Sound engineering design. 7. Proper weight and density. 8. Relatively inexpensive, reproducible, and easy to fabricate and process for large-scale production.
Generations of Biomaterials
- 1st generation: Goal: bioinertness (minimal reaction/interaction)
- 2nd generation: Goal: bioactivity (resorbable biomaterial; controlled reaction with the physiological environment (e.g. bone bonding, drug release)
- 3rd generation: Goal: regenerate functional tissue (biointeractive, integrative, resorb able; stimulate specific cell responses at the molecular level (e.g. proliferation, differentiation, ECM, production and organization)
Materials used in each generation
- 1st generation: silicone-rubber (elastomeric polymers), pyrolitic carbon (used today to coat mechanical components of heart valves).
- 2nd generation: PLA (Polylactic acid) and other biopolymers used to deliver drugs, calcium phosphate, hydroxyapatite-containing bone fillers, nano particles for drug delivery.
- 3rd generation: regeneration of functional tissue = tissue engineering – true replacement within living tissue.
Early biomaterials:
- Gold: Malleable, inert metal (does not oxidize); used in dentistry by Chinese, Aztecs and Romans - dates 2000 years.
- Iron, brass: High strength metals; rejoin fractured femur (1775).
- Glass: Hard ceramic; used to replace eye (purely cosmetic).
- Wood: Natural composite; high strength to weight; used for limb prostheses and artificial teeth.
- Bone: Natural composite; uses: needles, decorative piercings.
- Sausage casing: cellulose membrane used for early dialysis (W Kolff).
- Other: Ant pincers. Central American Indians used to suture wounds
Most biomaterials and medical devices perform satisfactorily in-vivo, so what can go wrong?
1) No manmade construct is perfect. All man- factored devices have a failure rate.
2) Also, all humans are different with differing genetics, gender, body chemistries, living environment, and degrees of physical activity.
3) physicians implant or use these devices with varying degrees of skill.
Dental restoration materials:
materials employed for the restoration of teeth include metal alloys (amalgams, gold, stainless steel, and cobalt-chrome) and ceramics (porcelain or alumina). Other uses of restorative materials include polymers as sealants for surface lamination.
Intraocular lenses (fabricated from ….)
fabricated from a variety of transparent materials including poly(methyl methacrylate), silicone elastomers, soft acrylic polymers, and hydrogels.
Dental implants today are fabricated from …..
Pure titanium (cpTi)
Cardiovascular stents:
1) design
2) materials (3 types)
1) Tubular scaffolds,
2) made from either 316L stainless steel, nitinol (NiTi alloy) or CoCr based alloys.
Total-hip replacement prostheses: fabricated from (5 types) ……
titanium alloys (Ti6Al4V), CoCr, CoCrMo, UHMWPE, ceramics.
- Femoral stem: titanium alloy
- Femoral head: alumina-zirconia ceramic
- Acetabular cup: UHMWPE infused with vitamin E antioxidant.
Total US biomaterials expenditures (2009)
$2.5 trillion per year
Heart valve prostheses: fabricated from ……….
from carbons, metals, elastomers, plastics, fabrics, and animal or human tissues chemically pretreated.
Cardiovascular assist devices: in devices such as pacemakers the cardiac pacing leads have to ……..
Biomaterials used in the design of the leads should be……..
…… have to survive in the harsh endocardial environment.
………………stable, flexible, possess adequate conductive and resistive properties, and provide endocardial contact with the heart.
Tissue engineering is:
cell seeding and and tissue implantation
How materials are structure (2 structures)?
- Surface structure: interface with biological environment (Interactive forces)
- Bulk structure: dictates mechanical performance such as how strong, ductil and elastic a material is (attractive forces)
- 4 Atrractive forces in the universe are:
- 3 aren’t important an 1 is important
- Gravitational, Weak nuclear, Strong nuclear, Electromagnetic
- Gravitational, Weak nuclear, Strong nuclear: Not high enough in magnitude to hold atoms together
2 Types of electromagnetic forces
- Weak electromagnetic leading to liquids
- Strong electromagnetic leading to solids
2 Forces athe the surface (interface) of materials
- Intermolecular
- Intramolecular
The bond-energy curve provides several important macroscopic material properties.
Specifically, one can estimate:
- the ____ _________
- the __________ _____ ________
- the __________ __________
- the _________ __ ________ __________
- bond energy,
- average bond length
- elastic modulus
- coefficient of thermal expansion
Electrostatic forces that hold atoms together (5)
- Van der Waals interactions
- Ionic
- Hydrogen (H) bonding
- Metalic
- Covalent
Van der Waals interactions
Once a random dipole is formed in one atom, an induced dipole is formed in the adjacent atom.
