Biomaterials Unit 5 Quiz

UBM008: Biomaterials - Odd Semester 2024-25

• Course: B.Tech. BME 2nd year
• Faculty: Debasmita Mondal
• Assistant Professor, DEIE, TIET, Patiala
• Email: [email protected]
• Phone: 9004008796

Unit 5: Syllabus

• Biomaterial Processing and Surface Properties
• Processing of metals, ceramics, and polymers to enhance bulk properties
• Techniques to improve biocompatibility
• Chemical, biological, and physical modifications of biomaterial surfaces
• Contact angle analysis
• Surface characterization techniques (light and electron microscopy)

Introduction

• Biomaterial processing aims to alter bulk or surface properties, for desired shapes, sterilization or improving biocompatibility
• The primary targeted bulk property is mechanical strength
• Stronger/harder materials require reduced dislocation motion, requiring more energy for plastic deformation
• Methods to improve bulk properties of metals:
  • Alloying
  • Strain Hardening
  • Grain size refinement
  • Annealing
  • Precipitation Hardening

Alloying

• An alloy is a solid solution of two or more elements in a solid solution form.
• Alloying introduces many substitutional point defects into the base metal.
• Alloys typically exhibit higher corrosion resistance and strength compared to pure metals.
• Point defects induce localized lattice strain, the strain field size depending on the size difference between alloying elements and host atoms.
• Strain field interactions influence already existing dislocations and impurities.
• Crystal distortion due to impurities may either increase or decrease lattice strains depending on the dislocation type (tensile or compressive) and impurity location relative to the dislocation.

Strain Hardening

• Adding point and line defects (dislocations) augments metal strength.
• Strain hardening, also known as cold working, occurs when a material is deformed plastically beyond its yield point.
• Increasing dislocation density leads to dislocations interacting and becoming tangled or pinned, hindering further movement and thus enhancing material strength (yielding to higher force requirements for continued deformation).
• Cold working enhances the material's strength but decreases ductility.

Grain Size Refinement

• Reducing the size of grains improves material quality and properties (mechanical strength, response to heat etc.)
• Metals are polycrystalline, meaning they are composed of grains with different orientations.
• Dislocations encounter difficulty crossing grain boundaries, impacting normal slip.
• For plastic deformation, dislocations need to move across grain boundaries; differences in grain orientation impede dislocation movement across these boundaries.
• Finer grain sizes introduce more grain boundaries, thereby hindering dislocation movement and increasing strength.
• Fewer dislocations pile up at the boundaries in fine-grained materials, leading to lower stress on the boundaries and higher yield stress, rendering the fine-grained material stronger.

Annealing

• Cold working (strain hardening) enhances material strength but reduces ductility and corrosion resistance.
• Annealing is a heat treatment process used to increase ductility, toughness, and reduce internal stresses in metals.
• Annealing comprises:
  • Heating to the required temperature
  • Maintaining the material at that temperature.
  • Controlled cooling (quenching)
• Factors affecting annealed material properties include material composition and quenching rate.

Precipitation Hardening

• Introducing volume defects (uniformly dispersed small particles of a second phase within the primary phase matrix) to a material can enhance its strength.
• Precipitate hardening, also known as age hardening or particle hardening, is a heat treatment that enhances strength and hardness of materials.
• Precipitates cause local lattice strains within the host crystal, acting as barriers to dislocation motion, thereby strengthening the material.
• Strengthening effect decreases as particle spacing increases.
• Dislocations are hindered by the closeness of closely spaced precipitate particles in the matrix.

Processing to Improve Bulk Properties

• Ceramics are strengthening by enhancing slip systems, while dislocation movement in covalently bonded crystals (like many polymers) is difficult.
• Polymer strength is sometimes increased by increasing percent crystallinity.
• Thermal processing techniques increase crystallinity, whereas pre-drawing procedures are similar to strain hardening seen in metals.

Processing Techniques for Improving Biocompatibility

• Biocompatibility implies that a material does not elicit harmful or unwanted host responses (infections, rejection).
• Sterilization of biomedical implants is crucial to prevent infections.
• It is often impossible to completely remove all pathogenic agents or eliminate their possible transfer to sterilized implants.
• Sterility assurance levels (SAL) are used to measure and control the level of sterilization, measuring the number of surviving bacterial colonies after different sterilization times.

