Pixin Wang

Fabrication of Size-Controlled Starch-Based Nanospheres by Nanoprecipitation

Nanometric and monodisperse starch acetate nanospheres can be prepared through a simple procedure of nanoprecipitation, by the dropwise addition of water to an acetone solution of starch acetate, without any stabilizing agent. This is the first report of the preparation of starch-based nanospheres by this method. The size of the nanospheres obtained can be easily controlled by a number of simple and efficient modifications, i.e., through regulation of the polymer concentration in acetone, the proportions of the water and organic phases, and the molecular weight and degree of substitution of the starch esters. A number of reasons are suggested to explain the observed transitions in the particle size. Fluorescence spectroscopic studies proved that these types of nanospheres could be potentially used for the encapsulation of hydrophobic drugs.

High mechanical strength and rapid response rate of poly(N-isopropyl acrylamide) hydrogel crosslinked by starch-based nanospheres

A novel approach is presented for fabricating the thermo-responsive poly(N-isopropylacrylamide) hydrogels (TPHs) using starch-based nanospheres with derivable allyl groups as nanoscale crosslinkers. The obtained hydrogels possess an extremely high mechanical strength and a remarkably rapid shrinking time, which is much shorter than the time it will take for a conventional hydrogel (CH). Moreover, the effects of the crosslink density on the swelling, deswelling, and mechanical properties are also systematically studied. The results show that TPHs can sustain strength two hundred times more than the CH, and the characteristic time of deswelling is almost eight times shorter than that of CH. These excellent properties of the TPH gels can be attributed to their special microstructure and the even distribution of the starch-based nanospheres in the hydrogel network.

Hydrophobically associated hydrogels based on acrylamide and anionic surface active monomer with high mechanical strength

In this work, a physically cross-linked hydrogel (HA gels) with high mechanical strength is synthesized via micellar copolymerization of acrylamide (AAm) and an anionic surface active monomer (surfmer), sodium 9 or 10-acrylamidostearic acid (NaAAS) without any adscititious surfactant or chemical cross-linkers. SEM and DLS characterizations indicate that the surfmer formed multi-micellar aggregates with 80–90 nm diameters above its critical micelle concentration, and serve as a crosslinked-center to endow the obtained hydrogel a robust three-dimensional architecture. Compared with the chemically cross-linked hydrogel, HA gels exhibit unusual swelling–deswelling behavior in water and a pulsatile swelling–deswelling behavior is exhibited with alternating pH changes from 5 to 10 because the presence of carboxyl in the surfmer, demonstrating a smart characteristic of the hydrogel. Moreover, the presence of the surfmer greatly improve the mechanical properties of HA gels. A hydrogel containing 20% (mol/mol) surfmer shows a compression strength of 22.50 MPa at a strain of 90% and can be elongated to 13 times its original length. Furthermore, the HA gels show a significant hysteresis recovery after large deformation, underlying a serious energy-dissipation mechanism. This uncommon swelling behavior and mechanical properties of the HA gels result from its special characteristic of cross-linked units. A self-healing ability is expected for this physical hydrogel in future applications in biotechnology.

A novel multi-responsive polyampholyte composite hydrogel with excellent mechanical strength and rapid shrinking rate

Series of hydrophilic core-shell microgels with cross-linked poly(N-isopropylacrylamide) (PNIPAAm) as core and poly(vinyl amine) (PVAm) as shell are synthesized via surfactant-free emulsion polymerization. Then, the microgels are treated with a small amount of potassium persulfate (KPS) to generate free radicals on the amine nitrogens of PVAm, which subsequently initiate the graft copolymerization of acrylic acid (AA), acryloyloxyethyl trimethyl ammonium chloride (DAC), and acrylamide (AAm) onto microgels to prepare multi-responsive composite hydrogels. The composite hydrogels consist of cross-linked ungrafted polyampholyte chains as the first network and microgels with grafted polyampholyte chains as graft point and second network and show surprising mechanical strength and rapid response rate. The investigation shows the compress strength of composite hydrogels is up to 17-30 MPa, which is 60-100 times higher than that of the hydrogel matrix. The composite hydrogel shows reversible switch of transmittance when traveling the lowest critical temperature (LCST) of microgels. When the composite hydrogel swollen in pH 2.86 solution at ambient condition is immersed into the pH 7.00 solution at 45 °C, a rapid dynamic shrinking can be observed. And the character time (τ) of shrinking dynamic of composite hydrogel is 251.9 min, which is less than that of hydrogel matrix (τ=2273.7 min).

