Thin Polymer Brush Decouples Biomaterial's Micro-/Nano-Topology and Stem Cell Adhesion

Surface morphology and chemistry of polymers used as biomaterials, such as tissue engineering scaffolds, have a strong influence on the adhesion and behavior of human mesenchymal stem cells. Here we studied semicrystalline poly(ε-caprolactone) (PCL) substrate scaffolds, which exhibited a variation of surface morphologies and roughness originating from different spherulitic superstructures. Different substrates were obtained by varying the parameters of the thermal processing, i.e. crystallization conditions. The cells attached to these polymer substrates adopted different morphologies responding to variations in spherulite density and size. In order to decouple substrate topology effects on the cells, sub-100 nm bio-adhesive polymer brush coatings of oligo(ethylene glycol) methacrylates were grafted from PCL and functionalized with fibronectin. On surfaces featuring different surface textures, dense and sub-100 nm thick brush coatings determined the response of cells, irrespective to the underlying topology. Thus, polymer brushes decouple substrate micro-/nano-topology and the adhesion of stem cells

Gradient polymers for tissue engineering

With increasing life expectancy, there is an constant demand for finding solutions to restore damaged or diseased tissues and organs. Regenerative medicine holds the promise to create continuous body-part replacements through the combination of cells, biological factors, and synthetic scaffolds. However, a better control over cell-material interactions needs to be achieved to fabricate better performing and long-lasting supports for tissue engineering (TE). In the human body chemical and physical gradients regulate cell migration and differentiation. It is thus crucial to control these processes in 3D artificial scaffolds used in regenerative medicine. In order to accomplish this, the modification of biomaterials’ interfaces represent a potentially successful approach. This includes the fabrication of synthetic extra-cellular matrices (ECMs) presenting interfacial gradients which can regulate the behavior of adhering cells. \ud Inspired by these approaches I fabricated chemical gradients on polymer substrates in order to determine cell response on both 2D and 3D supports. A specific focus is placed on polymer brush coatings with controllable properties (adhesion, flexibility, bioactivity) synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP). By this method poly(N-isopropyl acrylamide) (PNIPAM) and functionalizable poly(oligo(ethylene glycol) methacrylate (POEGMA) brushes were “grafted-from” 2D and 3D poly(ε-caprolactone) (PCL) supports. These thermoresponsive PNIPAM layers were applied to thermally control cellular adhesion while POEGMA-coated supports were used to vary the bioactivity of the coating via conjugated with fibronectin (FN) and growth factors. The peculiar physical properties of POEGMA brushes such as wettability and high functionality were exploited to obtain multi-dimensional surface gradients of (bio)chemical signals for spatially controlling cell behavior

Polymer Brush Coatings Regulating Cell Behavior: Passive Interfaces Turn Into Active

Material technology platforms able to modulate the communication with cells at the interface of biomaterials are being increasingly experimented on. Progress in the fabrication of supports is simultaneously introducing new surface modification strategies aimed at turning these supports from passive to active components in engineered preparations. Among these platforms, polymer brushes are arising not only as coatings determining the physical and (bio)chemical surface properties of biomaterials, but also as smart linkers between surfaces and biological cues. Their peculiar properties, especially when brushes are synthesized by “grafting-from” methods, enable closer mimicking of the complex and heterogeneous biological microenvironments. Inspired by the growing interest in this field of materials science, we summarize here the most prominent and recent advances in the synthesis of grafted-from polymer brush surfaces to modulate the response of adhering cells

Pulling angle-dependent force microscopy

In this paper, we describe a method allowing one to perform three-dimensional displacement control in force spectroscopy by atomic force microscopy (AFM). Traditionally, AFM force curves are measured in the normal direction of the contacted surface. The method described can be employed to address not only the magnitude of the measured force but also its direction. We demonstrate the technique using a case study of angle-dependent desorption of a single poly(2-hydroxyethyl methacrylate) (PHEMA) chain from a planar silica surface in an aqueous solution. The chains were end-grafted from the AFM tip in high dilution, enabling single macromolecule pull experiments. Our experiments give evidence of angular dependence of the desorption force of single polymer chains and illustrate the added value of introducing force direction control in AFM

Design and performance of flexible polymeric piezoelectric energy harvesters for battery-less tyre sensors

A piezoelectric energy harvester for battery-less tyre sensors has been developed. It consists of two key elements: (a) a piezoelectric material—polyvinylidene difluoride (PVDF) film and (b) an electrode—a conductive elastomer filled with carbon black and single-wall carbon nanotubes (SWCNTs). It was designed as a flexible patch in a sandwich-like configuration, which can be mounted onto the inner liner of a tyre. The patch was fabricated by inserting a PVDF film in between two conductive elastomer sheets. The development started with improving the conductivity of the elastomer by adding 6 wt% of SWCNT masterbatch. The adhesion between the interfaces was improved through surface modification of the PVDF film by introducing oxygen functional groups via a plasma treatment and further modification with a thiocyanate silane. The successful surface modification of the PVDF film was affirmed by x-ray photoelectron spectroscopy. T-peel and fatigue tests showed durable and stable adhesion between PVDF and conductive elastomer, confirming that the silane can effectively bridge the two components. A glueing method is proposed to adhere the patch to the tyre inner liner compound. The harvester is estimated to sufficiently power a reference tyre sensor, producing 28 μW cm−2

Dynamic measurement setups for validating piezoelectric energy harvesters in driving conditions

Sustainable power supply to flexible electronics is currently of high interest due to the transition to autonomous and self-driving vehicles. Piezoelectric Energy Harvesters (PEH) can be used as sustainable energy sources by harvesting the electrical power through the material deformation occurring in a tire. In this work, an analytical setup was developed to experimentally validate the energy harvesters for their use in tires. It was designed to measure the harvested electrical energy under simulated driving conditions. The setup includes a Dynamic Mechanical Analysis (DMA) as foundation to simulate the vibrations and dynamic responses occurring in a rolling tire. The dynamic properties and the output voltage from the harvesters were monitored under these sinusoidal conditions. For this, a PEH for tire applications was prepared in a sandwich configuration. It consists of a piezoelectric material, i.e. PolyVinyliDene diFluoride (PVDF) film, inserted in between two layers of electrodes, i.e. elastomers filled with conductive carbon black fillers. The electrical conductivity of elastomeric compounds was measured under dynamic conditions varying dynamic strain, frequencies, and temperatures. Dynamic strain and temperature resulted to be the most significant factors influencing the electrical conductivity of elastomers. Output power from the piezoelectric energy harvester was also measured at varied frequencies and temperatures. Both properties increase considerably the piezoelectric power. This development gives a promising method for analyzing the electro-mechanical properties of conductive and piezoelectric materials and optimizing their performance according to simulated tire-rolling conditions