Nonspherical assemblies generated from polystyrene-b-poly(L-lysine) polyelectrolyte block copolymers

Dental Composites with Calcium / Strontium Phosphates and Polylysine

Purpose

This study developed light cured dental composites with added monocalcium phosphate monohydrate (MCPM), tristrontium phosphate (TSrP) and antimicrobial polylysine (PLS). The aim was to produce composites that have enhanced water sorption induced expansion, can promote apatite precipitation and release polylysine.

Materials and Methods

Experimental composite formulations consisted of light activated dimethacrylate monomers combined with 80 wt% powder. The powder phase contained a dental glass with and without PLS (2.5 wt%) and/or reactive phosphate fillers (15 wt% TSrP and 10 wt% MCPM). The commercial composite, Z250, was used as a control. Monomer conversion and calculated polymerization shrinkage were assessed using FTIR. Subsequent mass or volume changes in water versus simulated body fluid (SBF) were quantified using gravimetric studies. These were used, along with Raman and SEM, to assess apatite precipitation on the composite surface. PLS release was determined using UV spectroscopy. Furthermore, biaxial flexural strengths after 24 hours of SBF immersion were obtained.

Results

Monomer conversion of the composites decreased upon the addition of phosphate fillers (from 76 to 64%) but was always higher than that of Z250 (54%). Phosphate addition increased water sorption induced expansion from 2 to 4% helping to balance the calculated polymerization shrinkage of ~ 3.4%. Phosphate addition promoted apatite precipitation from SBF. Polylysine increased the apatite layer thickness from ~ 10 to 20 μm after 4 weeks. The novel composites showed a burst release of PLS (3.7%) followed by diffusion-controlled release irrespective of phosphate addition. PLS and phosphates decreased strength from 154 MPa on average by 17% and 18%, respectively. All formulations, however, had greater strength than the ISO 4049 requirement of > 80 MPa.

Conclusion

The addition of MCPM with TSrP promoted hygroscopic expansion, and apatite formation. These properties are expected to help compensate polymerization shrinkage and help remineralize demineralized dentin. Polylysine can be released from the composites at early time. This may kill residual bacteria.

Janus Silica Hollow Spheres Prepared via Interfacial Biosilicification

A poly(ethylene glycol)-b-poly(L-lysine)-b-poly(styrene) (PEG-PLL-PS) triblock copolymer, which contains a cationic PLL block as the middle block, is synthesized via a combination of ring-opening polymerization (ROP) and atom-transfer radical polymerization (ATRP). The PEG-PLL-PS (ELS) triblock is employed as a macromolecular surfactant to form a stable oil-in-water (O/W) emulsion, which is subsequently used as the template to prepare Janus silica hollow spheres (JHS) via a one-pot biosilicification reaction. For the emulsion template, the middle PLL block assembles at the O/W interface and directs the biomimetic silica synthesis in the presence of phosphate buffer and silicic acid precursors. This biosilicification process takes place only in the intermediate layer between water and the organic interior phase, leading to the formation of silica JHSs with hydrophobic PS chains tethered to the inner surface and PEG attached to the outer surface. The three-layer JHSs, namely, PEG/silica-polylysine/PS composites, were verified by electron microscopy. Upon further breaking these JHSs into species, polymer-grafted Janus silica nanoplates (JPLs) can be obtained. Our studies provide an efficient one-step method for preparing hybrid silica Janus structures within minutes.

The analysis of solution self-assembled polymeric nanomaterials

This tutorial review provides a guide for the characterisation and analysis of soft nanomaterials based on polymeric self-assemblies using scattering and microscopic techniques.

