Dynamic materials from microgel multilayers

Guiding cell adhesion and motility by modulating cross-linking and topographic properties of microgel arrays

Biomaterial-driven modulation of cell adhesion and migration is a challenging aspect of tissue engineering. Here, we investigated the impact of surface-bound microgel arrays with variable geometry and adjustable cross-linking properties on cell adhesion and migration. We show that cell migration is inversely correlated with microgel array spacing, whereas directionality increases as array spacing increases. Focal adhesion dynamics is also modulated by microgel topography resulting in less dynamic focal adhesions on surface-bound microgels. Microgels also modulate the motility and adhesion of Sertoli cells used as a model for cell migration and adhesion. Both focal adhesion dynamics and speed are reduced on microgels. Interestingly, Gas2L1, a component of the cytoskeleton that mediates the interaction between microtubules and microfilaments, is dispensable for the regulation of cell adhesion and migration on microgels. Finally, increasing microgel cross-linking causes a clear reduction of focal adhesion turnover in Sertoli cells. These findings not only show that spacing and rigidity of surface-grafted microgels arrays can be effectively used to modulate cell adhesion and motility of diverse cellular systems, but they also form the basis for future developments in the fields of medicine and tissue engineering.

Ultrasonic Microplotting of Microgel Bioinks

Material scaffolds that mimic the structure, function, and bioactivity of native biological tissues are in constant development. Recently, material scaffolds composed of microgel particles have shown promise for applications ranging from bone regeneration to spheroid cell growth. Previous studies with poly N-isopropylacrylamide microgel scaffolds utilized a layer-by-layer (LBL) technique where individual, uniform microgel layers are built on top of each other resulting in a multilayer scaffold. However, this technique is limited in its applications due to the inability to control microscale deposition or patterning of multiple particle types within a microgel layer. In this study, an ultrasonic microplotting technique is used to address the limitations of LBL fabrication to create patterned microgel films. Printing parameters, such as bioink formulation, surface contact angle, and print head diameter, are optimized to identify the ideal parameters needed to successfully print microgel films. It was found that bioinks composed of 2 mg/mL of microgels and 20% polyethylene glycol by volume (v/v), on bovine serum albumin-coated glass, with a print head diameter of 50 μm resulted in the highest quality prints. Patterned films were created with a maximum resolution of 50 μm with the potential for finer resolutions to be achieved with alternative bioink compositions and printing parameters. Overall, ultrasonic microplotting can be used to create more complex microgel films than is possible with LBL techniques and offers the possibility of greater printing resolution in 3D with further technology development.

A study of the complex interaction between poly allylamine hydrochloride and negatively charged poly(N-isopropylacrylamide-co-methacrylic acid) microgels

Negatively charged poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAm-co-MAA)) microgels undergo size changes in response to changes in temperature and pH. Complexation of these microgels with positively charged polyelectrolytes can greatly affect their physical properties and their capacity for encapsulating active molecules. Here we study the interaction between (P(NIPAm-co-MAA)) microgels and a model positively charged polyelectrolyte, poly allylamine hydrochloride (PAH), with different molecular weights. Experiments were conducted at temperatures below and above the lower critical solution temperature (LCST) of the microgel (30-32 °C), at 20 and 40 °C, respectively, and for PAH at molecular weights of 15, 50, and 140 kDa. Below the LCST, dynamic light scattering and zeta potential measurements with molecular simulation show that for the 15 kDa PAH there is preferential accumulation of PAH inside the microgel, whereas for the higher molecular weight PAH, the polyelectrolyte deposits mainly on the microgel surface. Above the LCST, PAH is preferentially located on the surface of the microgels for all molecular weights studied as a result of charge segregation in the hydrogels. Confocal scanning laser microscopy and flow cytometry were used to quantify rhodamine labelled PAH associated with the microgel. Isothermal titration calorimetry studies give insight into the thermodynamics of the interaction of PAH with the hydrogels, and how this interaction is affected by the molecular weight of PAH. Finally, microgels with encapsulated doxorubicin were exposed to PAH, revealing that the drug is displaced from the microgel by the PAH chains.