Steam Sterilization/Autoclaving (High Temperature)

• Steam sterilization is achieved by subjecting items to high-pressure saturated steam at 121°C.
• It's highly effective and relatively quick, leaving no toxic residues.
• It is not suitable for materials with low melting points and/or susceptible to degradation from high temperatures.

Ethylene Oxide Sterilization

• Uses a specialized apparatus to expose implants to EtO gas.
• Often mixed with an inert gas, commonly at 30°C - 50°C and further purged with air.
• A suitable method for a wide range of materials because of its low-temperature application.
• Eto is toxic and flammable therefore appropriate safety measures are important.

Radiation Sterilization

• Gamma rays or electron beams from accelerators are used for radiation sterilization.
• Samples are monitored to ensure they have received the appropriate radiation dosage.
• Radiation ionization of cellular components (nucleic acids) kills microorganisms.
• Very rapid, effective and suitable for many materials, but requires high capital investment for sterilizing equipment and special care with some materials that might undergo degradation.

Concepts in Surface Chemistry and Biology

• Biomaterial surfaces significantly influence biological responses and implant success.
• Protein Adsorption and Biocompatibility
• Surface of a material possesses a higher energy density compared to its bulk, making it thermodynamically unstable.
• This instability drives the adsorption of atoms or molecules, including proteins, ions and water.
• Controlling protein adsorption is crucial for biocompatibility.
• Surface properties (hydorrophobicity, charge, roughness, steric concerns) heavily affect protein adsorption influencing favourable adsorption events.

Biocompatibility and surface Properties influencing protein adsorption

• Surface hydrophobicity is a key factor in determining the tendency of a surface to adsorb proteins, with more hydrophobic surfaces having a higher level of adsorption.
• Surface charge (positively or negatively charged areas) also plays a role in protein adsorption by attracting proteins with opposite charges, and repelling those with similar charges.
• Surface roughness causes physical trapping of proteins in crevices, which increases likelihood of adsorption.
• Adding steric considerations, like bulky hydrophilic chains (e.g poly(ethylene glycol ) (PEG)) to surfaces can reduce protein adsorption.

Physicochemical Surface Modification Techniques

• Surface modifications preserve bulk material characteristics, while optimizing surface properties.
• Ideal techniques:
  • Thin film to minimize volume property effects
  • Resistant to delamination (separation)
  • Simple & robust to promote widespread use
• Techniques are commonly grouped into physicochemical and biological modification approaches.

Physicochemical Surface Coatings

• Techniques include:
  • Plasma treatment
  • Chemical Vapor Deposition (CVD)
  • Physical Vapor Deposition (PVD) ( sputtering and thermal evaporation)
  • Self-assembled monolayers (SAMs).
  • Solution Coatings
  • Langmuir-Blodgett Films

Light Microscopy

• Used for largely qualitative analysis of surface topography to view thin sections under a microscope with visible light passing through these thin samples.

• The sample is placed on the stage and focused by the light condenser lens which passes through the sample and is magnified by the objective and ocular lenses.

• Resolution of light microscopy is around 0.2µm, with smaller features less resolved.

Electron Microscopy

• Electron microscopy (EM) achieves high resolutions using electron beams instead of light.

• Two main types are: Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM).

• TEM requires thin samples, (20-200um), for electron beams to efficiently pass through and create an image.

• SEM scans the surfaces of samples by the emitted secondary/backscattered electrons that are dependent on the surface topography.

• SEM provides surface topography images, and, if XRAY analysis is performed, additional information about the sample’s chemical composition

Contact Angle Analysis

• Used for determining surface hydrophobicity of a material using the contact angle (θ)
• The surface free energy or surface tension can be derived by calculating the contact angle of liquids on the material surface, using Young's equation for liquid-solid interfaces
• The critical surface tension can be extrapolated by plotting contact angle versus liquid surface tension.
• Contact angles can range from 0° to 180°, with 180° indicating complete non-wetting and high hydrophobicity.
• Instrumentation includes:
  • Solid sample holder
  • Liquid holder
  • Contact angle measurement device (automated)
• Providing information about the material surface, as a number that describes the contact angle.

Test your knowledge on biomaterial processing and surface properties in this Unit 5 quiz for B.Tech. BME. You'll cover techniques to enhance biocompatibility, processing methods for metals, ceramics, and polymers, and key characterization techniques. This quiz aims to reinforce your understanding of crucial concepts in biomaterials.