Stimuli-sensitive synthetic polypeptide-based materials for drug and gene delivery

Stimuli-sensitive synthetic polypeptides are unique biodegradable and biocompatible synthetic polymers with structures mimicking natural proteins. These polymers exhibit reversible secondary conformation transitions and/or hydrophilic-hydrophobic transitions in response to changes in environmental conditions such as pH and temperature. The stimuli-triggered conformation and/or phase transitions lead to unique self-assembly behaviors, making these materials interesting for controlled drug and gene delivery applications. Therefore, stimuli-sensitive synthetic polypeptide-based materials have been extensively investigatid in recent years. Various polypeptide-based materials, including micelles, vesicles, nanogels, and hydrogels, have been developed and tested for drug- and gene-delivery applications. In addition, the presence of reactive side groups in some polypeptides facilitates the incorporation of various functional moieties to the polypeptides. This Review focuses on recent advances in stimuli-sensitive polypeptide-based materials that have been designed and evaluated for drug and gene delivery applications. In addition, recent developments in the preparation of stimuli-sensitive functionalized polypeptides are discussed.

Biohybrid block copolymers: towards functional micelles and vesicles

This critical review covers the elaboration of micelles and vesicles made from block copolymers containing peptide or oligonucleotide blocks with a focus on recent developments toward responsive and functional assemblies (166 references).

Self-Assembly of Poly(L-glutamate)-block-poly(2-(diethylamino)ethyl methacrylate) in Aqueous Solutions

The self-assembly and gene delivery applications of poly(l-glutamate)-block-poly(2-(diethylamino)ethyl methacrylate) (PLG18-b-PDEAEMA37) were investigated. Owing to the functional groups on the polymer, an amine and a carboxylic acid group, the self-assembly process is dependent on the solution pH, where the critical micelle concentration was determined to be 61 and 320 μg mL–1 at high and low pH, respectively. The block copolymer forms positively charged vesicles at low pH with a hydrodynamic radius of 90 nm and negatively charged vesicles at high pH with a hydrodynamic radius of 50 nm. At the isoelectric point of 4.9, PLG-b-PDEAEMA was found to form larger micellar aggregates with a hydrodynamic radius of 180 nm because of the presence of both positive and negative charges. The complexation between PLG-b-PDEAEMA and DNA was studied at physiological pH as well as at low and high pH. DNA is condensed most effectively at pH 3, at an N/P ratio of 5, whereas at pH 7 and 10, an N/P ratio of 20 is required. However, in-vitro studies at physiological pH using neuroblastoma cells did not show substantial gene expression.

Oxygen carrier based on hemoglobin/poly(L-lysine)-block-poly(L-phenylalanine) vesicles

An oxygen carrier was prepared by encapsulating carbonylated hemoglobin (CO-Hb) molecules into polypeptide vesicles made from poly(L-lysine)-block-poly(L-phenylalanine) (PLL-b-PPA) diblock copolymers in aqueous medium at pH 5.8. The encapsulation was confirmed by confocal laser scanning microscopy (CLSM). The morphology and size of the vesicles were studied by field-emission scanning electron microscopy (ESEM). They had a spherical shape with a mean diameter of about 4 to 5 microm. The encapsulation efficiency of hemoglobin was 40 wt %, and the hemoglobin content in the vesicles was 32 wt %. The CO-Hb encapsulated in the PLL-b-PPA vesicles was more stable than free CO-Hb under ambient conditions. In the presence of a O(2) atmosphere, the CO-Hb in the vesicle could be converted into oxygen-binding hemoglobin (O(2)-Hb) under irradiation of visible light for 2 h. Therefore, the CO-Hb/PLL-b-PPA vesicles are expected to be used as red blood cell substitutes.

Soft hydrogels from nanotubes of poly(ethylene oxide)-tetraphenylalanine conjugates prepared by click chemistry

A new poly(ethylene oxide)-tetraphenylalanine polymer-peptide conjugate has been prepared via a "click" reaction between an alkyne-modified peptide and an azide-terminated PEO oligomer. Self-assembled nanotubes are formed after dialysis of a THF solution of this polymer-peptide conjugate against water. The structure of these nanotubes has been probed by circular dichroism, IR, TEM, and SAXS. From these data, it is apparent that self-assembly involves the formation of antiparallel beta-sheets and pi-pi-stacking. Nanotubes are formed at concentrations between 2 and 10 mg mL(-1). Entanglement between adjacent nanotubes occurs at higher concentrations, resulting in the formation of soft hydrogels. Gel strength increases at higher polymer-peptide conjugate concentration, as expected.

