Biomaterial Surface Modification Quiz

Biocompatible Materials - Surface Modification

• Course: 376-1714-00L, Biocompatible Materials
• Date: 13.11.2024
• Lecturer: Prof. Dr. Katharina Maniura, Empa

Teaching Objectives

• Students will learn different techniques to control biomolecule/protein adsorption to biomaterials.
• Students will understand the advantages and disadvantages of physical adsorption compared to covalent protein coupling.
• Students will be able to identify various chemical pathways and functional groups for biomolecule conjugation.
• Students will grasp the basic degradation mechanisms of biomaterial surface coatings.

Surface Modifications - Key Parameters

• Density: Crucial for the amount of biomolecules or proteins on the surface.
• Stability: The longevity of biomolecule adhesion on the surface.
• Specificity: The selectivity of biomolecule interactions with the surface.
• Activity: The biological activity of the adsorbed biomolecules.
• Chemical Binding: Defines the specificity of interactions with biomolecules.
• Physical Adsorption: Defines the specific response of implanted materials (e.g., tissue growth).

Bioactive Surface Functionalization

• Physical Adsorption: Includes the diffusion, adsorption, and adhesion processes of biomolecules.
• Chemical Conjugation: Combining biomolecules to the surface via chemical reactions, like forming covalent bonds.
• Bioinspired Functionalization: Mimicking natural systems (e.g., biological surfaces) to adjust surface properties for specific biomolecule interactions.

Passivation: Anti-fouling Surfaces

• Self-assembled Monolayers (SAMs): Organized molecular layers forming on surfaces.
• Supported Lipid Bilayers (SLBs): Mimic cell membranes with two lipid layers.
• Polymer Brushes: Polymer chains extending from the surface.

Stability and Degradability

• Surface stability is a key factor in biomaterial design.
• Degradation of surfaces can be passive (e.g., water-induced cleavage) or active (e.g., biological processes).
• Control over degradation can be achieved by introducing cleavable bonds.

Physical Adsorption of Biomolecules - Electrostatic Interactions

• Electrostatic interactions are influenced by introducing charges to surfaces (e.g., plasma treatment).
• Oxidizing agents (e.g., Piranha solution) modify surfaces (e.g., hydroxylating them).
• Plasma treatment acts as a "molecular sandblast".
• Reactions involving radicals are key.
• Surfaces become more hydrophilic.

Physical Adsorption of Biomolecules - Types of Interactions

• Relevant interactions are:
  • Hydrophobic interactions.
  • Electrostatic/Coulomb interactions.
  • Van der Waals interactions.
  • π−π interactions.
  • Ion bridging.

Control of Physical Adsorption

• Electrostatic Interactions: Introducing charges to the surface (e.g., plasma treatment).
• Hydrophobic Interactions: Modifying materials to become hydrophobic or adjusting the entropic interactions between biomolecules and water.
• Affinity Recognition: Adding relevant end groups (e.g., biotin-streptavidin) to allow for specific binding and recognition.

Physical Adsorption - Plasma Treatment

• Plasma is a partially ionized gas at high temperatures.
• It creates a means for surface modification via ionization of gas atoms as "molecular sandblast".
• Molecular processes are triggered at the surface including bond-breaking and activation/chemical reaction.
• Surfaces become more energetic and charged (increasing hydrophilicity).

Physical Adsorption - Electrostatic Interactions

• Coating with a charged polymer (e.g., Poly-L-Lysine):
  • A polymer comprised of repeating amino acid units is used.
  • Positively charged amino groups bind to negatively charged residues.
  • The surface is modified and made more attractive to negatively charged molecules.

Control of Physical Adsorption - Summary

• Electrostatic interactions are affected by introducing charges using various methods like plasma treatment or applying charged molecules.
• Hydrophobic interactions can be manipulated by altering hydrophobicity or regulating protein mobility.
• Affinity recognition leverages specific interactions (such as biotin-streptavidin) to target bonding.

