CHAPTER
7
Surface Properties
of Biomaterials
7.1 Introduction: Concepts in Surface Chemistry and Biology
biomaterial surface --- biological response and implant success
1) physicochemical and biological modifications
2) surface patterning
7.1.1 Protein Adsorption and Biocompatibility
atoms on the surface with extra-energy ---- surface tension
adsorption to minimize the surface tension (driving force)
adsorbates under physiological conditions
coated surface with proteins
controlling protein adsorption to the surface --- biocompatibility
thermodynamics of protein adsorption
7.1.2 Surface Properties Governing Protein Adsorption
(1) Surface hydrophobicity
contact angle analysis
polymers > metals & ceramics
hydrophobicity --- protein adsorption
(2) Surface charge
charge interaction with proteins
(3) Physical characteristics of the surface
steric concerns (hindrance & roughness)
1) large, hydrophilic polymers --- adsorption 감소
(quick movement & bulky chain)
2) roughness --- adsorption 증가
(physical trapping)
Surface modifications --- Biocompatibility
7.2 Physicochemical Surface Modification Techniques
7.2.1 Introduction to Surface Modification Techniques
post-fabrication vs. pre-designed
ideal tech.
1) thin
2) resistant to delamination
3) simple & robust
4) discourage
surface rearrangement
Physicochemcial
& Biological
Modifications
7.2.2 Physicochemcial Surface Coatings: Covalent Surface Coatings
(1) Plasma treatment
plasma: assembly of species
in an atomically/molecularly dissociated gaseous environment
electric potential across a gas between cathode (-) and anode (+)
electrons from cathode (sample) to anode
positive ions to the sample on cathode
1) at the surface
competition between deposition and ablation/etching
very energetic species: etching
deposition (free radicals or small molecules)
2) plasma discharge for surface pre-treatment
cleaning or addition of –OH or –NH2 groups
X-linking the copolymer on the surface
[advantages]
1) conformal; 2) free of voids/pinhole defects; 3) easily prepared;
4) sterile; 5) low amount of leachable substances; 6) good adhesion;
7) unique film chemistry; 8) easy characterization
[polymeric, metallic, and pyrolytic carbon-based materials]
[disadvantages]
1) ill-defined chemistry; 2) expensive equipment; 3) difficult uniform reaction;
4) contamination
cf) plasma torch
plasma spray coatings [ceramic coatings to metallic implants]
(2) Chemical vapor deposition (CVD)
mix of gases – high temp – decomposition of one/more components of the gas
--- substrate deposition
plasma assisted chemical vapor deposition
pyrolytic carbon coatings on substrates
[tantalum, molybdenum/rhenium, graphite]
(3) Physical vapor deposition (PVD)
sputtering
1) energetic ions or atoms --- target bombardment --ejection of target surface atoms
2) target atoms --- sample surface --- thin film formation via condensation
plasma-assisted PVD
target (-) & sample (+)
(4) Radiation grafting/photografting
radiation on the surface
--- reactive species
--- covalent coating
on the underlying material
ex) hydrogel to hydrophobic substrates
1) mutual irradiation
a) biomaterial substrate in monomer solution
b) substrate irradiation first – then coating
2) radiation grafting in air
ROS generation --- heating ---- polymerization
3) photografting with photoresponsive chemical moiety [phenyl azide, benzophenone]
(5) Self-assembled monolayers (SAMs)
Amphiphilic molecules
a) attachment group; b) long hydrocarbon (alkyl) chain;
3) functional (polar) head group
1) strong exothermic reaction between substrate and attachment group
2) alkyl chains
van der Waals force --- crystallization
high molecular mobility --- tight packing
[advantages of SAM]
1) easy formation; 2) chemical stability; 3) variety of chemical moieties
4) physical property control (smooth surface)
7.2.3 Physicochemical Surface Coating: Noncovalent Surface Coating
(1) Solution coating
dipping (polymer in organic solvent) --- dry --- polymer deposition
cf) bioactive molecule coating in aqueous solvent
(2) Langmuir-Blodgett films
amphipathic molecules in Langmuir trough
compression to critical area [constant minimum value of the area per molecule]
size and type of hydrophobic tail
