Quiz 3 Flashcards
Surface Properties
Surfaces
atoms and molecules make up the outermost surface of a biomaterials
- characterize
- tailor
- drive many of the biological reactions that occur in response to the biomaterial (protein adsorption, cell adhesion, cell growth, blood compatibility etc.)
Surface-driven biointeractions
Atoms and molecules residing at the surface have special activity and a direct biological response
- the body “reads” the surface structure and responds to the particular chemistry and organization
General surface considerations and definitions
- surfaces have unique reactivity
- the surface is inevitably different from the the bulk
- the mass of material that makes up the surface zone is very small
- surfaces readily contaminate
- surface molecules can exhibit considerable mobility
Biomaterial surface
have unique reactivities and properties since they are pulled upon asymmetrically by the units beneath them.
- molecules in bulk of material have a relatively low energy state due to nearest neighbor interactions
- molecules at the surface are in a state of higher free energy than those in bulk due to the lack of nearest neighbor interactions at the surface (bonding)
Energy minimzation
Hydrophobic environment (air): more hydrophobic (lower energy) components may migrate to the surface of a material (reducing interfacial energy)
Aqueous environment: surface may reverse its structure and point polar (hydrophilic groups) outward to interact with the polar water molecules
Surface parameters
roughness
patterns
wettability
surface mobility
chemical composition
electrical charge
crystallinity
modulus
heterogeneity to biological reaction
Surface analysis techniques: principles and methods
Sample preparation
- sample should resemble the material or device in the form it is used for in biological testing or placement
- packaging materials may deliver surface contamination (samples can be analyzed prior to and after storage in containers to ensure surface composition measured is not due to the container)
- polyethylene press-close bags used in electron microscopy and cell culture plasticware are clean storage containers (aluminum foil should be checked for surface contamination layers by surface analysis methods prior to wrapping specimens, some are treated with surface layer of stearic acid that can contaminate)
Surface analysis
General principle guide sample analysis
1. all methods used to analyze surfaces also have the potential to alter the surface
2. more than one method should be used whenever possible (to construct a complete picture of the surface)
Common methods to characterize biomaterial surface
Contact angles
ESCA (XPS)
Augar electron spectroscopy
SIMS
FTIR-ATR
STM
SEM
Contact angles methods
liquid wetting of surfaces to estimate the energy of surfaces, has been used to predict the performance of vascular grafts and adhesion of cells to surface
surface energy (related to wettability) has been correlated with biological interaction
drop analyzers or contact angle goniometers
equation on slides
Wettability
refers to how easily a liquid spreads over a solid surface
contact angle is an inverse measure of wettability
contact angle of 0 indicates perfect wettability
Critical surface tension
value of the liquid-vapor surface tension when contact angle is 0
property of solid surface that describes the interaction between the liquid and solid surface
higher critical surface tension indicates the solid surface is hydrophilic (tends to be wetted by liquids with higher surface tension (water))
Example of critical surface tension
hydrophobic surface: leaves, teflon, lotus leaves
hydrophilic: clean glass or metal
as wettability increases, the contact angle decreases
Surface chemical composition: X-ray photoelectron spectroscopy (XPS)
purpose: quantifies elemental composition and chemical states on the surface
application: analysis of polymers, metals, and coatings
Surface chemical composition: Secondary ion mass spectrometry (SIMS)
purpose: provides molecule and elemental composition of the outermost surface
application: protein adsorption and biomaterial surface chemistry
Surface chemical composition: Fourier transform infrared spectroscopy (FTIR)
purpose: identifies functional groups and chemical bonds at the surface
application: characterizing surface modifications and chemical treatments
Surface chemical composition: Auger electron spectroscopy (AES)
purpose: provides surface-sensitive elemental and chemical state information
application: analysis of metallic and ceramic biomaterials
Surface morphology
Scanning electron microscope (SEM): high-resolution imaging of surface topography for detailed structural analysis
Atomic force microscopy (AFM): nanoscale surface roughness and 3D imaging at atomic-level resolution
Transmission electron microscopy (TEM): high-resolution surface structure for thin and nano-structured biomaterials
Confocal microscopy: 3D imaging of surfaces, particularly useful in biointerfaces
ESEM: imaging of hydrated and soft biomaterials in their native state, can modulate environmental conditions (humidity, temp etc), use for hydrogels
Surface analysis general comments
organic and polymeric materials are more easily damaged by surface analysis methods (compared to metals, ceramics, glasses)
polymeric systems exhibit greater surface molecular mobility than inorganic systems
