Introduction to ceramics, their structure, tissue attachment mechanisms, classification of ceramics and non-absorbable or relatively bioinert bioceramics.

Introduction to ceramics, their structure, tissue attachment mechanisms, classification of ceramics and non-absorbable or relatively bioinert bioceramics

Ceramics Structure of Ceramics
Since the bonds in ceramics are partially to totally ionic in nature (i.e. Pure ionic bonding cannot exist: all ionic compounds have some degree of covalent bonding. Thus, an ionic bond is considered a bond where the ionic character is greater than the covalent character) Crystal structures in ceramic materials are thought of as being composed of ions rather than atoms The variety of chemical compositions of ceramic materials results in a wider range of crystal structures than with metals Ceramic crystal structure is affected by two parameters that are not concerns in metallic structures; (i) the magnitude of the electrical charge on the constituent ions, and (ii) the particle size of these ions

Ceramics Ceramics Ceramics Structure of Ceramics
The magnitude of the electrical charge is important because the crystal must remain electrically neutral (i.e. Sum of cation and anion charges in unit cell should be zero) The second characteristic requires the knowledge of the radii of both the cations (rc) and anions (ra) composing a ceramic material Cations are generally smaller, positively charged and usually metals Anions are usually O, C, N, larger and negative charge For an optimally stable structure, cations prefer to contact the maximum allowable number of anions (and vice versa for the anions) Certain ratios does not allow close contact between cations and anions and thus produce unstable structures

Ceramics Ceramics Ceramics Structure of Ceramics

Ceramics Structure of Ceramics
In this case, ion’s coordination number refers to the number of nearest neighbors with opposite charge and it depends on the rc/ra ratio This ratio of ionic radii dictates the coordination number of anions around each cation As the ratio gets larger, the coordination number gets larger For example, for a coordination number of 4, the cation is found at the center of a tetrahedron, with anions at each corner The most common coordination numbers for ceramics are 4,6 and 8 Silicate Structure

Ceramics Structure of Ceramics AX Crystal Structures
For ceramics in which both the cation and anion have the same charge, an equal number of each is required for a stable crystal structure These are called AX crystals, with A representing the cation and X representing the anion The most common AX structure is the Sodium Chloride (NaCl) structure, and the coordination number for both cations (Na+) and anions (Cl-) is 6 The NaCl structure can be thought of as two interpenetrating FCC-type crystals, one composed of anions and the other of cations

Ceramics Structure of Ceramics AmXp Crystal Structures
Ceramic materials are often composed of cations and anions that do not have equal charges, leading to compounds with the formula AmXp A common example is found in Fluorite (CaF2) The coordination number for Ca2+ is 8, and the system exhibits a cubic coordination geometry The cations are at the center of the cube, with the anions at the corners

Ceramics Structure of Ceramics AmXp Crystal Structures
The difference between these structures is due to the relative size of the ions If the positive and negative ions are about the same size, the structure becomes a simple cubic (CsCl) structure. The face-centered cubic structure arises if the relative size of the ions is quite different

Ceramics Structure of Ceramics Carbon-based Materials
Although carbon-based materials do not neatly fall into any of the classes of materials (metals, ceramic or polymers) One common form, Graphite, is sometimes considered a ceramic Even though it does not possess a standard unit cell, Graphite is crystalline Crystal structure of Graphite

The structure consists of planes of hexagonally arranged carbon atoms Within the planes, each carbon atom is bonded covalently to three neighbors While the fourth valence electron participates in van der Waals interactions with plane above it

A property of graphite that is important to the biomaterialist is its ability to adsorb gases This is used in the formation of Pyrolytic carbon, in which carbon in the gaseous state is deposited onto another material (such as graphite) Pyrolytic carbon has been used in a number of cardiovascular devices, including replacement heart valves

An additional synthetic form of carbon can be found in Single-walled nanotubes (SWNT) and Multi- walled nanotubes (MWNT) A single-walled nanotube can be visualized as a single sheet of graphite rolled to form a tube Similarly, a multi-walled nanotube can be visualized as a tube rolled from multiple layers of graphite sheets These carbon nanotubes are generally a few nanometers in diameter and on the order of a micron in length