In situ hydrogel constructed by starch-based nanoparticles via a Schiff base reaction.

Polysaccharide-based hydrogels are remarkable materials for the biomedical fields because of its excellent biodegradation and biocompatibility. In this work, a novel polysaccharide-based hydrogel was fabricated by in situ crosslinking of starch-based nanoparticles and polyvinylamine. Starch was decorated with cholesterol group and aldehyde groups. TEM and DLS showed that the cholesterol modified oxidation starch (OCS) exhibited a core-shell nanoparticles with mean size of ∼143 nm in aqueous. The hydrogel was then synthesized via Schiff base reaction. Rheological measurements demonstrated the incorporation of cholesterol groups not only reduced the gel time but also improved the storage modulus of the hydrogel compared with the oxide starch crosslinked hydrogel. SEM showed the OCS based hydrogels possess a well-defined porous structure. Furthermore, doxorubicin (DOX) was used as model drug to investigate the control and release properties of OCS hydrogels. This OCS hydrogel would be a promising drug carrier for biomedical applications.

Spontaneous volume transition of polyampholyte nanocomposite hydrogels based on pure electrostatic interaction

A circular system is employed in this paper to investigate the swelling behaviors of polyampholyte hydrogels; this circular system can effectively eliminate the disturbance of various factors and keep the surrounding environment constant. It is found that there exists a spontaneous volume transition to the collapsed state of polyampholyte hydrogels, which is attributed to the overshooting effect, and the transition can occur repeatedly under certain conditions. (13)C NMR is employed to investigate the swelling behavior of polyampholyte hydrogels. The swelling kinetics of polyampholyte hydrogels under various circular media and various circular runs are also investigated in this paper. All the results suggest that the spontaneous volume transition to the collapsed state of polyampholyte hydrogels is dominated by pure electrostatic interaction between different charges in polymer chains.

Synthesis of poly(2-(2-methoxyethoxy)ethyl methacrylate) hydrogel using starch-based nanosphere cross-linkers

Biodegradable thermosensitive hydrogels have attracted great interest because of their potential in biomedical applications. Herein, we present a novel, thermoresponsive poly(2-(2-methoxyethoxy)ethyl methacrylate) hydrogels with starch-based nanospheres as cross-linkers (NMH). NMHs exhibit a narrow lower critical phase transition temperature (LCST) range and high mechanical strength compared with conventional, small molecular cross-linked hydrogels (CMH). Fourier transform infrared (FT-IR) spectroscopy confirms that the NMHs are degradable in aqueous medium. The phase transition temperature of the NMHs is ∼4°C compared with ∼25°C for CMH. The NMHs can sustain strength of 12.2MPa, 10 times more than that of CMH. Moreover, the deswelling rate of NMHs is faster than CMH. The different concentrations of nanospheres can efficiently regulate the various properties of NMHs. The NMHs have excellent properties because of its even network structure formed by nanosphere cross-linkers.

Pickering polymerization of styrene stabilized by starch-based nanospheres

Here, we present a novel nanoparticle derivate from natural polysaccharide as a stabilizer for the Pickering polymerization of styrene. In this process, amphiphilic starch-based nanospheres (SNPs) were fabricated from starch octenyl succinic ester through a nanocoprecipitation process as a Pickering stabilizer. The effects of the SNP concentration, size and pH value on the Pickering polymerization are investigated in detail. The polystyrene (PS) particle morphology transforms from bare PS particles to raspberry-like structures with an increase in the SNP content. The linear relationship between the inverse diameter of the PS particles and the SNP content allows for an estimation of the coverage of SNPs on the PS particle surface. Moreover, the size of the PS particles can be regulated by the SNP size. The microstructure of the PS particles can also be regulated by the pH value of the reaction medium. Finally, a possible mechanism for the formation of the PS particles with different morphologies is proposed.

