A review on thermoresponsive cell culture systems based on poly(N-isopropylacrylamide) and derivatives

A Spontaneous In Situ Thiol-Ene Crosslinking Hydrogel with Thermo-Responsive Mechanical Properties

The thermo-responsive behavior of Poly(N-isopropylacrylamide) makes it an ideal candidate to easily embed cells and allows the polymer mixture to be injected. However, P(NiPAAm) hydrogels possess minor mechanical properties. To increase the mechanical properties, a covalent bond is introduced into the P(NIPAAm) network through a biocompatible thiol-ene click-reaction by mixing two polymer solutions. Co-polymers with variable thiol or acrylate groups to thermo-responsive co-monomer ratios, ranging from 1% to 10%, were synthesized. Precise control of the crosslink density allowed customization of the hydrogel's mechanical properties to match different tissue stiffness levels. Increasing the temperature of the hydrogel above its transition temperature of 31 °C induced the formation of additional physical interactions. These additional interactions both further increased the stiffness of the material and impacted its relaxation behavior. The developed optimized hydrogels reach stiffnesses more than ten times higher compared to the state of the art using similar polymers. Furthermore, when adding cells to the precursor polymer solutions, homogeneous thermo-responsive hydrogels with good cell viability were created upon mixing. In future work, the influence of the mechanical micro-environment on the cell's behavior can be studied in vitro in a continuous manner by changing the incubation temperature.

Thermo/redox-responsive dissolvable gelatin-based microsphere for efficient cell harvesting during 3D cell culturing

The use of microspheres for culturing adherent cells has been proven as an important method, allowing for obtaining adequate number of cells in limited space and volume of medium for the intended cell-based medical applications. However, the use of proteolytic enzymes for cell harvesting from the microsphere resulted in cell damage and loss of functionality. Therefore, in this study, we developed a novel redox/thermo-responsive dissolvable gelatin-based microsphere for successful cell proliferation and harvesting adequate high-quality cells using non-enzymatic cell detachment methods. Initially, a redox-induced dissolvable gelatin-based microsphere was successfully prepared using disulfide bonds as crosslinking agent, firmly stabilizing gelatin networks and forming a stable microsphere at physiological temperature. The optimized concentration of the crosslinking agent was 1.2 mM, which kept the microsphere stable for >120 h. The microsphere was then coated with PNIPAm-ALA copolymer via physical or chemical means, resulting in a positively charged thermosensitive surface. The positive charge derived from ALA in PNIPAm-ALA copolymer enhanced cell attachment, while the thermosensitive property of the copolymer enabled for temperature induced cell harvesting. When the temperature dropped below the LCST value of PNIPAm-ALA₅ (33.4°C), the copolymer swelled and became more hydrophilic, allowing cells to be readily separated. The addition of reducing agents such as GSH, DTT and L-cysteine resulted in further cleavage of the disulfide bond in the microsphere and dissolution of the microsphere for complete cell detachment. Interestingly, cell attachment and proliferation were enhanced on microspheres coated with PNIPAm-ALA₅ using diselenide as a crosslinking agent, and complete cell detachment was occurred within 15 min after adding 25 mM DTT followed by lowering the temperature (4°C). Therefore, the microsphere fabricated in this study was worthwhile for non-enzymatic cell detachment and has the potential to be used for cell expansion and harvesting adequate live cells of high quality and functionality for tissue engineering or cell therapy.