[advantages]
1) uniformity; 2) chemical alteration
[disadvantage]
1) instability [X-linking]
(3) Surface-modifying additives (SMA)
atoms and molecules in the material bulk
spontaneously rise to the surface to reduce the surface free energy
surface coating
1) difference in surface tension with and without the SMA
2) mobility of SMA; 3) surrounding environment (air, water)
not post-fabrication, but a part of the formation/synthesis of the biomaterials
metal; ceramics
polymers
Block copolymer
A: anchor [bulk polymer interaction]
B: surface property
[incompatible with the bulk]
[surface affinity]
SMA coating: final environment 고려
ex) hydrophobic group
7.2.4 Physicochemical Surface Modification Methods with No Overcoat
modify existing atoms at the surface
--- hydrophobicity, protein affinity, wear resistance
Ion beam implantation
accelerated high energy ions --- surface penetration
1) formation of vacancies and interstitials
2) sputtering of atoms
3) localized heating
ex) nitrogen to titanium ---- wear resistance
boron & carbon into stainless steel ---- fatigue life
[advantages]
1) hardness; 2) wear; 3) corrosion;
4) biocompatibility
[disadvantages]
1) vacancy & interstitial defects
heat treatment 필요
(2) Plasma treatment
plasma discharge – etching and cleaning processes --- surface properties
(3) Conversion coatings (not overlay coating)
oxide layer on the metallic surface
chemically inert & barrier to electron transfer (preventing corrosion)
acid treatment (steel); anodization (aluminum & titanium)
(4) Bioactive glasses
biological responses like fibrous encapsulation to material dissolution
depending on the ratio of CaO, Na2O, SiO2
IB index: measurement of bioactivity
high number --- quick integration
in vivo modification of a bulk material
7.2.5 Laser Methods for Surface Modification
laser --- high energy --- reactions of annealing/alloying, etching, film deposition
polymerization
laser selection
pulsed or continuous wave beam
heating on the surface
[advantages]
1) atmospheric conditions; 2) precise wavelength; 3) reaction time control
& spatial location control; 4) heat and light-induced excitation
7.3 Biological Surface Modification Techniques
attachment of biologically active molecules on the substrate
retention of biological activity
orientation and rotation ability of individual molecules after coating
Biomolecules on polymer substrates
soluble polymers
solid polymers with or without pores
three-dim scaffolds
hydrogels
7.3.1 Covalent Biological Coatings
surface with –OH, -COOH, -NH2 groups
or introduction of functional groups
spacer arm
rotational freedom to improve the biological activity
biodegradable spacer arm [selective release of biomolecules]
attachment of biomolecule
1) post-fablication reaction
glue or releasable bond
2) combined conjugation/synthesis reactions
a) biomolecule + monomer --- polymerization
b) activated precursor --- polymerization --- biomolecule coupling
7.3.2 Noncovalent Biological Coatings
adsorption and X-linking
via hydrophobic and electrostatic
interactions
heparin on the surface
1) hydrophobic region
2) negative charge
7.3.3 Immobilized Enzymes
biosensor, controlled release devices, protein analysis
bioactivity of enzymes
1) immobilization tech
hydrophilic hydrogel: polyacrylamide, PEG
hydrophobic carrier: nylon, poly(styrene)
2) geometry of the carrier
7.4 Surface Properties and Degradation
degradation
surface coating
or underlying substrate
conversion coatings (oxide films)
biodegradable polymers
removal of the supporting material
degradation by-products
7.5 Patterning Techniques for Surfaces
surface or substrate patterning
geometric design of well-defined regions
(1) Microcontact printing
mold (photolithography on silicon wafer)
silicone rubber --- stamp
--- inked --- press on the surface
--- mutifunctional surface 제작
hydrophilicity of portions of a surface
protein immobilization
--- cell attachment
(2) Microfluidics
positive mold --- PDMS polymerization --- glass slide & plasma treatment ----- PDMS on the substrate + protein solution --- PDMS removal
multifunctional surface 제작
different surface-active molecules
physicochemcial and biological
coating methods
very little fluid volume