19 research outputs found
“Clickable” Polymeric Nanofibers through Hydrophilic–Hydrophobic Balance: Fabrication of Robust Biomolecular Immobilization Platforms
Fabrication
of hydrophilic polymeric nanofibers that undergo facile
and selective functionalization through metal catalyst-free Diels–Alder
“click” reaction in aqueous environment is outlined.
Electrospinning of copolymers containing an electron-rich furan moiety,
hydrophobic methyl methacrylate units and hydrophilic polyÂ(ethylene
glycol)Âs as side chains provide specifically functionalizable yet
antibiofouling fibers that remain stable in aqueous media due to appropriate
hydrophobic hydrophilic balance. Efficient functionalization of these
nanofibers is accomplished through the Diels–Alder reaction
by exposing them to maleimide-containing molecules and ligands. Diels–Alder
conjugation based functionalization is demonstrated through attachment
of fluorescein-maleimide and a maleimide tethered biotin ligand. Biotinylated
nanofibers were utilized to mediate immobilization of the protein
streptavidin, as well as streptavidin coated quantum dots. Facile
fabrication from readily available polymers and their effective functionalization
under mild and reagent-free conditions in aqueous media make these “clickable”
nanofibers attractive candidates as functionalizable scaffolds for
various biomedical applications
“Clickable” Nanogels via Thermally Driven Self-Assembly of Polymers: Facile Access to Targeted Imaging Platforms using Thiol–Maleimide Conjugation
Multifunctionalizable
nanogels are fabricated using thermally driven
self-assembly and cross-linking of reactive thermoresponsive copolymers.
Nanogels thus fabricated can be easily conjugated with various appropriately
functionalized small molecules and/or ligands to tailor them for various
applications in delivery and imaging. In this study, a polyÂ(ethylene
glycol)-methacrylate-based maleimide-bearing copolymer was cross-linked
with a dithiol-based cross-linker to synthesize nanogels. Because
of lower critical solution temperature (LCST) around 55 °C in
aqueous media, these copolymers assemble into nanosized aggregates
when heated to this temperature, and they are cross-linked using the
thiol–maleimide conjugation. Nanogels thus fabricated contain
both thiol and maleimide groups in the same cross-linked nanogels.
Postgelation functionalization of the residual maleimide and thiol
groups is demonstrated through conjugation of a thiol-bearing hydrophobic
dye (BODIPY-SH) and <i>N</i>-(fluoresceinyl) maleimide,
respectively. In addition, to demonstrate the utility of multifunctionality
of these nanogels, a thiol-bearing cyclic-peptide-based targeting
group, cRGDfC, and <i>N</i>-(fluoresceinyl)-maleimide-based
fluorescent tag was conjugated to nanogels in aqueous media. Upon
treatment with breast cancer cell lines, MDA-MB-231, it was deduced
from cellular internalization studies using fluorescence microscopy
and flow cytometry that the peptide carrying constructs were preferentially
internalized. Overall, a facile synthesis of multifunctionalizable
nanogels that can be tailored using effective conjugation chemistry
under mild conditions can serve as promising candidates for various
applications
Hooked on Cryogels: A Carbamate Linker Based Depot for Slow Drug Release
PolyÂ(ethylene
glycol) (PEG) based bulk hydrogels and cryogels containing
activated carbonate groups as amine reactive handles to facilitate
drug conjugations through carbamate linkages were fabricated and evaluated
as slow releasing drug reservoirs. As an initial approach, photopolymerization
of <i>N</i>-hydroxysuccinimide (NHS)-activated carbonate
functional group containing monomer and PEG-methacrylate in the presence
of a cross-linker was utilized to obtain bulk hydrogels with high
gel conversions. The resultant hydrogels possessed moderate water
uptake (170–340%) which was dependent on the monomer ratios.
