23 research outputs found
Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces
The
nanotribological properties of hydrophilic polymer brushes
are conveniently analyzed by lateral force microscopy (LFM). However,
the measurement of friction for highly swollen and relatively thick
polymer brushes can be strongly affected by the tendency of the compliant
brush to be laterally deformed by the shearing probe. This phenomenon
induces a âtiltingâ in the recorded friction loops,
which is generated by the lateral bending and stretching of the grafts.
In this study we highlight how the brush lateral deformation mainly
affects the friction measurements of swollen PNIPAM brushes (below
LCST) when relatively short scanning distances are applied. Under
these conditions, the energy dissipation recorded by LFM is almost
uniquely determined by stretching and bending of the compliant brush
back and forth along the scanning direction, and it is not correlated
to dynamic friction between two sliding surfaces. In contrast, when
the scanning distance applied during LFM is relevantly longer than
the brush lateral deformation, sliding of the probe on the brush interface
becomes dominant, and a correct measurement of dynamic friction can
be accomplished. By increasing the temperature above the LCST, the
PNIPAM brushes undergo dehydration and assume a collapsed morphology,
thereby hindering their lateral deformation by scanning probe. Hence,
at 40 °C in water the recorded friction loops do not show any
tilting and LFM accurately describes the dynamic friction between
the probe and the polymer surface
Controlled Crosslinking Is a Tool To Precisely Modulate the Nanomechanical and Nanotribological Properties of Polymer Brushes
Covalent crosslinking of weak polyelectrolyte
brushes widens the
tuning potential for their swelling, nanomechanical, and nanotribological
properties, which can be simultaneously adjusted by varying the crosslinker
content and the pH of the surroundings. We demonstrate that this is
especially valid for polyÂ(hydroxyethyl methacrylate) (PHEMA) brushes
and brush hydrogels, and their ionizable, succinate-modified derivatives
(PHEMA-SA), covalently crosslinked with different amounts of diÂ(ethylene
glycol) dimethacrylate (DEGDMA) during surface-initiated atom transfer
radical polymerization (SI-ATRP). Atomic force microscopy (AFM) methods
highlight how pristine PHEMA films are stiff and display high coefficients
of friction in water. Their succinate derivatives swell profusely
in aqueous media. Under acidic conditions they are neutral, compliant,
and lubricious, with apparent Youngâs moduli (<i>E</i>*) lying between 10 and 30 kPa. Their contact mechanical behavior
can be described by either the JohnsonâKendallâRoberts
(JKR) or the DerjaguinâMuÌllerâToporov (DMT) model,
depending on crosslinker content. In contrast, under basic conditions,
brushes and brush hydrogels become charged, expand, and present a
rigid, electrostatic barrier toward the AFM probe. Friction is extremely
low at relatively low applied loads, whereas it increases at higher
loads, to an extent that is regulated by the number of crosslinks
within the films
Nanoassemblies of Tissue-Reactive, Polyoxazoline Graft-Copolymers Restore the Lubrication Properties of Degraded Cartilage
Osteoarthritis
leads to an alteration in the composition of the
synovial fluid, which is associated with an increase in friction and
the progressive and irreversible destruction of the articular cartilage.
In order to tackle this degenerative disease, there has been a growing
interest in the medical field to establish effective, long-term treatments
to restore cartilage lubrication after damage. Here we develop a series
of graft-copolymers capable of assembling selectively on the degraded
cartilage, resurfacing it, and restoring the lubricating properties
of the native tissue. These comprise a polyglutamic acid backbone
(PGA) coupled to brush-forming, poly-2-methyl-2-oxazoline (PMOXA)
side chains, which provide biopassivity and lubricity to the surface,
and to aldehyde-bearing tissue-reactive groups, for the anchoring
on the degenerated cartilage <i>via</i> Schiff bases. Optimization
of the graft-copolymer architecture (<i>i.e</i>., density
and length of side chains and amount of tissue-reactive functions)
allowed a uniform passivation of the degraded cartilage surface. Graft-copolymer-treated
cartilage showed very low coefficients of friction within synovial
fluid, reestablishing and in some cases improving the lubricating
properties of the natural cartilage. Due to these distinctive properties
and their high biocompatibility and stability under physiological
conditions, cartilage-reactive graft-copolymers emerge as promising
injectable formulations to slow down the progression of cartilage
degradation, which characterizes the early stages of osteoarthritis
Modulation of Surface-Initiated ATRP by Confinement: Mechanism and Applications
The mechanism of surface-initiated
atom transfer polymerization
(SI-ATRP) of methacrylates in confined volumes is systematically investigated
by finely tuning the distance between a grafting surface and an inert
plane by means of nanosized patterns and micrometer thick foils. The
polymers were synthesized from monolayers of photocleavable initiators,
which allow the analysis of detached brushes by size-exclusion chromatography
(SEC). Compared to brushes synthesized under âopenâ
polymerization mixtures, nearly a 4-fold increase in brush molar mass
was recorded when SI-ATRP was performed within highly confined reaction
volumes. Correlating the SI-ATRP of methyl methacrylate (MMA), with
and without âsacrificialâ initiator, to that of lauryl
methacrylate (LMA) and analyzing the brush growth rates within differently
confined volumes, we demonstrate faster grafting kinetics with increasing
confinement due to the progressive hindering of Cu<sup>II</sup>-based
deactivators from the brush propagating front. This effect is especially
noticeable when viscous polymerization mixtures are generated and
enables the synthesis of several hundred nanometer thick brushes within
relatively short polymerization times. The faster rates of confined
SI-ATRP can be additionally used to fabricate, in one pot, precisely
structured brush gradients, when volume confinement is continuously
varied across a single substrate by spatially tuning the vertical
distance between the grafting and the confining surfaces
Engineering Lubricious, Biopassive Polymer Brushes by Surface-Initiated, Controlled Radical Polymerization
Surface-initiated controlled radical
polymerization enables the
fabrication of biopassive polymer brushes with interfacial, physicochemical
properties that can be independently varied across a single substrate.