Avidin-Biotin Cross-Linked Microgel Multilayers as Carriers for Antimicrobial Peptides

Herein, we report on the formation of cross-linked antimicrobial peptide-loaded microgel multilayers. Poly(ethyl acrylate- co-methacrylic acid) microgels were synthesized and functionalized with biotin to enable the formation of microgel multilayers cross-linked with avidin. Microgel functionalization and avidin cross-linking were verified with infrared spectroscopy, dynamic light scattering, and z-potential measurements, while multilayer formation (up to four layers) was studied with null ellipsometry and quartz crystal microbalance with dissipation (QCM-D). Incorporation of the antimicrobial peptide KYE28 (KYEITTIHNLFRKLTHRLFRRNFGYTLR) into the microgel multilayers was achieved either in one shot after multilayer formation or through addition after each microgel layer deposition. The latter was found to strongly promote peptide incorporation. Further, antimicrobial properties of the peptide-loaded microgel multilayers against Escherichia coli were investigated and compared to those of a peptide-loaded microgel monolayer. Results showed a more pronounced suppression in bacterial viability in suspension for the microgel multilayers. Correspondingly, LIVE/DEAD staining showed promoted disruption of adhered bacteria for the KYE28-loaded multilayers. Taken together, cross-linked microgel multilayers thus show promise as high load surface coatings for antimicrobial peptides.

Functional Stimuli-Responsive Gels: Hydrogels and Microgels

One strategy that has gained much attention in the last decades is the understanding and further mimicking of structures and behaviours found in nature, as inspiration to develop materials with additional functionalities. This review presents recent advances in stimuli-responsive gels with emphasis on functional hydrogels and microgels. The first part of the review highlights the high impact of stimuli-responsive hydrogels in materials science. From macro to micro scale, the review also collects the most recent studies on the preparation of hybrid polymeric microgels composed of a nanoparticle (able to respond to external stimuli), encapsulated or grown into a stimuli-responsive matrix (microgel). This combination gave rise to interesting multi-responsive functional microgels and paved a new path for the preparation of multi-stimuli "smart" systems. Finally, special attention is focused on a new generation of functional stimuli-responsive polymer hydrogels able to self-shape (shape-memory) and/or self-repair. This last functionality could be considered as the closing loop for smart polymeric gels.

Preparation and ion sensing property of the self-assembled microgels by QCM

The polyanion polystyrene sulfonate (PSS), the polycation poly (allylamine hydrochloride) (PAH), and the anionic poly (N-isopropylacrylamide-co-acrylic acid) [P(NIPAM-co-AA)] microgels were self-assembled onto the polyethylene imine (PEI) adsorbed gold surfaces of quartz crystal microbalance (QCM) because of the electrostatic attractions. The interactions of various metal particles including Ca2+, Bi3+, Cu2+, Zn2+, Ni2+, Sn2+, Co2+, and Cd2+with the obtained PEI/PSS/PAH/microgel layer in aqueous solutions were evaluated by QCM. The PEI/PSS/PAH/Microgel covered QCM sensor demonstrates the lowest detection limit of 0.1 ppm in aqueous solutions and the obviously linear connection between the frequency response and Ni2+concentration from 0.1 to 20 ppm, which is due to the complexation of Ni2+with the carboxyl groups of microgels. Atomic force microscopy (AFM) was used to reveal the morphology and stability of the self-assembled polyelectrolyte/microgel layer before and after adsorbing heavy metal ions. These self-assembled materials of polyelectrolyte/microgel layer will be helpful for manufacturing ion-selective materials for separation and identification purposes.

Supramolecular polymer networks: hydrogels and bulk materials

Supramolecular polymer networks are materials crosslinked by reversible supramolecular interactions, such as hydrogen bonding or electrostatic interactions. Supramolecular materials show very interesting and useful properties resulting from their dynamic nature, such as self-healing, stimuli-responsiveness and adaptability. Here we will discuss recent progress in polymer-based supramolecular networks for the formation of hydrogels and bulk materials.

Confinement of thermoresponsive microgels into fibres via colloidal electrospinning: experimental and statistical analysis

Colloidal electrospinning allow confining microgels within polymer fibre. Optimization (DoE) to minimize fibre diameter gives rise to nanofibres (63 nm).

Unusual temperature-induced swelling of ionizable poly(N-isopropylacrylamide)-based microgels: experimental and theoretical insights into its molecular origin

In the traditional view of temperature-driven volume phase transitions in PNIPAM-based microgel solutions, a monotonic and sharp decrease in the particle size occurs upon heating the solution to above the volume phase transition temperature (VPTT). However, at sufficiently high microgel concentrations and under low salt conditions, our dynamic light scattering experiments reveal an unexpected non-monotonic evolution of the particle size when increasing the solution temperature. These findings show that poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAM-co-MAA)) microgels swell upon heating the solution in the temperature range where NIPAM is water-soluble (i.e., below the VPPT). Further heating the microgel solution leads to microgel collapse as typically observed at temperatures above the VPTT. This novel behavior depends on the particle and salt concentration. We have observed the expected monotonic temperature-response of P(NIPAm-co-MAA) microgel solutions at low particle density and high salt concentration. To gain insights into the molecular origin of the unusual behavior of these microgel solutions, we have combined nuclear magnetic resonance studies and molecular-level theoretical calculations of the system. A delicate balance between inter-particle steric compressions and intra-microgel physical interactions and chemical equilibria determines the size of these microgels. Both steric compression, due to finite density, and hydrogen bond formation in the interior of the microgels favors a more compact particle. On the contrary, at the pH of the experiments the acid-base equilibrium constrains the polymer charge to increase, which favors particle swelling due to intra-microgel electrostatic repulsions. This interplay between physical interactions and chemical equilibria occurring at the nanometer length-scale determines the unusual thermal-induced swelling of P(NIPAM-co-MAA) microgels.