In situ crosslinkable hydrogels formed from modified starch and O-carboxymethyl chitosan

An in situ hydrogel based on oxidation cholesterol starch (OCS) and O-carboxymethyl chitosan (CMCT) that is completely devoid of potentially cytotoxic small molecule cross-linkers and does not require complex manoeuvres or catalysis has been formulated and characterized. The network structure was created by Schiff base formation. The mechanical properties, internal morphology and swelling ability of the injectable hydrogel were examined. Rheological measurements demonstrated that increasing the concentration of the monomer improved the storage modulus. SEM showed that the hydrogel possessed a well-defined porous structure. In addition, the Schiff base reaction was acid sensitive. Under acid conditions, the hydrogel could hydrolyse quickly compared with high pH conditions. Doxorubicin (DOX) was used as a model drug to investigate the control and release properties of the hydrogel. The cytotoxic potential of the hydrogel was determined using an in vitro viability assay with L929 cells as a model and the results revealed that the hydrogel was non-cytotoxic.

Thermally Induced Multimicellar-Aggregate-to-Vesicle Transition for a Dentritic Starch Ester

Smart polymer nanoparticles are of great interest owing to their response for internal or external stimuli, such as pH, temperature, magnetic field, redox potential, light, and so on. Moreover, the introduced intelligent groups can be used as a reversible switch to modulate the hydrophilic/hydrophobic balance, resulting in drastic changes in the sizes and shapes of the final self-assembly aggregates. Some excellent examples have been reported and, based on amphiphatic copolymers, show that the reversible micelle-to-vesicle transition is responsive towards the stimuli of temperature; the time taken for this thermally induced transition can be in the order of weeks. Waxy corn starch is a naturally occurring dentritic polysaccharide that has been a fantastic material in the biomedical field due to its biodegradability, biocompatibility, and high disease site selectivity. However, because of its irregular molecular architecture and higher polydispersity, it is rarely involved in the supramolecular self-assembly system. Herein, we report a fast, thermally induced micelle-tovesicle transition based on natural polysaccharide derivatives (S-NIPAM-AC). The waxy corn starch ester, decorated with a temperature-sensitive group of poly(N-isopropylacrylamide) (PNIPAM), can undergo self-assembly in aqueous media to form a nanosphere. In addition, this large nanoaggregate is composed from unimolecular micelles units, which act as the basic responsive units and reorganize at high temperature to transform the microstructure of nanoaggregates from micelles to vesicles. The commercially available waxy corn starch was firstly grafted with NIPAM by free radical copolymerization using ammonium cerium nitrate as an initiator, and the obtained polymer was then esterified by using acetic anhydride. The structural feature of S-NIPAM-AC (MN (GPC)= 345,000 Da, MW/MN= 3.75, see the Supporting Information, Figure S1) was characterized by H NMR spectroscopy (see the Supporting Information, Figure S2) and illustrated in Scheme 1. The graft ratio of NIPAM and degree of substitution (DS) of acylation (AC) is 25.7 % and 2.41, respectively. The self-assembly of S-NIPAM-AC was conducted by using a nanoprecipitation procedure. Deionized water was added into the stirred THF solution of S-NIPAM-AC followed by volatilization at 20 8C to exhaustively remove the THF, giving a final concentration of approximately 2 mgmL . A typical TEM image of the spherical micelles is shown in Figure 1 A. The amplified TEM photos revealed the fine structures within a large micelle and provided direct evidence that the large micelle is composed by small spherical building units. For dentritic polymers, Yan and Haggs have pointed out that the large micelles are multimicellar aggregates (MA) with the basic building units of unimolecular micelles, and this aggregation may be maintained by the cooperative interaction between hydrogen bond and hydrophobic association interaction during the self-assembly process. In addition, dynamic light scattering (DLS) characterization also indicated the coexistence of nanoparticles with dimensions of around 11 and 175 nm in the aqueous solution of obtained micelles (Figure 1 B). Therefore, we could identify the larger ones as the MA, and the smaller ones as the [a] Dr. Y. Tan, Dr. K. Xu, Dr. C. Lu, Dr. C. Liu, Prof. Dr. P. Wang Key Laboratory of Ecomaterial Polymers Changchun Institute of Applied Chemistry Chinese Academy of Sciences 130022 Changchun (P.R. China) Fax: (+86) 431-526-2629 E-mail : pxwang@ciac.jl.cn Supporting information for this article (including experimental details and additional H NMR/DLS/TEM data) is available on the WWW under http://dx.doi.org/10.1002/chem.201101060. Scheme 1. The potential structural feature of a dentritic starch ester. PNIPAM =poly(N-isopropylacrylamide), Ac= acyl.