Ceramics Tissue Attachment Mechanisms
No one material is suitable for all biomaterial applications As a class of biomaterials, ceramics, glasses and glass- ceramics are generally used for repair or replacement of musculoskeletal hard connective tissues Their use depends on achieving a stable attachment to connective tissue Carbon-base ceramics are also used for replacement heart valves, where resistance to blood clotting and mechanical fatigue are essential characteristics Diamond-like carbon (DLC) coated

Ceramics Tissue Attachment Mechanisms Implant Material Characteristics
The mechanism of tissue attachment is directly related to the type of tissue response at the implant interface No material implanted in living tissues is practically inert; all materials elicit a response from living tissues Four types of possible tissue responses to biomedical implants, given below Implant Material Characteristics Tissue Response Toxic Surrounding tissue dies Nontoxic, biologically inactive Fibrous tissue of variable thickness forms Nontoxic, bioactive Interfacial bond forms Nontoxic, dissolves Surrounding tissue replaces material

Ceramics Tissue Attachment Mechanisms Type of Attachment Example
These types of tissue responses (mentioned in the previous slide) allow four different means of achieving attachment of prostheses to the musculoskeletal system Therefore, tissue attachment mechanisms for bioceramic implants, are Type of Attachment Example Dense, nonporous, nearly inert ceramics attached by bone growth into surface irregularities by cementing the device into the tissues, by press-fitting into a defect, or attachment via a sewing ring (Morphological fixation) Alumina (Aluminium oxide Al2O3) Pyrolytic Carbon For porous inert implants, bone ingrowth occurs, which mechanically attaches the bone to the materials (Biological fixation) Hydroxyapatite-coated porous metals Dense, nonporous, surface-reactive ceramics, glasses and glass ceramics attach directly by chemical bonding with the bone (Bioactive fixation) Bioactive glasses Hydroxyapatite Dense, nonporous (or porous), resorbable ceramics are designed to be slowly replaced by bone Calcium sulfate (Plaster of Paris) Tricalcium Phosphate

Ceramics Tissue Attachment Mechanisms
When biomaterials are nearly inert and the interface between an implant and bone is not chemically or biologically bonded There is relative movement and progressive development of a fibrous capsule in soft and hard tissues The presence of movement at the biomaterial-tissue interface eventually leads to deterioration in function of the implant or the tissue at the interface or both

Ceramics Tissue Attachment Mechanisms

Ceramics Classification of Ceramics
Ceramics used in fabricating implants can be classified as Nonabsorbable (relatively inert) Bioactive or Surface Reactive (semi-inert) Biodegradable or Resorbable (non-inert) Calcium phosphate Putty for coating/filling purpose of various Grafts (which enables excellent cell infiltration, vascularization and resorption) Alumina, zirconia, silicone nitrides and carbons are inert bioceramics Certain glass ceramics and dense hydroxyapatite are semi-inert (bioreactive/bioactive) Calcium phosphates and calcium aluminates are resorbable ceramics Collagen ceramic osteoconductive scaffolds are engineered to mimic the composition and pore structure of natural bone

Ceramics Classification of Ceramics

Ceramics Nonabsorbable (Relatively inert) Ceramics
Relatively bioinert ceramics maintain their physical and mechanical properties while in the host They resist corrosion and wear have properties, such as Non-toxic Non-carcinogenic Non-allergenic Non-inflammatory Biofunctional for its lifetime in the host Examples of relatively bioinert ceramics are dense and porous Alumina (Aluminium oxides) and Zirconia (Zirconium dioxides) etc. Relatively bioinert ceramics are typically used as structural-support implants, such as bone plates, bone screws and femoral heads Examples of non-structural support uses are ventilation tubes, sterilization devices and drug delivery devices