These hydrogels were functionalized with an anticancer drug, namely,
doxorubicin. Surprisingly, while negligible drug release was observed
from the bulk hydrogels under normal pH, only about 6% drug release
was observed under acidic condition. Limited swelling of these hydrogels
as well as lack of porous structure as deduced from scanning electron
microscopy analysis might explain the poor drug release. To enhance
the drug releasing capacity of these hydrogels that might stem from
the increased porosity, reactive carbonate group bearing cryogels
were synthesized. Compared to the bulk hydrogels, cryogels were highly
porous in structure and also possessed much higher swelling capacity
(1150–1500%). As a result of these distinctions, a 7-fold enhancement
in drug release was observed for the cryogel system compared to the
relating hydrogel. In vitro studies demonstrated that the anticancer
drug doxorubicin conjugated through carbamate linkers to the cryogels
was released and proved effective against MDA-MB-231 human breast
cancer cells. Overall, a novel class of slow releasing nontoxic hydrogel
and cryogel scaffolds with potential applications as anticancer drug
reservoirs was realized
Surface-Anchored Thiol-Reactive Soft Interfaces: Engineering Effective Platforms for Biomolecular Immobilization and Sensing
Fabrication of antibiofouling,
specifically reactive polymeric coatings that undergo facile functionalization
with thiol-bearing small molecules and ligands, yields effective platforms
for biomolecular immobilization and sensing. PolyÂ(ethylene glycol)
(PEG)-based copolymers containing alkoxysilyl groups to enable surface-anchoring
and furan-protected maleimide groups as latent thiol-reactive moieties
as side-chains were synthesized. Reactive interfaces were obtained
by coating these copolymers onto Si/SiO<sub>2</sub> or glass surfaces
and activating the maleimide groups to their thiol-reactive forms
via thermal treatment. A series of surfaces modified with copolymers
containing varying amounts of maleimide groups were synthesized. Effectiveness
of surface modification was probed using Fourier transform infrared
spectroscopy, contact angle goniometry, ellipsometry and X-ray photoelectron
spectroscopy. Facile surface modification through thiol-maleimide
conjugation was established by attachment of a thiol-containing fluorescent
dye, namely BODIPY-SH. It was demonstrated that these surfaces allow
spatially localized modification through microcontact printing. Importantly,
the extent of surface modification could be tuned by varying the initial
composition of the copolymer used for coating. Using fluorescence
microscopy, it was observed that increasing amount of fluorescent
dye was attached onto surfaces fabricated with copolymers with increasing
amount of masked maleimide groups. Thereafter, the thiol–maleimide
conjugation was utilized to decorate these surfaces with biotin, a
protein-binding ligand. It was observed that though these biotinylated
surfaces were able to bind Streptavidin effectively, some nonspecific
binding was observed on places that were not in conformal contact
with the stamp during microcontact printing. This nonspecific binding
was eliminated upon neutralizing the residual maleimide units on the
printed surface using thiol-containing PEG. Notably, fluorescence
analysis of Streptavidin immobilized onto biotinylated surfaces fabricated
using varying amounts of maleimide demonstrated that the amount of
immobilized protein could be tuned by varying surface composition.
It can be envisioned that facile fabrication of these maleimide-containing
polymeric surfaces, their effective functionalization in a tunable
manner to engineer interfaces for effective immobilization or sensing
of biomolecules in a spatially controlled manner would make them attractive
candidates for various biotechnological applications
Indispensable Platforms for Bioimmobilization: Maleimide-Based Thiol Reactive Hydrogels
PolyÂ(ethylene
glycol)-based hydrogels containing thiol-reactive
maleimide functional groups is prepared via a Diels–Alder/retro
Diels–Alder reaction sequence using a masked maleimide monomer.
Bulk and micropatterned hydrogels containing varying amounts of the
thiol-reactive maleimide functional group are fabricated at ambient
temperature. During the fabrication, the reactive maleimide functional
group in the monomer is masked with a furan moiety and then unmasked
to its reactive form via the retro-Diels–Alder reaction. The
reactive maleimide groups embedded within the hydrogel are amenable
to facile and efficient functionalization with thiol-containing molecules
such as fluorescent dyes. Furthermore, these hydrogels are readily
biotinylated using the nucleophilic thiol–ene conjugation to
enable immobilization of streptavidin onto the hydrogel patterns to
achieve facile bioimmobilization. Notably, the extent of functionalization
of these hydrogels can be easily tailored by varying the amount of
reactive handles incorporated during their fabrication
Redox-Responsive “Catch and Release” Cryogels: A Versatile Platform for Capture and Release of Proteins and Cells
Macroporous cryogels are attractive
scaffolds for biomedical
applications,
such as biomolecular immobilization, diagnostic sensing, and tissue
engineering. In this study, thiol-reactive redox-responsive cryogels
with a porous structure are prepared using photopolymerization of
a pyridyl disulfide poly(ethylene glycol) methacrylate (PDS-PEG-MA)
monomer. Reactive cryogels are produced using PDS-PEG-MA and hydrophilic
poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) monomers,
along with a PEG-based cross-linker and photoinitiator. Functionalization
of cryogels using a fluorescent dye via the disulfide-thiol exchange
reactions is demonstrated, followed by release under reducing conditions.