Limitations of Physical Adsorption

• Stability: Interactions are typically weak and reversible.
• Specificity: Different molecules can adsorb depending on the surrounding environment (e.g., different solutions).
• Activity: The orientation of adsorbed biomolecules is frequently random.
• Enhanced control via covalent conjugation of biomolecules is necessary as this allows controlled orientation and better stability.

Covalent Coupling of Biomolecules

• Advantages:
  • Stable and specific immobilization.
  • Applicable to small molecules.
  • Controlled orientation is feasible.
• Disadvantages:
  • Requires specific functional groups on both surfaces.
  • Can be more expensive with lower throughput.

(Bio-)Conjugation Chemistry - Functional Groups

• Hydroxyl (OH): Polar, hydrophilic.
• Amino (NH₂): Polar, basic.
• Carbonyl (CO, CHO): Polar, can be acidic.
• Carboxyl (COOH): Polar, acidic.
• Thiol (SH): Polar.
• Phosphate (PO₄): Polar, acidic, charged.
• Vinyl (CH₂CH): Hydrophobic or hydrophilic depending on the attached groups.

Methods to Create Reactive Groups on Surfaces

• Ionizing Radiation
• Plasma Activation
• Photochemistry
• Ozone Grafting
• Chemical Derivatization
• Polymeric Adlayers (e.g., PLL)
• CVD/PVD
• Silanization

Silanization of Biomaterials

• Coupling of silanes to hydroxylated surfaces is common.
• Common substrates are rich in hydroxyl groups (e.g., glass, silica, PDMS, etc.).
• Various silane linkers allow diverse functionalities.
• The process is straightforward, fast, and cost-effective.

Surface Coupling of Biomolecules Using a Spacer

• Bioreactive end groups are crucial for coupling to surfaces.
• Covalent bonding enables attachment.
• Advantages include: steric freedom, accessibility, modularity, and tunable degradability.
• PEG linkers (of varying lengths) are typical spacers.

Comparison of Surface Immobilization Strategies

• Physical Adsorption: Simpler, but less stable, weaker and unspecific binding. Lower cost.
• Chemical Bonding: More complex, stronger and specific binding. Higher cost.

Example: Albumin

• Illustrates both physical adsorption (hydrophobic, electrostatic interactions) and covalent tethering methods.

Bioinspired Strategies: Using Attraction

• Mimicking nacre (mother-of-pearl) for creating 3D structures via alternating deposition of layers.

Bioinspired Strategies: Mussel Adhesion and DOPA

• Shows bioinspired strategies employing natural adhesive mechanisms like mussel adhesion and dopamine-based (DOPA) materials to attach different surfaces.

Surface Passivation - Facts

• Ideal non-fouling surfaces resist protein adsorption.
• Low protein adsorption leads to reduced cell adhesion on the surface.
• Hydrogels are generally non-fouling while hydrophobic surfaces are prone to fouling.
• Non-fouling surfaces typically have strong water interactions that shield surface charges.

How to Avoid Protein Adsorption

• Reduce enthalpic interactions (electrostatic, van der Waals, ionic).
• Add an entropic penalty to the surface (e.g., using hydrophobic interactions).

Summary on Surface Passivation

• Crucial to consider enthalpic and entropic contributions to bioadhesion and interaction with a solvent.
• PEG is a prime example of an anti-fouling polymer.
• Self-assembled monolayers (SAMs) are a popular technique for controlling molecular organization and chemical properties on surfaces.
• Supported lipid bilayers (SLBs) mimic cell membranes and provide anti-fouling properties.
• Polymer brushes offer a readily-tunable technique with versatile functionality.

Stability of Functionalized Surfaces

• Passive: Typically involves desorption (e.g. water-induced cleavage).
• Active: Includes cellular adhesion, and enzymatic degradation of bonds on the surface.