Composition (weight %)
Ceramics Nonabsorbable (Relatively inert) Ceramics Aluminium oxide Al2O3 (Alumina) The main source of high purity Alumina is Bauxite and Corundum The chemical composition of commercially pure Alumina are given below Bauxite rock Chemicals Composition (weight %) Al2O3 99.6 SiO2 0.12 Fe2O3 0.03 Na2O 0.04 The American Society for Testing and Materials (ASTM) specifies that Alumina for implant use should contain 99.5% pure Alumina and less than 0.1% combined SiO2 and alkali oxides (mostly Na2O) Corundum rock

Polycrystalline structure
Ceramics Nonabsorbable (Relatively inert) Ceramics Aluminium oxide Al2O3 (Alumina) The single crystal form of Alumina has been used successfully to make implants The strength of polycrystalline Alumina depends on its grain size and porosity Generally, the smaller the grains, the lower the porosity and the higher the strength The ASTM standards requires Elastic Modulus of around 380GPa and a flexural Strength greater than 500MPa Structure of Single Crystal Polycrystalline structure

Aluminium oxide Al2O3 (Alumina) Single crystal Alumina has been used in orthopedics and dental surgery for almost 20 years Alumina is usually a quite hard material, its hardness varies from 20 to 30 GPa The high hardness is accompanied by low friction and wear and inertness to the in vivo environment. These properties make alumina an ideal material for use in joint replacements

Zirconium dioxide ZrO2 (Zirconia) Zirconium dioxide (ZrO2), sometimes known as zirconia, is a white crystalline oxide of zirconium Zirconium is taken from the mineral zircon (the most important source of zirconium). It is a lustrous, grey-white, strong transition metal that resembles titanium Zirconia (ZrO2) is an oxidized form of the zirconium metal, just as alumina (Al2O3) is an oxidized form of aluminum metal. Zirconium Metal

Zirconium dioxide ZrO2 (Zirconia) Zirconia may exist in several crystal types (phases), depending on the addition of minor components such as calcia (CaO), magnesia (MgO), yttria (Y2O3), or ceria (CeO2) Pure zirconia exists in 3 crystal phases at different temperatures, i.e. Cubic, Tetragonal and Monoclinic. The transformation between phases occurs during temperature change and can lead to structural failure in the material. A number of different oxides can be dissolved into the zirconia crystal structure to prevent or control these structural changes. Y2O3, MgO and CeO2 are the most common.

Zirconium dioxide ZrO2 (Zirconia) These phases are said be stabilized at room temperature by the minor components If the right amount of component is added, one can produce a fully stabilized Cubic phase If smaller amounts are added, 3 wt% to 5wt%, a partially stabilized zirconia is produced. The tetragonal zirconia phase is stabilized, but under stress, the phase may change to monoclinic, with a subsequent 3% volumetric size increase. This dimensional change takes energy away from the crack and can stop it in its tracks. This is called “transformation toughening”

Zirconium dioxide ZrO2 (Zirconia)

Bioactive or surface-reactive ceramics, biodegradable or resorbable ceramics, processing of ceramics and application of bioceramics By: Dr. Murtaza Najabat Ali (Ceng MIMechE)

Bioactive or Surface Reactive Ceramics
Some Basics Biocompatibility Objective is to minimize inflammatory responses and toxic effects Bioactivity - Evolving concept The characteristic that allows the material to form a bond with living tissue The ability of a material to stimulate healing and trick the tissue system into responding as if it were a natural tissue Advantages: Bone tissue – implant interface, enhanced healing response, extends implant life Biodegradability Breakdown of implant due to chemical or cellular actions Negates issues of stress shielding, implant loosening, long term stability

A different approach to solving problems of interfacial attachment (bone-implant) is the use of Bioactive materials The concept of Bioactive materials is intermediate between Degradable and Bioinert A Bioactive material is one that elicits a specific biological response at the interface, which results in the formation of a bond between the tissue and the material This concept has now been expanded to include a large number of Bioactive materials with a wide range of rates of bonding and thickness of interfacial bonding layers Relatively small changes in the composition of a biomaterial can dramatically affect whether it is Bioinert, Degradable or Bioactive