Architecturally Induced Multiresponsive Vesicles from Well-Defined Polypeptides. Formation of Gene Vehicles

A series of novel, partially labeled amphiphilic triblock copolypeptides, PLL-b-PBLG-d7-b-PLL, has been synthesized, where PLL and PBLG-d7 are poly(L-lysine hydrochloride) and poly(gamma-benzyl-d7-L-glutamate), respectively. The synthetic approach involved the sequential ring-opening polymerization (ROP) of gamma-benzyl-L-glutamate and epsilon-Boc-L-lysine N-carboxy anhydrides by a diamino initiator using high-vacuum techniques, followed by the selective deprotection of the Boc groups. Combined characterization results showed that the copolypeptides exhibit high degrees of molecular and compositional homogeneity. The synthesized copolypeptides had similar molecular weights, while the composition of the middle block ranged between 19 and 74% with respect to the monomeric units. Due to the macromolecular architecture of the copolypeptide and the rigid nature of the middle block, the formation of monolayers was favored, and, surprisingly, vesicles were formed in water at neutral pH over the entire compositional range. The vesicular structures were extensively characterized by static and dynamic light scattering, small-angle neutron scattering, atomic force microscopy, cryo-transmission electron microscopy, scanning electron microscopy, UV and Fourier transform infrared spectroscopy, and circular dichroism. In contrast to other vesicular structures derived from conventional polymers, the formed polypeptidic vesicles possess the unique feature of being stimuli-responsive to pH and temperature. When the copolypeptides were mixed with plasmid DNA (pDNA), large vesicular structures were also formed. The molecular characterization of the vectors was performed with most of the methods mentioned above, and indicated that the pDNA is both partially condensed on the PLL phase and partially encapsulated inside the vesicle. Consequently, the synthesized vectors combine the advantages of the polylysine-DNA systems to condense large amounts of genes, as well as those of the liposome-DNA systems to better protect the encapsulated DNA. These vectors are expected to present better gene transfection efficiency to the cell nucleus.

pH responsiveness of block copolymer vesicles with a polypeptide corona

The aggregation behavior of polybutadiene165-block-poly(L-lysine)88 in saline solution was studied by combined static and dynamic light scattering analyses. Vesicles were observed if the polypeptide segment was in a 100% coil conformation (pH 7.0) or in an 80% alpha-helical conformation (pH 10.3). At the higher pH, aggregates were smaller in size (hydrodynamic radius: 364 nm --> 215 nm) and chains were more densely packed at the core-corona interface (interchain distance: 3.2 nm --> 2.4 nm). Changes in size and structure could be explained in basic terms of colloid stabilization without considering a secondary structure effect.

Bioinspired functional block copolymers

The diversity and complexity of structures and functions in synthetic polymer systems can be increased through conjugation with biological segments or, in other words, through generation of "polymer-bioconjugates" or "macromolecular chimeras". The present contribution highlights major synthetic approaches toward sophisticated functional hybrid block copolymers and analyses of structure-function relationships.

Rod-sphere transition in polybutadiene-poly(L-lysine) block copolymer assemblies

This paper describes the synthesis and characterization of poly(butadiene)m-poly(L-lysine)n (m-n = 107-200, 107-100, and 60-50) block copolymers. The polymers are prepared in a two-step process whereby amine-terminated polybutadiene is used to initiate the ring-opening polymerization of the epsilon-benzyloxycarbonyl L-lysine N-carboxyanhydride. After deprotection, the self-assembly of the block copolymers in aqueous media were studied using dynamic light scattering, transmission electron microscopy, and circular dichroism spectroscopy. These block copolymers were found to form either spherical micelles or rod-like micelles at high pH depending on the composition of the block copolymer. As the pH is decreased, the micelles swell due to charge-charge repulsions between corona chains and from the helix-coil transition of the polypeptide block. The two systems that form rod-like micelles at high pH also exhibit a pH-induced rod-sphere transition at low pH. This transition was verified from Kratky analysis of the static light scattering data and via CONTIN analysis of the dynamic light scattering data, which shows a bimodal distribution in particle sizes.

