Antti Nykänen

Miktoarm stars of poly(ethylene oxide) and poly(dimethylaminoethyl methacrylate): manipulation of micellization by temperature and light

A novel method for preparation of miktoarm stars is presented, first employing Williamson ether synthesis with protected dipentaerythritol and preformed poly(ethylene oxide) (PEO) as reactants. This arm-first reaction gave, after modification, PEO macroinitiators with 4 or 6 initiation sites, which are located in the center of the main chain. The initiators were used for atom transfer radical polymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA; core-first method). Heteroarm stars were obtained with two hydrophilic PEO chains and 4 or 3 stimuli responsive PDMAEMA chains respectively. Both polymers had almost the same molecular weights. The star-shaped polymers were analyzed by NMR, size exclusion chromatography SEC, osmometry and mass spectrometry. The micellization of the polymers was investigated by light scattering, fluorescence spectroscopy and cryogenic transmission electron microscopy. At the conditions used (0.1 g/L in pH 8 buffer), PDMAEMA turns hydrophobic around 55 °C, forming micelles at higher temperatures. At low temperature, trivalent counterions like hexacyanocobaltate(III) allow additional micellization of the weak polyelectrolyte PDMAEMA, with PEO as the solubilizing agent. For this unique behavior the notion “confused micellization” is introduced, which is in analogy to schizophrenic micelles. The morphology of the aggregates depends strongly on the macromolecular architecture, giving spherical micelles for the star with 4 shorter PDMAEMA arms and vesicles for the star with 3 longer arms. The diameter of the vesicles, varying between 200 nm and 4000 nm at 10 °C, can be tuned by the cooling rate. This ionically induced micellization can then be reversed by UV-illumination, leading to disaggregation upon a photoinduced valency change of the counterion.

Hybrid Supramolecular and Colloidal Hydrogels that Bridge Multiple Length Scales

Hybrid nanocomposites were constructed based on colloidal nanofibrillar hydrogels with interpenetrating supramolecular hydrogels, displaying enhanced rheological yield strain and a synergistic improvement in storage modulus. The supramolecular hydrogel consists of naphthyl-functionalized hydroxyethyl cellulose and a cationic polystyrene derivative decorated with methylviologen moieties, physically cross-linked with cucurbit[8]uril macrocyclic hosts. Fast exchange kinetics within the supramolecular system are enabled by reversible cross-linking through the binding of the naphthyl and viologen guests. The colloidal hydrogel consists of nanofibrillated cellulose that combines a mechanically strong nanofiber skeleton with a lateral fibrillar diameter of a few nanometers. The two networks interact through hydroxyethyl cellulose adsorption to the nanofibrillated cellulose surfaces. This work shows methods to bridge the length scales of molecular and colloidal hybrid hydrogels, resulting in synergy between reinforcement and dynamics.

Molecular Engineering of Fracture Energy Dissipating Sacrificial Bonds Into Cellulose Nanocrystal Nanocomposites

Even though nanocomposites have provided a plethora of routes to increase stiffness and strength, achieving increased toughness with suppressed catastrophic crack growth has remained more challenging. Inspired by the concepts of mechanically excellent natural nanomaterials, one-component nanocomposites were fabricated involving reinforcing colloidal nanorod cores with polymeric grafts containing supramolecular binding units. The concept is based on mechanically strong native cellulose nanocrystals (CNC) grafted with glassy polymethacrylate polymers, with side chains that contain 2-ureido-4[1H]-pyrimidone (UPy) pendant groups. The interdigitation of the grafts and the ensuing UPy hydrogen bonds bind the nanocomposite network together. Under stress, UPy groups act as sacrificial bonds: simultaneously providing adhesion between the CNCs while allowing them to first orient and then gradually slide past each other, thus dissipating fracture energy. We propose that this architecture involving supramolecular binding units within side chains of polymer grafts attached to colloidal reinforcements opens generic approaches for tough nanocomposites.