PolyÂ[(oligoethylene glycol) methacrylate] (POEGMA) brushes were synthesized
by surface-initiated atom transfer radical polymerization (SI-ATRP),
locally varying the exposure of initiator-functionalized surfaces
to the polymerization solution to yield POEGMA brush thickness gradients.
A combination of variable-angle spectroscopic ellipsometry (VASE)
and atomic force microscopy (AFM) demonstrated that brush swelling,
grafting density, nanomechanical properties, and biopassivity towards
protein adsorption all remained constant within a thickness range
between 20 and 90 nm. However, the nanotribological properties of
POEGMA brushes, investigated by lateral force microscopy (LFM), were
found to vary progressively along the gradient, thinner brushes showing
significantly lower friction than thicker and more viscoelastic grafts.
The independent variation of lubricity across a biopassive brush gradient
shows how SI-ATRP can be used to tailor surfaces destined for applications
involving both contact with biological media and exposure to shear
stresses, as is the case for tissue-replacement implants and scaffolds
for tissue engineering
Hairy and Slippery Polyoxazoline-Based Copolymers on Model and Cartilage Surfaces
Comb-like polymers
presenting a hydroxybenzaldehyde (HBA)-functionalized
polyÂ(glutamic acid) (PGA) backbone and polyÂ(2-methyl-2-oxazoline)
(PMOXA) side chains chemisorb on aminolized substrates, including
cartilage surfaces, forming layers that reduce protein contamination
and provide lubrication. The structure, physicochemical, biopassive,
and tribological properties of PGA-PMOXA-HBA films are finely determined
by the copolymer architecture, its reactivity toward the surface,
i.e. PMOXA side-chain crowding and HBA density, and by the copolymer
solution concentration during assembly. Highly reactive species with
low PMOXA content form inhomogeneous layers due to the limited possibility
of surface rearrangements by strongly anchored copolymers, just partially
protecting the functionalized surface from protein contamination and
providing a relatively weak lubrication on cartilage. Biopassivity
and lubrication can be improved by increasing copolymer concentration
during assembly, leading to a progressive saturation of surface defects
across the films. In a different way, less reactive copolymers presenting
high PMOXA side-chain densities form uniform, biopassive, and lubricious
films, both on model aminolized silicon oxide surfaces, as well as
on cartilage substrates. When assembled at low concentrations these
copolymers adopt a âlying downâ conformation, i.e. adhering
via their backbones onto the substrates, while at high concentrations
they undergo a conformational transition, assuming a more densely
packed, âstanding upâ structure, where they stretch
perpendicularly from the substrate. This specific arrangement reduces
protein contamination and improves lubrication both on model as well
as on cartilage surfaces
Amplified Responsiveness of Multilayered Polymer Grafts: Synergy between Brushes and Hydrogels
The responsive properties of surface-grafted
polymer films in aqueous
media can be amplified by covalently layering thermosensitive brushes
and hydrogels. This was demonstrated by synthesizing layers of linear
polyÂ(<i>N</i>-isopropylacrylamide) (PNIPAM) brushes, alternating
with cross-linked, polyÂ(hydroxyethyl)Âmethacrylate (PHEMA) hydrogels
via sequential surface-initiated atom-transfer radical polymerization
(SI-ATRP) steps. Below the lower critical solution temperature (LCST)
of PNIPAM, brush/hydrogel multilayered films swell similarly to linear
PNIPAM homopolymer brushes, as measured by liquid ellipsometry. In
contrast, above the LCST, the PHEMA hydrogel interlayer acts as stiffening
element within the collapsed multilayered film, as monitored by atomic
force microscopy (AFM) nanoindentation and lateral force microscopy
(LFM). This translates into a 10-fold increase in Youngâs modulus
by the collapsed, layered films compared
to PNIPAM homopolymer analogues. The (macro)Âmolecular continuity between
the brush main chains and hydrogel constituents thus enables a chemically
robust layering to form graded, quasi-3D grafted polymer architectures,
which display a concerted and amplified temperature-triggered transition