Self-Assembly of Polypeptide-Containing ABC-Type Triblock Copolymers in Aqueous Solution and Its pH Dependence

Self-assembling of novel biodegradable ABC-type triblock copolymer poly(ethylene glycol)-poly(L-lactide)-poly(L-glutamic acid) (PEG-PLLA-PLGA) is studied. In aqueous media, it self-assembles into a spherical micelle with the hydrophobic PLLA segment in the core and the two hydrophilic segments PEG and PLGA in the shell. With the lengths of PEG and PLLA blocks fixed, the diameter of the micelles depends on the length of the PLGA block and on the volume ratio of H(2)O/dimethylformamide (DMF) in the media. When the PLGA block is long enough, morphology of the self-assembly is pH-dependent. It assembles into the spherical micelle in aqueous media at pH 4.5 and into the connected rod at or below pH 3.2. The critical micelle concentration (cmc) of the copolymer changes accordingly with decreasing solution pH. Both aggregation states can convert to each other at the proper pH value. This reversibility is ascribed to the dissociation and neutralization of the COOH groups in the LGA residues. When the PLGA block is short compared to the PEG or PLLA block, it assembles only into the spherical micelle at various pH values.

Morphology of hybrid polystyrene-block-poly(ethylene oxide) micelles: Analytical ultracentrifugation and SANS studies

Morphology and structure of aqueous block copolymer solutions based on polystyrene-block-poly(ethylene oxide) (PS-b-PEO) of two different compositions, a cationic surfactant, cetyl pyridinium chloride (CPC), and either platinic acid (H2PtCl6.6H2O) or Pt nanoparticles were studied using a combination of analytical ultracentrifugation (AUC), transmission electron microscopy (TEM), and small angle neutron scattering (SANS). These studies combining methods contributing supplemental and analogous structural information allowed us to comprehensively characterize the complex hybrid systems and to discover an isotope effect when H2O was replaced with D2O. In particular, TEM shows formation of both micelles and larger aggregates after incorporation of platinic acid, yet the amount of aggregates depends on the H2PtCl6.6H2O concentration. AUC reveals the presence of micelles and micellar clusters in the PS-b-PEO block copolymers solution and even larger (supermicellar) aggregates in hybrids (with CPC). Conversely, SANS applied to D2O solutions of the similar species indicates that micelles are spherical and no other micellar species are found in block copolymer solutions. To reconcile the SANS and AUC data, we carried out AUC examination of the corresponding D2O block copolymer solutions. These measurements demonstrate a pronounced isotope effect on micelle aggregation and micelle size, i.e., no micelle aggregation in D2O solutions, revealing good agreement of AUC and SANS data.

Spontaneous condensation in DNA-polystyrene- b-poly(l-lysine) polyelectrolyte block copolymer mixtures

We investigated the condensation of calf thymus DNA by amphiphilic polystyrene(m)-b-poly(l-lysine)(n) block copolymers ( PS(m)-b- PLys(n), m, n = degree of polymerization), using small-angle X-ray scattering, polarized optical microscopy and laser scanning confocal microscopy. Microscopy studies showed that the DNA condenses in the form of fibrillar precipitates, with an irregular structure, due to electrostatic interactions between PLys and DNA. This is not modified by the presence of hydrophobic PS block. Scattering experiments show that the structure of the polyplexes corresponds to a local order of DNA rods which becomes more compact upon increasing n. It can be concluded that for DNA/ PS(m)-b- PLys(n) polyplexes, the balance between the PLys block length and the excess charge in the system plays an essential role in the formation of a liquid crystalline phase.