the surface of inorganic materials is contaminated more rapidly than polymeric materials because of their higher surface energy
electrically conductive metals and carbons will often be easier to characterize than insulators using electron, x-ray, and ion interaction methods
insulators accumulate a surface electrical charge that requires special methods (low-energy electron beam) to neutralize
Surface modification
biological response to biomaterials and devices is influenced by their surface chemistry and structure
rationale: to retain the key physical properties of a biomaterial while modifying only the outermost surface to influence the biointeraction
General principles of surface modifcation
Surface modifications fall into three categories
1. chemically or physically altering the atoms, compounds, or molecules in the existing surface (chemical mods, etching, mechanical roughening)
2. overcoating the existing surface with a material having a different compositions (coating, grafting, thin film deposition)
3. creating surface textures or patterns
picture on slides
Surface coating: langmuir-blodgett deposition
overcoats a surface with one or more highly ordered layers of surfactant molecules
each of the molecules that assemble into this layer contains a polar “head” group and a nonpolar “tail” group
advantage: high degree of order and uniformity, and resemblance to the lipid bilayer membranes surrounding living cells. also the possibility to incorporate new chemistries at the surface
picture on slides
Which of the following techniques is commonly used to measure the contact angle of a liquid on a biomaterial surface?
Atomic force microscopy (AFM)
X-Ray photoelectron spectroscopy (XPS)
Contact angle goniometry
Scanning electron microscopy (SEM)
Contact angle goniometry
What is the primary purpose of grafting Poly(N-isoproplyacrylamide) (PNIPAM) onto a biomaterial surface?
To increase electrical conductivity
To provide the temperature-resistive cell adhesion and detachment
To make the surface hydrophobic
To improve protein adsorption
To provide the temperature-resistive cell adhesion and detachment
What does the critical surface tension of a material indicate?
The mechanical strength of the material
The threshold below which a liquid will completely wet the surface
The thermal conductivity of the material
The optical transparency of the material
The threshold below which a liquid will completely wet the surface
Which method is most commonly used to measure surface roughness at the nanometer scale?
X-ray diffraction (XRD)
Atomic force microscopy (AFM)
Contact angle goniometry
Secondary ion mass spectrometry (SIMS)
Atomic force microscopy (AFM)
True or false: X-ray photoelectron spectroscopy (XPS) reveals the elemental composition and chemical state of a biomaterial surface.
True
True or false: surface energy can be measured directly using contact angle measurements with different liquids
true
Synthetic materials vs. Donor tissues/organs
materials are not attacked by the immune system unlike donor tissues to organs. this difference arises from the presence of immunologically recognizable biologic motifs on donor tissue, and their absense on synthetic materials
Protein and cellular response to biomaterial implantation
implantation
protein adsorption
cellular infiltration
release of cytokines and chemokines from cells
recruitment of tissue repair cells
fibrous encapsulation and granulation tissue formation
biological responses to biomaterials
basis of reactions: adsorption of adhesion proteins to the surface of the biomaterials that are recognized by the integrin receptors present on most cells
the adsorption of adhesion proteins to the biomaterials:
convert it into a biologically recognizable material
the protein adsorption event is rapid (seconds) and generally happens on all materials implanted into biological systems with few exceptions
cell interactions with foreign surfaces are mediated by integrin receptors with adsorbed adhesion proteins that sometimes change their biological activity when they adsorb
the receptor proteins recognize and cause the cell to adhere to only the surface0bound form of one protein
Adsorption
a substance adheres to the surface of another material, forming a thin layer. the molecules are not absorbed into the bulk of the material but only accumulate on the surface. adsorption is typically a surface phenomenon
ex. activated charcoal adsorbs impurities and toxins on its surface
tldr: sticks to the surface
Absorption
a subsyance is taken up into the bulk or volume of another material. the molecules of the absorbed substance penetrate into the entire structure of the material
ex. a sponge absorbing water, where the water penetrates and fills the pores throughout the sponge
tldr: substance penetrates into the material’s bulk
in vitro
most studies of protein interactions with biomaterials and effects on cells have been done in vitro after relatively short contact periods, leading to the effects in the shorter term that involve undegraded adhesion proteins that mediate cell interactions
adsorbed protein layer on biomaterials implanted for longer times are presumably due to proteolytic attack. The functional role of any of these changes to the adsorbed proteins on the interaction of biomaterials with the body remains to be
elucidated.