Physical Networks of Metal-Ion-Containing Polymer Brushes Show Fully Tunable Swelling, Nanomechanical and Nanotribological Properties
The interaction between weak polyacid
brushes and metal ions can
lead to the formation of a wide variety of complex structures across
the polymer grafts. In the case of polyÂ(hydroxyethyl methacrylate)
brushes derivatized with succinate side groups (PHEMA-SA), the coordination
with Zn<sup>2+</sup> or Ca<sup>2+</sup> species can be tuned by varying
the solution pH, below and above the p<i>K</i><sub>a</sub> of the polyacid brush. Attenuated total reflection Fourier transform
infrared spectroscopy (ATR-FTIR) indicates that Zn<sup>2+</sup> ions
generate relatively weak, localized monodentate bridges along PHEMA-SA
grafts at basic pH. These Zn<sup>2+</sup>âbrush conjugates
swell profusely in water, are compliant and very lubricious, as observed
by combining variable angle spectroscopic ellipsometry (VASE), quartz
crystal microbalance with dissipation (QCM-D), and atomic force microscopy
(AFM) methods. In contrast, incubation of PHEMA-SA brushes with Ca<sup>2+</sup> or Zn<sup>2+</sup> at acidic pH leads to the formation of
more extended, bidentate linkages forming a physical network between
the metal centers and the surrounding grafts. This type of coordination
causes brush dehydration and the stiffening of the films, as well
as high friction, due to the energy dissipation required to perturb
the dense, brush physical network by the shearing AFM probe. Regulating
the interaction between metal ions and ionizable polymer brushes emerges
as a versatile and easily accessible tool to control the interfacial
properties of grafted polymer films. The achieved modulation of nanomechanical
and nanotribological characteristics is technologically relevant,
in that it allows both function and performance to be tuned for widely
applicable polymer coatings
Poly(2-oxazoline)âPterostilbene Block Copolymer Nanoparticles for Dual-Anticancer Drug Delivery
Functional block
copolymers based on polyÂ(2-oxazoline)Âs are versatile
building blocks for the fabrication of dual-drug delivery nanoparticles
(NPs) for anticancer chemotherapy. Coreâshell NPs are
fabricated from diblock copolymers featuring a long and hydrophilic
polyÂ(2-methyl-2-oxazoline) (PMOXA) block coupled to a relatively short
and functionalizable polyÂ(2-methylsuccinate-2-oxazoline) (PMestOXA)
segment. The PMOXA block stabilizes the NP dispersions, whereas the
PMestOXA segment is used to conjugate pterostilbene, a natural bioactive
phenolic compound that is used as lipophilic model-drug and constitutes
the hydrophobic core of the designed NPs. Subsequent loading of the
NPs with clofazimine (CFZ), an inhibitor of the multidrug resistance
pumps typically expressed in a large variety of cancer cells, provides
an additional function to their formulation. Optimization of the copolymer
composition allows the design of polymer scaffolds showing low toxicity
and capable of assembling into highly stable NPs dispersions at physiologically
relevant pH. In addition, the incorporation of CFZ increases the stability
of the NPs and stimulates their internalization by RAW 264.7 cells
Thin Polymer Brush Decouples Biomaterialâs Micro-/Nanotopology and Stem Cell Adhesion
Surface morphology
and chemistry of polymers used as biomaterials,
such as tissue engineering scaffolds, have a strong influence on the
adhesion and behavior of human mesenchymal stem cells. Here we studied
semicrystalline polyÂ(Δ-caprolactone) (PCL) substrate scaffolds,
which exhibited a variation of surface morphologies and roughness
originating from different spherulitic superstructures. Substrates
were obtained by varying the parameters of the thermal processing,
that is, crystallization conditions. The cells attached to these polymer
substrates adopted different morphologies responding to variations
in spherulite density and size. In order to decouple substrate topology
effects on the cells, sub-100 nm bioadhesive polymer brush coatings
of oligoÂ(ethylene glycol) methacrylates were grafted from PCL and
functionalized with fibronectin. On surfaces featuring different surface
textures, dense and sub-100 nm thick brush coatings determined the
response of cells, irrespective to the underlying topology. Thus,
polymer brushes decouple substrate micro-/nanoscale surface topology
and the adhesion of stem cells