Actually the term Bioactive was first used to describe certain glass compositions developed in the late 1960s These were found to produce a reaction layer on their surface and thereby form a bond with bone tissue Its is now known that other Bioactive ceramic materials that form a bond with bone develop a surface that is different from the original surface before implantation It has been shown that this Bioactive surface is made up of calcium phosphate layer, and is common to all Bioactive ceramic materials

Types of Bioactive Ceramics Generally they fall into two main material groups: Hydroxyapatites Bioactive Glasses Osseointegration

Hydroxyapatites (HA) The greatest potential for bone substitution is shown by materials based on HA (Ca10(PO4)6(OH)2 HA can develop tight bonding with bone tissue It exhibits osteoconductive behavior It is stable toward bioresorption and has no adverse effects on the human organism The biological behavior of HA ceramics depends on many factors, in particular, on their Chemical and phase composition Microstructure Pore size and pore volume

Hydroxyapatites (HA) Bone is a ceramic-organic composite consisting mainly of collagen (20%), calcium phosphate (69%) and water (9%) Collagen is located in bone tissue and has the form of fibrils Calcium phosphate in the form of crystallized HA ensures bone rigidity The mineral component of bone is similar to HA but contains fluoride, magnesium, sodium and other ions as impurities

Hydroxyapatites (HA)

Hydroxyapatites (HA) The mineral apatite, can be represented by the general formula, M10(ZO4)6X2 Each component (M, ZO4, and X) of the common equation M10(ZO4)6X2 can be replaced by a large number of different elements or solid states M or X can also be absent The most common form found in nature is calcium phosphate apatite, whereby the M and ZO4 are Ca2+ and PO43- groups. If the X is F, it is given the name Fluorapatite, the main source of phosphorus. When the X is OH-, i.e. Ca10(PO4)6(OH)2, it is given the name hydroxyapatite, HA. HA is the key inorganic component of the hard tissues of vertebrae, and is an important substance in bioactive ceramic materials.

Hydroxyapatites (HA) HA ceramics have already been used as filling agents in bone defects in the form of a sintered body, a porous body, granules (agglomerates), and cement. Improvements to these materials are being made with regards to their mechanical properties and bioactivity by compounding with organic polymers. A number of promising materials are currently under development for use as regenerative medicines, (e.g. cell scaffolds), or as vehicles for sustained release of drugs. Typical HA ceramics in current use have a coating material over titanium-based implants, as this has been discovered to be invaluable due to its mechanical strength characteristics.

Hydroxyapatites (HA)

Bioactive Glasses Certain compositions of glasses, ceramics, glass-ceramics and composites have been shown to bond to bone These materials have become known as Bioactive ceramics Some even more specialized compositions of Bioactive Glasses will bond to soft tissues as well as bone

Bioactive Glasses A common characteristic of Bioactive Glasses is a time- dependent, kinetic modification of the surface that occurs on implantation The surface forms a biologically active HA layer that provides the bonding interface with tissues Materials that are Bioactive develop an adherent interface with tissues that resists substantial mechanical forces In many cases, the interfacial strength of adhesion is equivalent or greater than the cohesive strength of the implant material or the tissue bonded to the Bioactive implant

Bioactive Glasses The traditional composition of ceramics such as Soda-Lime-Silica (i.e. Na2O- CaO-SiO2), are commonly called as Glasses Since the discovery of stable Na2O-CaO-SiO2 ceramic formulations several thousand years ago, most silicate ceramics used by man have an SiO2 content of 65 wt.% or more These 65 wt.% silica glasses are extremely Bioinert, weak and shatter easily

Bioactive Glasses The first Bioactive glass contained Na2O (20-25 wt.%), CaO (20-25 wt.%) and SiO2 (45-55 wt.%) Unfortunately, this new type of glass was still very weak and brittle The addition of Phosphorous oxide (P2O5) to the Soda-Lime-Silica (i.e. Na2O- CaO-SiO2) matrix makes the glass extremely Bioactive Bioactive glass is a cleverly designed glass that contains silicon, calcium and sodium oxides