Effects of adhesion proteins on cellular interactions with biomaterials
protein adsorption to materials can be performed with a single protein or complex multiprotein solutions
single protein: to study fundamental aspects (adsorption rates or conformational changes) of protein adsorption and to study biological responses (cell adhesion to each protein)
multiprotein: to approximate the adsorption in vivo for a more realistic insight into the functional role of adsorbed proteins.
Protein-mediated cell adhesion
preadsorption with purified adhesion proteins
adhesion proteins
preadsorption of certain proteins onto a solid substrate greatly increases its adhesiveness to many types of cells and such proteins are called adhesion proteins
integrins
the increased adhesiveness is because many cells have receptors in their cell membranes that bind specifically to these specialized proteins. the adhesion receptors involved in cell adhesion to biomaterials and ECM are called integrins.
Protein-mediated cell adhesion
fibronectin preadsorption greatly increases the adhesion of fibroblasts, while albumin preadsorption prevents it (albumin is a adhesive protein)
adhesion proteins also promote the flattening out of spreading of the cell onto the surface
slide 16 (on quiz)
non-specific cell adhesion
the proteins involved here are the membrane-bound proteins that are presented from a cell’s surface (blocking with albumin or IgG)
adsorption behavior of proteins at solid-liquid interfaces
adsorption transforms the interface
characteristics of protein adsorption to solid surface
rapid adsoption of proteins
most of the adsorbed proteins is irreversibly bound, as indicated by the fact that washing the surface with buffer does not remove the protein
the adsorbed protein is only removed when a strong surfactant (sds) is used
slides 18
Non-fouling surfaces
development of surface chemistries that are highly resistant to protein adsorption oin an attempt to eliminate biological recognition of the biomaterials altogether
surface chemistries highly resistant to protein adsorption
since adsorbed proteins are required for platelet adhesion, an obvious approach to improve blood compatibility is to produce biomaterials that prevent or at least greatly reduce protein adsorption.
protein-repellent materials
poly(ethylene oxide) (PEO)
poly(ethylene glycol) (PEG)
which of the following is the most common driving force for initial protein adsorption onto a surface?
hydrogen bonding
electrostatic interactions
hydrophobic interactions
covalent bonding
hydrophobic interactions
Non-fouling surfaces are designed to:
promote strong protein adsorption for biomedical applications
reduce or prevent protein adsorption to avoid biofouling
attract proteins to increase cell adhesion
inhibit cell attachment by promoting protein denaturation
reduce or prevent protein adsorption to avoid biofouling
Which of the following materials is commonly used to create non-fouling surfaces?
PEG
PVC
Collagen
Fibrinogen
PEG
PEG (bc very hydrophilic)
PVC (promotes adsorption)
fibrinogen (helps with blood clotting, promotes adsorption)
collagen (promotes adsorption)
Why are hydrophilic surfaces often considered non-fouling?
hydrophilic surfaces repel water, which prevents protein adsorption
hydrophilic surfaces promote strong ionic interactions with proteins
hydrophilic surface reduce driving force in hydrophobic protein interaction
hydrophilic surfaces bind proteins tightly and prevent further adsorption
Hydrophilic surfaces reduce the driving force in hydrophobic protein interactions.