Tailoring cellular microenvironments using scaffolds based on magnetically-responsive polymer brushes

A variety of polymeric scaffolds with the ability to control cell detachment has been created for cell culture using stimuli-responsive polymers. However, the widely studied and commonly used thermo-responsive polymeric substrates always affect the properties of the cultured cells due to the temperature stimulus. Here, we present a different stimuli-responsive approach based on poly(3-acrylamidopropyl)trimethylammonium chloride) (poly(APTAC)) brushes with homogeneously embedded superparamagnetic iron oxide nanoparticles (SPIONs). Neuroblastoma cell detachment was triggered by an external magnetic field, enabling a non-invasive process of controlled transfer into a new place without additional mechanical scratching and chemical/biochemical compound treatment. Hybrid scaffolds obtained in simultaneous surface-initiated atom transfer radical polymerization (SI-ATRP) were characterized by atomic force microscopy (AFM) working in the magnetic mode, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS) to confirm the magnetic properties and chemical structure. Moreover, neuroblastoma cells were cultured and characterized before and after exposure to a neodymium magnet. Controlled cell transfer triggered by a magnetic field is presented here as well.

Preparation and Characterization of Thermoresponsive Poly(N-Isopropylacrylamide) for Cell Culture Applications

Poly(N-isopropylacrylamide) (PNIPAAm) is a typical thermoresponsive polymer used widely and studied deeply in smart materials, which is attractive and valuable owing to its reversible and remote "on-off" behavior adjusted by temperature variation. PNIPAAm usually exhibits opposite solubility or wettability across lower critical solution temperature (LCST), and it is readily functionalized making it available in extensive applications. Cell culture is one of the most prospective and representative applications. Active attachment and spontaneous detachment of targeted cells are easily tunable by surface wettability changes and volume phase transitions of PNIPAAm modified substrates with respect to ambient temperature. The thermoresponsive culture platforms and matching thermal-liftoff method can effectively substitute for the traditional cell harvesting ways like enzymatic hydrolysis and mechanical scraping, and will improve the stable and high quality of recovered cells. Therefore, the establishment and detection on PNIPAAm based culture systems are of particular importance. This review covers the important developments and recommendations for future work of the preparation and characterization of temperature-responsive substrates based on PNIPAAm and analogues for cell culture applications.

Stacking up: a new approach for cell culture studies

Traditional cell culture relies mostly on flat plastic surfaces, such as Petri dishes and multiwell plates. These commercial surfaces provide limited flexibility for experimental design. In contrast, cell biology increasingly demands surface customisation, functionalisation, and cell monitoring in order to obtain data that is relevant in vivo. The development of research areas such as microfluidics and electrochemical detection methods greatly promoted the customised design of cell culture platforms. However, the challenges for mass production and material limitations prevent their widespread usage and commercialisation. This article presents a new cell culture platform based on stacks of a transparent flexible printable substrate. The arrangement introduces multi-layered stacks for possible manipulation and access to the cells. The platform is highly compatible with current technologies, such as colorimetric imaging and fluorescence microscopy. In addition, it can potentially integrate, e.g., biomaterials, patterning, microfluidics, electrochemical detection and other techniques to influence, monitor, and assess cell behaviour in a multitude of different settings. More importantly, the platform is a low-cost alternative customisable through functional printing and coating technologies. The device shown in this manuscript represents a prototype for more sophisticated variations that will expand the relevance of in vitro studies in cell biology.

Understanding the effects of symmetric salt on the structure of a planar dipolar polymer brush

The effects of added salt on a planar dipolar polymer brush immersed in a polar solvent are studied using a field theoretic approach. The field theory developed in this work provides a unified framework for capturing effects of the inhomogeneous dielectric function, translational entropy of ions, crowding due to finite sized ions, ionic size asymmetry, and ion solvation. In this paper, we use the theory to study the effects of ion sizes, their concentration, and ion-solvation on the polymer segment density profiles of a dipolar brush immersed in a solution containing symmetric salt ions. The interplay of crowding effects, translational entropy, and ion solvation is shown to exhibit either an increase or decrease in the brush height. Translational entropy and crowding effects due to finite sizes of the ions tend to cause expansion of the brush as well as uniform distribution of the ions. By contrast, ion-solvation effects, which tend to be stronger for smaller ions, are shown to cause shrinkage of the brush and inhomogeneous distribution of the ions.