Highly Stable, Protein-Resistant Surfaces via the Layer-by-Layer Assembly of Poly(sulfobetaine methacrylate) and Tannic Acid

Zwitterionic materials have received great attention because of the non-fouling property. As a result of the electric neutrality of zwitterionic polymers, their layer-by-layer (LBL) assembly is generally conducted under specific conditions, such as very low pH values or ionic strength. The formed multilayers are unstable at high pH or in a high ionic strength environment. Therefore, the formation of highly stable multilayers of zwitterionic polymers via the LBL assembly process is still challenging. Here, we report the LBL assembly of poly(sulfobetaine methacrylate) (PSBMA) with a polyphenol, tannic acid (TA), for protein-resistant surfaces. The assembly process was monitored by a quartz crystal microbalance (QCM) and variable-angle spectroscopic ellipsometry (VASE), which confirms the formation of thin multilayer films. We found that the (TA/PSBMA)n multilayers are stable over a wide pH range of 4-10 and in saline, such as 1 M NaCl or urea solution. The surface morphology and chemical composition were characterized by specular reflectance Fourier transform infrared spectroscopy (FTIR/SR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Furthermore, (TA/PSBMA)n multilayers show high hydrophilicity, with a water contact angle lower than 15°. A QCM was used to record the dynamic protein adsorption process. Adsorption amounts of bovine serum albumin (BSA), lysozyme (Lys), and hemoglobin (Hgb) on (TA/PSBMA)20 multilayers decreased to 0.42, 52.9, and 37.9 ng/cm(2) from 328, 357, and 509 ng/cm(2) on a bare gold chip surface, respectively. In addition, the protein-resistance property depends upon the outmost layer. This work provides new insights into the LBL assembly of zwitterionic polymers.

Hollow microgel based ultrathin thermoresponsive membranes for separation, synthesis, and catalytic applications

Thermoresponsive core-shell microgels with degradable core are synthesized via surfactant based free radical polymerization using N,N'-(1,2-dihydroxy-ethylene)bis(acrylamide) (DHEA) as a cross-linker for core preparation. The 1,2-glycol bond present in DHEA is susceptible to NaIO4 solution, and thus, the structure can be cleaved off resulting in hollow microgel. Ultrathin membranes are prepared by suction filtration of a dilute suspension of core-shell microgels over a sacrificial layer of Cd(OH)2 nanostrand coated on track etched membrane. After removal of the degraded cores from microgels, the membranes are cross-linked with glutaraldehyde and the nanostrands are removed by passing a 10 mM HCl solution. The prepared membranes are thoroughly characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and dynamic contact angle for morphology, thermoresponsive, and hydrophilic properties, respectively. The prepared membranes showed thermoresponsive permeation behavior and remarkable separation performance for low molecular weight dyes and lysozyme protein. These membranes are also used to synthesize gold nanoparticles and immobilize lactate dehydrogenase enzyme for catalytic and biocatalytic application. The results for water permeation, solute rejection, and ability to immobilize gold nanoparticles and enzymes showed its wide range of applicability. Furthermore, the synthesis of hollow microgel is simple and environmentally friendly, and the membrane preparation is easy, scalable, and other microgel systems can also be used. These responsive membranes constitute a significant contribution to advanced separation technology.

Tunable swelling and rolling of microgel membranes

The tunable swelling and rolling of films assembled via layer-by-layer (LbL) methods from poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels and poly(ethylenimine) (PEI) have been systematically studied. Microgel/PEI films assembled at pH 7.4 display a high degree of in-plane swelling at low pH that dramatically increases the film area and drives self-delamination from the substrate to form a free-standing film. The degree of film swelling can be controlled by the size of microgels used in film fabrication. Taking advantage of this feature, self-rolled scrolls can be easily obtained from microgel/PEI films prepared from microgels of two different sizes. The rolling direction can be controlled by the assembly of different size microgels in different film strata, and the final shape of the scrolls can be controlled by scratching the desired film edges. The present work contributes to a deeper understanding of microgel/PEI film swelling properties and introduces a facile and novel method to prepare free-standing films and self-rolled scrolls.