True or false: proteins typically adsorb to hydrophobic surfaces more readily than hydrophilic surfaces
true
True or false: non-fouling surfaces are primarily designed to promote blood clotting and immune responses
false
True or false: non-fouling surfaces completely prevent all types of biomolecule adsorption under all conditions
false
hydrophobic effect
oil in water it will round into a sphere as it floats to water surface, decrease in water entropy (it is energetically unfavorable), oil minimizes interfacial area with water, driven by the necessity to minimize the more structured organization of water molecules
Hydrophobic effect-driven structures that lead to free-energy minimization:
liposome, polymeric micelles
hydrophobic region surrounded by hydrophilic shell/outer layer
protein adsorption: Why do proteins bind rapidly and tenaciously to almost all surfaces?
If a protein can displace the ordered water in binding to the surface, the entropy of the system will increase, and the free energy will decrease as the water molecules are released and gain freedom in bulk water. This is probably the driving force for protein adsorption at most interfaces
Nonfouling Surfaces (NFSs), protein-resistant surfaces and “stealth” surfaces
surfaces that resist the adsorption of proteins and/or adhesion of cells
surfaces that strongly adsorb proteins will generally,
bind cells
surfaces that resist protein adsorption will
resist cell adhesion
hydrophilic surfaces are more likely to
resist protein adsorption
hydrophobic surfaces will usually
adsorb a monolayer of tightly adsorbed protein.
potential mechanisms of action of NFSs
strong interactions with water- This water-polymer interaction highly hydrates and expands surface hydrophilic polymer chains.
bind water tightly, water shield separates the proteins from the materials of the surface
NFSs resist protein adsorption by binding or structuring water so strongly that the protein molecule cannot displace the organized or tightly bound water, and thus, there is no driving force for adsorption.
common properties shared by non-fouling groups
Structure-property relationships of surfaces that resist protein adsorption:
hydrophilic
electronically neutral
containing groups that are hydrogen bond acceptors but not hydrogen bond donors
retention of bound water by the surface molecules
At high protein concentrations such as in the bloodstream or in the body fluids, many of these surfaces will
no longer appear nonfouling.
Longer, flexible hydrophilic chains with high surface packing density will
help the nonfouling effect.
Nonfouling Materials
strongly interact with water
Two types of nonfouling materials:
Neutral and hydrophilic (interacting with water via hydrogen bonding) – containing hydroxyl, ether, or amide groups.
Ionic nature (interacting with water via electrostatically induced hydration) - Zwitterionic materials with an overall neutral charge demonstrate strong resistance to nonspecific protein adsorption.
Poly(ethylene glycol) (PEG)
-(CH2CH2O)n-
when n is in the range of 15-3500 (MW of ~400-100000), the PEG designation is used.
when MW>100000 it is commonly referred to as PEO.
when n is in the range of 2-15, the term olgio(ethylene glycol) (OEG) is often used.
NFS are important in medical devices where they may
inhibit bacterial colonization and blood cell adhesion.
Zwitterionic materials
contain both positively and negatively charged groups in the same unit and exhibit excellent non-fouling properties.
can bind water molecules even more strong via electronically induced hydration that hydration achieved by hydrogen bonding
demonstrate superhydropholicity and strong repulsion to nonspecific protein adsorption
Zwitterionic materials applications
contact lenses
artificial joint implants
coat membranes to improve blood compatibility
implant material to prevent foreign body reaction
Zwitterionic hydrogels implanted in mice
PCBMA hydrogels elicited less inflammation than PHEMA hydrogels. this weaker inflammatory response may be due to the superior ability of PCBMA hydrogels to resist nonspecific protein adsorption and thereby avoid recognition by macrophages.