For ligand-mediated protein immobilization, first, thiol-containing
biotin or mannose is conjugated onto the cryogels. Subsequently, fluorescent
dye-labeled proteins streptavidin and concanavalin A (ConA) are immobilized
via ligand-mediated conjugation. Furthermore, we demonstrate that
the mannose-decorated cryogel could capture ConA selectively from
a mixture of lectins. The efficiency of protein immobilization could
be easily tuned by changing the ratio of the thiol-sensitive moiety
in the scaffold. Finally, an integrin-binding cell adhesive peptide
is attached to cryogels to achieve successful attachment, and the
on-demand detachment of integrin-receptor-rich fibroblast cells is
demonstrated. Redox-responsive cryogels can serve as potential scaffolds
for a variety of biomedical applications because of their facile synthesis
and modification
Diels-Alder “Clickable” Polymer Brushes: A Versatile Catalyst-Free Conjugation Platform
Polymeric brushes
provide an attractive functional interface for
a variety of applications in materials and biomedical sciences. Facile
access to functionalized brushes can be realized through effective
postpolymerization functionalization of reactive brushes. Over the
past decade, efficient chemical transformations based on various “click”
reactions have been employed for functionalization of polymeric brushes.
This paper reports the first example of utilization of the Diels–Alder
cycloaddition reaction based functionalization strategy that allows
efficient conjugation of maleimide-containing molecules onto furan-containing
polymer brushes under mild and reagent-free conditions. Polymers incorporating
furan groups as side chains are “grafted from” silicon
oxide surfaces and investigated toward their functionalization. Brushes
are fabricated using atom transfer radical polymerization with varying
amounts of furfuryl methacrylate to enable control over extent of
functionalization, along with a polyÂ(ethylene glycol) chain containing
methacrylate as a comonomer to impart hydrophilic and antibiofouling
characteristics. Functionalization of these reactive brushes were
investigated through the immobilization of a model compound <i>N</i>-ethylmaleimide, a fluorescent dye BODIPY-maleimide, and
a maleimide-containing biotin based ligand to direct the immobilization
of streptavidin-coated quantum dots
Embedding Well-Defined Responsive Hydrogels with Nanocontainers: Tunable Materials from Telechelic Polymers and Cyclodextrins
Design, synthesis, and application
of cyclodextrin (CD) containing
thermoresponsive hydrogels fabricated from thiol-reactive telechelic
polymers are reported. Hydrophilic polymers containing 2-hydroxyethyl
methacrylate and/or diÂ(ethylene glycol)Âmethylether methacrylate monomers
as side chains and thiol-reactive groups at chain ends were synthesized.
A series of hydrogels was fabricated using thiol–ene conjugation
of these thiol-reactive polymers with multivalent thiol-containing
CDs as crosslinkers. Clear and transparent hydrogels were obtained
with good conversion (79–89%) by utilizing the “nucleophilic”
and “radical” thiol–ene “click”
reactions. Analysis of the amount of residual thiol groups in these
hydrogels using Ellman’s reagent suggested that gels with a
moderately well-defined network structure were obtained. Hydrogels
fabricated using different telechelic polymers were examined for their
properties such as morphology, equilibrium water uptake, and rheological
characteristics. Cytocompatibility of these hydrogels was ascertained
by a cell viability assay that demonstrated low toxicity toward fibroblast
cells. Thereafter, the CD-containing hydrogels were evaluated for
the loading and controlled release of puerarin, an antiglaucoma drug.
Utilization of thermoresponsive polymers as the matrix for these hydrogels
allows use of temperature as a stimulus to modulate the drug release.