Alanine-rich amphiphilic peptide containing the RGD cell adhesion motif: a coating material for human fibroblast attachment and culture

We studied the self-assembly of peptide A6RGD (A: alanine, R: arginine, G: glycine, D: aspartic acid) in water, and the use of A6RGD substrates as coatings to promote the attachment of human cornea stromal fibroblasts (hCSFs). The self-assembled motif of A6RGD was shown to depend on the peptide concentration in water, where both vesicle and fibril formation were observed. Oligomers were detected for 0.7 wt% A6RGD, which evolved into short peptide fibres at 1.0 wt% A6RGD, while a co-existence of vesicles and long peptide fibres was revealed for 2-15 wt% A6RGD. A6RGD vesicle walls were shown to have a multilayer structure built out of highly interdigitated A6 units, while A6RGD fibres were based on β-sheet assemblies. Changes in the self-assembly motif with concentration were reflected in the cell culture assay results. Films dried from 0.1-1.0 wt% A6RGD solutions allowed hCSFs to attach and significantly enhanced cell proliferation relative to the control. In contrast, films dried from 2.5 wt% A6RGD solutions were toxic to hCSFs.

Dual-responsive and super absorbing thermally cross-linked hydrogel based on methacrylate substituted polyphosphazene

Stimuli-responsive hydrogels have played a crucial role in biomaterials being extensively studied for applications such as tissue engineering and drug delivery. Here, we report on the synthesis and characterization of a new hydrogel based on methacrylate substituted polyphosphazene (PMAPhos). The hydrogel was obtained spontaneously after the polyphosphazene substitution reaction and its mechanical properties were improved after annealing. By annealing the hydrogel in the presence of a 6% w/w methacrylic acid solution, we could obtain a poly(methacrylic acid) (PMAA) branched and slightly cross-linked polyphosphazene network, which showed ultra-high absorbency being able to absorb distilled water, at maximum, as much as 870 times of its own weight. Moreover, PMAPhos showed dual-responsive behaviour responding to both pH and temperature changes. Unlike many other similar systems, the hydrogel structure collapsed not only at lower but also at higher pHs. The gel volume phase transition as a function of temperature was determined by both differential scanning calorimetry (DSC) and by swelling experiments. However, after the hydrogel annealing, the thermal response was suppressed and overridden by the pH-response behaviour of the PMAA branches. Hydrogel morphology and swelling behaviour were also analyzed directly by Cryo-transmission electron microscopy, which revealed the fibrous polymer network. Remarkable features of this new hydrogel are: the biodegradation offered by the polyphosphazene backbone and simplicity of preparation where no co-polymerizations were used to reach the dual-responsive behaviour. Besides, the particular pH-responsive behaviour found for this hydrogel is a useful characteristic for applications aiming for controlled delivery of substances in specific sites of the human body.

Cilia-mimetic hairy surfaces based on end-immobilized nanocellulose colloidal rods.

We show a simple method toward nanoscale cilia-like structures, i.e., functional hairy surfaces, upon topochemically functionalizing nanorods of cellulose nanocrystals (CNCs) with thiol end groups (CNC-SHs), which leads to their immobilization onto a gold surface from one end, still allowing their orientational mobility. CNCs having a lateral dimension of 3-5 nm and length of 50-500 nm incorporate the native crystalline structure with hydrogen-bonded cellulose chains in the parallel configuration. This facilitates asymmetric, selective chemical modification of the reducing ends through reductive amination. Successful thiol functionalization is demonstrated using cryo transmission electron microscopy based on selective attachment of silver nanoparticles to the CNC-SH ends to form Janus-like colloidal rod-sphere adducts. The extent of thiol modification of CNC-SHs is quantified using X-ray photoelectron spectroscopy. The promoted binding of CNC-SHs on gold surfaces is shown by atomic force microscopy and quartz crystal microbalance, where the high dissipation suggests pronounced orientational mobility due to flexible joints at one rod end onto the surfaces. That the joints are flexible is also shown by the bending and realignment of the CNC-SH rods using a receding triple-phase evaporation front of a drying drop of water. The ability of the hairy surface to size-selectively resist particle binding was also investigated. As the CNCs are piezoelectric and allow magnetic functionalization by nanoparticles, we foresee a general platform for nanosized artificial cilia for fluid manipulation and controlled adsorption/desorption.