device-related infections
introduction of organisms during the device insertion/implantation or attachment of bloodborne organisms to the device
indwelling devices increase the risk of infection
infection rates generally increase with duration
- catheter-associated urinary tract infection
prosthetic join (hip and knee) infections
cardiovascular implantable device infections
nosocomial infections (healthcare-associated infections)
infections that patients acquire while receiving treatment in a healthcare setting, healthcare professionals give it to their clients
infection can occur after insertion from bacteria or urine
infectious agents
microscopic organisms penetrate the body’s natural barriers and multiply to create symptoms:
bacteria
viruses
funhi
biofilm
communities of bacteria that attach and grow on surfaces of abiotic materials as well as host tissues
when attached, they embed themselves in highly hydrated and protective materials (extracellular polymeric substances (EPS))
biofilm development
- initial attachment of single cells and cell aggregates in the overlying fluid
- initiation of microcolony formation where EPS production more firmly adheres cells to the surface
- early development of biofilm clusters by clonal expansion (mixed or single species)
- mature biofilm
- dispersion of single cells and detachment of biofilm aggregates containing cells and EPS
Extracellular polymeric substances (EPS)
biofilm can modify their local environment largely because transport through the biofilm EPS is diffusion limited
- nutrients, such as oxygen (consumed at the periphery)
- acid fermentation deeper within biofilm structures can lead to anaerobic and acidic conditions at the base of the biofilm
bacterially produced polymers
- extracellular DNA
- polysaccharide
- lipids
- proteins
antibiotic and antimicrobial tolerance of bacteria in biofilms
mechanisms of tolerance:
bacteria in the interior of the biofilm enter a dormant state due to nutrient depletion
reaction of antimicrobial agents with the EPS through binding and/or degradation
development of recalcitrant (difficult to control) populations, such as slow-growing small colony variants or “persister” cells
defense mechanisms of biofilm
EPS matrix production protects bacteria within the biofilm from phagocytes due in part to physical size and viscoelastic properties, depending on the biofilm species and age
EPS matrix also reduces the ability of antibodies to penetrate the biofilm
biofilm formation facilitates the evasion of host immune effectors, and many biofilm-associated infections are associated with chronic inflammation
process of bacterial adhesion to surfaces
affected by bacterial density, flow conditions, and organic conditioning layers.
process occurring in integrated phases, involving bacteria approaching and bacteria interacting with the biomaterial surface.
Approaching: Long-range, non-specific interactions like van der Waals
Interacting: Once in proximity to the surface (< 5 nm), short-range interactions, such as hydrogen bonding and hydrophobic, ionic, and dipole interactions, can be established between the biomaterial surface and the bacterial cell.
antibiofilm surface
Antibiofilm surface would prevent bacteria attachment in the first place.
Inherently nonadherent to bacteria yet being biocompatible for its application.
influence of wettability on bacterial adhesion
bacterial adhesion is more pronounced on hydrophobic surfaces
influence of roughness on bacterial adhesion
Generally, an increase in surface roughness promotes bacterial adhesion since bacteria have a larger surface available and are protected from shear forces.
control of biofilm formation
antimicrobial approaches
- kill microorganisms in proximity to or contacting the surface
antifouling approaches
- repel microbes by physical or chemical modalities
approaches that affect biofilm architecture
- target biofilm virulence
which of the following best describes a non-fouling surface?
a surface that promotes bacteria adhesion
a surface that resists protein and cell adhesion
a surface designed to enhance biofilm formation
a surface that increases corrosion resistance
a surface that resists protein and cell adhesion
what is the most common cause of biofilm formation on indwelling medical devices?
low nutrient availability
poor material selection
the presence of microbial contamination
mechanical failure of the device
the presence of microbial contamination
which of the following is not a key factor contributing to the virulence of biofilms in medical devices?
increased resistance to antibiotics
slowed metabolic activity of microorganims
enhanced immune system penetration
protection by extracellular matrix
enhanced immune system penetration
which of the following materials is commonly used to reduce biofilm formation on medical devices?
titanium alloy
silicone
hydrogel coatings
stainless steel
hydrogel coatings
true or false: biofilms on medical devices increase resistance to antibiotics compared to planktonic (drift or floating) bacterias
true
true or false: the extracellular matrix in biofilms helps protect microorganisms from the immune system and antibiotics
true
true or false: biofilm formation on medical devices typically results in the need to remove or replace the device
true
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