Bioactive Glasses One formulation commercially named as Bioglass 45S5 contains Na2O-CaO-SiO2- P2O wt.% respectively In warm water Bioglass 45S5 (i.e. in the body), the sodium at the surface dissolves. The remaining material is not stable and reorganizes into silica (some of this dissolves) and tiny crystals of hydroxy- carbonate apatite (HCA) This porous surface (reactive/bioactive) layer is a very favorable substrate for the re-growth of bone tissue Although these materials are Bioactive, Resorbable usually refers to those cement formulations which have Calcium Phosphate or Calcium Sulfate

Bioactive Glasses Their Applications Bioactive glasses have many applications but these are primarily in the areas of bone repair and bone regeneration via tissue engineering Synthetic bone graft materials for general orthopedic, craniofacial (bones of the skull and face), maxillofacial and periodontal (the bone structure that supports teeth) repair. These are available to surgeons in a particulate form As Cochlear implants Bone tissue engineering scaffolds. These are being investigated in many forms, in particular as porous (contains pores into which cells can grow and fluids can travel) 3-dimensional scaffolds Treating dentine hypersensitivity and promoting enamel re-mineralization

Biodegradable Ceramics
The temporary nature of the material is ideal to promote localized tissue healing or release of a bioactive agent without the need for a second surgery to remove the implant It also avoids the late device complications Biodegradable ceramics are usually a type of Calcium Phosphate Such as Calcium Hydroxyapatite (Ca10(PO4)6(OH)2) or Tricalcium Phosphate (Ca3(PO4)2)

Calcium Phosphate Calcium phosphate has been used in the form of artificial bone This material has been synthesized and used for manufacturing various forms of implants, as well as for solid or porous coatings on other implants Calcium phosphate can be crystallized into salts such as Hydroxyapatite and Whitlockite depending on the Ca:P ratio, presence of water, impurities and temperature The mineral part of bone and teeth is made of a crystalline form of calcium phosphate similar to hydroxyapatite

Calcium Phosphate In a wet environment and at lower temperatures, it is more likely that hydroxyl or hydroxyapatite will form While in a dry atmosphere and at a higher temperature, Whitlockite will be formed Both forms are very tissue compatible and are used as Bone substitutes in a granular form or a solid block The apatite form of calcium phosphate is considered to be closely related to the mineral phase of bone and teeth

Factors that influence Ceramic Degradation Biodegradable ceramics actually erode under physiological conditions due to a combination of dissolution and physical disintegration Factors that control the erosion of ceramics are; Chemical susceptibility of the material Amount of Crystallinity Amount of media (water) available Material surface area to volume ratio

Factors that influence Ceramic Degradation The chemical composition of the ceramic can have a significant impact on the degradation rate, with hydrated forms eroding faster than their non-hydrated counterparts A more tightly packed crystalline material is less susceptible to dissolution than a ceramic that is mainly amorphous Polycrystalline materials degrade more quickly than ceramics created from a single crystal due to the presence of grain boundaries

Factors that influence Ceramic Degradation Degradation rates are also affected by the total amount of aqueous media available as well as the surface area of the implant Therefore, highly porous materials will dissolve more quickly than the ceramic with fewer pores due to the increase in area for interaction with the environment Additionally, ceramic Bioerosion is encouraged in areas of high mechanical stress, either due to the implant site location or the presence of stress raisers in the device Dissolution can also be mediated by biological factors such as pH drop caused by the presence of inflammatory cells

Processing of Ceramics
There are several types of ceramic biomaterials including Glasses and crystalline ceramics Each has different processing methods as well as distinct final properties Layering of ceramics on top of preshaped materials is also possible through different coating techniques such as plasma spray coating techniques

Introduction to ceramics, their structure, tissue attachment mechanisms, classification of ceramics and non-absorbable or relatively bioinert bioceramics.

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Introduction to ceramics, their structure, tissue attachment mechanisms, classification of ceramics and non-absorbable or relatively bioinert bioceramics.

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Presentation on theme: "Introduction to ceramics, their structure, tissue attachment mechanisms, classification of ceramics and non-absorbable or relatively bioinert bioceramics."— Presentation transcript:

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