Relative strength: Weak
Ionic forces
Atoms with a permanent positive (+) charge attract atoms with a permanent negative (-) charge.
Relative strength: Very strong
Hydrogen (H) bonding
The interaction of a covalently bound hydrogen with an electronegative atom, such as oxygen or fluorine.
Relative strength: Medium
Metallic interatomic force
The attractive force between a “sea” of positively charged atoms and delocalized electrons.
Covalent Force
A sharing of electrons between two atoms.
Relative strength: strong
Strong intermolecular forces arise from ____
the sharing of electrons between two or more atoms
Characteristics of Covalent and metallic bonds:
- Valency
- Directionality
- Short range (1 -2 Å)
- Relatively strong (100 - 300 kT/bond)
Simple bonding models assume that the total bonding results from the sum of two forces:
The repulsive force dominates at ______ ________and the attractive force dominates at _______ _________ At equilibrium they are ____ ________
an attractive force (FA ) and a repulsive (FR).
FN =FA+FR
small distances,
larger distances.
just equal.
r = r0
Equilibrium separation distance
Equilibrium separtion distance:
r = r0
Ions occupy finite amount of space so this is the closest separation ===> bond length
Equilibrium separtion distance:
Ions ____ finite ________ __ _____ so this is the closest ______ ===> _______ length
r = r0
Ions occupy finite amount of space so this is the closest separation ===> bond length
Equilibrium separtion distance:
Ions occupy ________ amount __ _____ so this is the ______ separation ===> bond _______
r = r0
Ions occupy finite amount of space so this is the closest separation ===> bond length
At equilibrium when r = r0
Fnet =
Unet =
Fnet = 0
Unet = 0
Fa =
attractive forces (Coulombic force)
Ionic bond: Once charge transfer has occurred a ______ __ ________ occurs between ions ___
force of atraction
Fa
Fa =
€0 = permittivity of vacuum
€0 = 8.85 x 10-12 C2/Nm2
Repulsive force equation
Fr= - B / (rm)
B and m are constants 9<m>
</m>
Force of attraction and Force of repulsion
Fa= A / r2 —-> A = q1q2 /(4π€0)
Fr = - B / (rm)
B and m are constants 9 < m < 12
Fr is dominant at small r values
Fa is dominant at larger r values
Net force at the equilibrium separation distance is:
Bond-Force curve: it gives ___ _____ _______ by ___ ____ as a _________ of __
Fnet = Fa + Fr
Fnet = 0
the force experienced by the ions as a function of r
The bond-force curve will provide us with the information about __________ __ ___________ or ________ ___________
The bond-force curve will provide us with the information modulus of elasticity or Young’s modulus (E)
The slope of the bond-force curve is ______ and it is the measure of ___ _____ (__) required to _______ _____ from ____ _________ ______ ( )
dF/dr
it is the measure of the force 9F) required to displace atoms from their equilibrium position (r0)
Near r0 we have:
F α
F α Δr
F / Δr = aE
Force is proporcional to separation distance
by a factor E
E = Young’s modulus
a = geometric factor
Young’s modulus:
is the a measure of the resistance of the material to relative atomic separation (stiffness)
Different classes of Materials will exhibit _________ ____________ and thus __________ ___________
Compare the curves for materiasl A and B (slopes and E values):
will exhibit different curves and thus different slopes:
The steeper the slope, the greater the force required to move atoms from their equilibrium position, the higher the value of E
Fadoes work as it ______ ______ _______ from an ________ ________ _________
Energy = ______ = U(a) =
Fa does work as it draws ions together from an infinite separation distance.
Energy = work = U(a) =
U(a) = integral ∞ to r (Fa δr) = -A/r
A = (Q1Q2) / 4Πε0
U(a) = integral ∞ to r (Fa δr) = - (Q1Q2) / 4Πε0r
Repulsion Energy (Ur)
U(r) = integral r to ∞ (Fa δr) = C/rn
n = m-1
C = B/n
U(net) =
F(net) at the bottom of the energy well =
U(net) = U(a) + U(r)
F(net) = 0
There is not thermal energy and not vibration.
bond length
equilibrium separation distance
What happends to bond length when thermal energy increases?
desparation distance increases —> this give ions mobility (vibration)
What does the depht of the well means (on the energy-bond curve)?
Measure of inherent bond strength
Coefficient of thermal expansion:
Definition:
Equation:
Ast temperature increases, atoms gain energy and move up the sides of the energy well.
(re - ro) / ro = αth (T – To)
re = equilibrium separation at temperature T
ro = equilibrium separation at To
αth = coefficient of linear expansion