Aluminum-Induced Photoluminescence Red Shifts in Core–Shell GaAs/AlxGa1–xAs Nanowires

We report a new phenomenon related to Al-induced carrier confinement at the interface in core-shell GaAs/Al(x)Ga(1-x)As nanowires grown using metal-organic vapor phase epitaxy with Au as catalyst. All Al(x)Ga(1-x)As shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to ~66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron-hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell-core-shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.

Delivery of Suramin as an Antiviral Agent through Liposomal Systems

Norovirus RNA‐dependent RNA polymerase (RdRp) is a promising target enzyme for the development of new antiviral drugs. Starting from the crystal structure of norovirus RdRp, we had previously performed an in silico docking search using a library of low‐molecular‐weight compounds that enabled us to select molecules with predicted enzyme inhibitory activity. Among these, the polysulfonated naphthylurea suramin proved to inhibit in vitro both murine and human norovirus polymerases, with IC50 values in the low micromolar range. The negatively charged inhibitor, however, displayed poor cell permeability in cell‐based experiments. Therefore, we produced different suramin‐loaded liposome formulations and evaluated their activities in cell‐based assays using murine norovirus cultivated in RAW 264.7 macrophages, as a model for norovirus genus. The results obtained show that suramin, when delivered through liposomes, can effectively inhibit murine norovirus replication.

An efficient and stable star-shaped plasticizer for starch: cyclic phosphazene with hydrogen bonding aminoethoxy ethanol side chains

Starch is a sustainable polysaccharide with many existing and foreseen applications. Even if a plethora of routes have been described to plasticize it, e.g. with glycerol, sorbitol, and polyols, there remain major challenges related to retrodegradation, stability, and plasticizer leaching. Here we describe plasticization of starch using star-shaped molecules to combine the seemingly conflicting requirements: efficient plasticization typically assigned to low molecular weight hydrogen bonding plasticizers and reduced leaching typical for high molecular weight molecules. Efficient plasticization is allowed by the short, flexible, and hydrogen bonding dangling side chains, which are connected to the core of the plasticizer, leaving crystalline starch domains to allow self-reinforcement. The star-shaped plasticizer, a cyclic phosphazene having six covalently bound aminoethoxy ethanol side groups, was synthesized via nucleophilic substitution, and a series of films using rice starch and different weight fractions of the plasticizer were prepared by drop casting. Incorporating ≥ca. 10 wt% of the plasticizer leads to transparent plasticized films. FTIR indicates that the samples having ≤ca. 60 wt% of the plasticizer involve both plasticized amorphous domains and reinforcing crystalline domains, suggesting self-reinforced nanocomposite structures. The composition with 20 wt% of the plasticizer shows a high tensile modulus of 1.12 GPa and a yield strength of 20.9 MPa, still showing a large strain of 8.8%. No retrodegradation is observed after two months and no clear tackiness is observed even during aging of one year, suggesting stability and suppressed plasticizer leaching. We suggest that the star-shaped hydrogen bonding plasticizers show promising potential to optimize starch-based materials for advanced applications.

Calcium Phosphate Formation on Ethylphosphoester of Poly(ε-Caprolactone) and Poly[Bis(Methacrylate)]Phosphazene In Vitro

Phosphorus containing biopolymers have been synthesized and studied as polymeric candidates for potential tissue engineering applications. The presence of phosphorus in the polymeric structure may improve the biocompatibility of polymers by enhancing their tissue contact. One aim of this study was to examine the chain extending reaction of poly(ε-caprolactone), PCL, using ethyldichlorophosphate as a coupling agent. A preliminary survey was done to find out whether the presence of phosphoester units in a rapidly degradable polymeric structure improves the Ca phosphate formation on PCL. Another aim of this study was to synthesize one kind of polyphosphazene, i.e. poly[bis(methacrylate)]phosphazene, PMAP. In addition, a preliminary biomineralization study for PMAP polymer was carried out. The results of the biomineralization studies indicated some bioactivity of both biopolymers.