A slower and more sustained drug release was observed at physiological
temperatures compared to ambient conditions. The effect of temperature
on the elasticity of the hydrogel was investigated rheologically to
demonstrate that the collapse of the network structure occurs near
physiological temperatures. The increased hydrophobicity and compactness
of the gel matrix at higher temperatures results in a slower drug
release. The strategy employed here demonstrates that tuning the matrix
composition of hydrogels with well-defined network structures through
appropriate choice of responsive copolymers allows design of materials
with control of their physical properties and drug-release behavior
Designing Dendron–Polymer Conjugate Based Targeted Drug Delivery Platforms with a “Mix-and-Match” Modularity
Polymeric
micellar systems are emerging as a very important class
of nanopharmaceuticals due to their ability to improve pharmacokinetics
and biodistribution of chemotherapy drugs, as well as to reduce related
systemic toxicities. While these nanosized delivery systems inherently
benefit from passive targeting through the enhanced permeation and
retention effect leading to increased accumulation in the tumor, additional
active targeting can be achieved through surface modification of micelles
with targeting groups specific for overexpressed receptors of tumor
cells. In this project, nontoxic, biodegradable, and modularly tunable
micellar delivery systems were generated using two types of dendron–polymer
conjugates. Either an AB type dendron–polymer construct with
2K PEG or an ABA type dendron–polymer–dendron conjugate
with 6K PEG based middle block was used as primary construct; along
with an AB type dendron–polymer containing a cRGDfK targeting
group to actively target cancer cells overexpressing α<sub>υ</sub>β<sub>3</sub>/α<sub>υ</sub>β<sub>5</sub> integrins.
A set of micelles encapsulating docetaxel, a widely employed chemotherapy
drug, were prepared with varying feed ratios of primary construct
and targeting group containing secondary construct. Critical micelle
concentrations of all micellar systems were in the range of 10<sup>–6</sup> M. DLS measurements indicated hydrodynamic size distributions
varying between 170 to 200 nm. An increase in docetaxel release at
acidic pH was observed for all micelles. Enhanced cellular internalization
of Nile red doped micelles by MDA-MB-231 human breast cancer cells
suggested that the most efficient uptake was observed with targeted
micelles. <i>In vitro</i> cytotoxicity experiments on MDA-MB-231
breast cancer and A549 lung carcinoma cell lines showed improved toxicity
for RGD containing micelles. For A549 cell line EC<sub>50</sub> values
of drug loaded micellar sets were in the range of 10<sup>–9</sup> M whereas EC<sub>50</sub> value of free docetaxel was around 10<sup>–10</sup> M. For MDA-MB-231 cell line EC<sub>50</sub> value
of free docetaxel was 6 × 10<sup>–8</sup> M similar to
EC<sub>50</sub> of nontargeted AB type docetaxel doped micellar constructs
whereas the EC<sub>50</sub> value of its targeted counterpart decreased
to 5.5 × 10<sup>–9</sup> M. Overall, in this comparative
study, the targeting group containing micellar construct fabricated
with a 2 kDa PEG based diblock dendron–polymer conjugate emerges
as an attractive drug delivery vehicle due to the ease of synthesis,
high stability of the micelles, and efficient targeting
Modular Fabrication of Polymer Brush Coated Magnetic Nanoparticles: Engineering the Interface for Targeted Cellular Imaging
Development of efficient and rapid
protocols for diversification
of functional magnetic nanoparticles (MNPs) would enable identification
of promising candidates using high-throughput protocols for applications
such as diagnostics and cure through early detection and localized
delivery. Polymer brush coated magnetic nanoparticles find use in
many such applications. A protocol that allows modular diversification
of a pool of parent polymer coated nanoparticles will lead to a library
of functional materials with improved uniformity. In the present study,
polymer brush coated parent magnetic nanoparticles obtained using
reversible addition–fragmentation chain transfer (RAFT) polymerization
are modified to obtain nanoparticles with different “clickable”
groups. In this design, trithiocarbonate group terminated polymer
brushes are “grafted from” MNPs using a catechol group
bearing initiator. A postpolymerization radical exchange reaction
allows installation of “clickable” functional groups
like azides and maleimides on the chain ends of the polymers. Thus,
modified MNPs can be functionalized using alkyne-containing and thiol-containing
moieties like peptides and dyes using the alkyne–azide cycloaddition
and the thiol–ene conjugation, respectively. Using the approach
outlined here, a cell surface receptor targeting cyclic peptide and
a fluorescent dye are attached onto nanoparticle surface. This multifunctional
construct allows selective recognition of cancer cells that overexpress
integrin receptors. Furthermore, the approach outlined here is not
limited to the installation of azide and maleimide functional groups
but can be expanded to a variety of “clickable” groups
to allow nanoparticle modification using a broad range of chemical
conjugations