Functional polymers currently represent a basic component of a large range of biological and biomedical applications including molecular release, tissue engineering, bio-sensing and medical imaging. Advancements in these fields are driven by the use of a wide set of biodegradable polymers with controlled physical and bio-interactive properties. In this context, microscopy techniques such as Atomic Force Microscopy (AFM) are emerging as fundamental tools to deeply investigate morphology and structural properties at micro and sub-micrometric scale, in order to evaluate the in time relationship between physicochemical properties of biomaterials and biological response. In particular, AFM is not only a mere tool for screening surface topography, but may offer a significant contribution to understand surface and interface properties, thus concurring to the optimization of biomaterials performance, processes, physical and chemical properties at the micro and nanoscale. This is possible by capitalizing the recent discoveries in nanotechnologies applied to soft matter such as atomic force spectroscopy to measure surface forces through force curves. By tip-sample local interactions, several information can be collected such as elasticity, viscoelasticity, surface charge densities and wettability. This paper overviews recent developments in AFM technology and imaging techniques by remarking differences in operational modes, the implementation of advanced tools and their current application in biomaterials science, in terms of characterization of polymeric devices in different forms (i.e., fibres, films or particles).

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Complex architecture of natural tissues such as nerves requires the use of multifunctional scaffolds with peculiar topological and biochemical signals able to address cell behavior towards specific events at the cellular (microscale) and macromolecular (nanoscale) level. In this context, the electrospinning technique is useful to generate fiber assemblies having peculiar fiber diameters at the nanoscale and patterned by unidirectional ways, to facilitate neurite extension via contact guidance. Following a bio-mimetic approach, fully aligned polycaprolactone fibers blended with gelatin macromolecules have been fabricated as potential bioactive substrate for nerve regeneration. Morphological and topographic aspects of electrospun fibers assessed by SEM/AFM microscopy supported by image analyses elaboration allow estimating an increase of fully aligned fibers from 5 to 39% as collector rotating rate increases from 1,000 to 3,000 rpm. We verify that fully alignment of fibers positively influences in vitro response of hMSC and PC-12 cells in neurogenic way. Immunostaining images show that the presence of topological defects, i.e., kinks--due to more frequent fiber crossing--in the case of randomly organized fiber assembly concurs to interfere with proper neurite outgrowth. On the contrary, fully aligned fibers without kinks offer a more efficient contact guidance to direct the orientation of nerve cells along the fibers respect to randomly organized ones, promoting a high elongation of neurites at 7 days and the formation of bipolar extensions. So, this confirms that the topological cue of fully alignment of fibers elicits a favorable environment for nerve regeneration.

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The discovery of new drugs to treat pathological cells in the case of aggressive liver primary cancer is imposing the identification of high-throughput screening systems to predict the in vivo response of new therapeutic molecules, in order to reduce current use of animals and drug testing costs. Recently, micro/nanostructured scaffolds have been adopted to reproduce the hepatic microenvironment due to their higher similarity to the biological niche with respect to the traditional two-dimensional culture plate, so providing novel in vitro models for reliably understanding molecular mechanisms related to cancer cells activity. Herein, we propose the study of electrospun scaffolds made of polycaprolactone as in vitro model that can mimic the morphological organization of native extracellular matrix and the co-culture of hepatic cell lines-i.e., HepG2, human healthy hepatocytes (HHH). The micro- and nano-scale morphological features of fibers with diameter equal to (3.22 ± 0.42) μm and surface roughness of (17.84 ± 4.43) nm-allow the reproduction of the in vivo scenario influencing the adhesion and proliferation rate of the cultured cells. A much lower proliferation rate is observed for the HepG2 cells compared to the HHH cells, when cultured on the fibrous scaffolds over a time course of 4 weeks. Moreover, results on oxidative stress mechanisms indicate an antioxidant effect of fibers mainly in the case of co-colture, thus suggesting a promising use as new in vitro models to explore alternative therapeutic strategies in hepatocarcinoma treatment.

Surface topography and chemistry both play a crucial role on influencing cell response in 3D porous scaffolds in terms of osteogenesis. Inorganic materials with peculiar morphology and chemical functionalities may be proficiently used to improve scaffold properties-in the bulk and along pore surface-promoting in vitro and in vivo osseous tissue in-growth. The present study is aimed at investigating how bone regenerative properties of composite scaffolds made of poly(Ɛ-caprolactone) (PCL) can be augmented by the peculiar properties of Mg(2+) ion doped hydroxyapatite (dHA) crystals, mainly emphasizing the role of crystal shape on cell activities mediated by microstructural properties. At the first stage, the study of mechanical response by crossing experimental compression tests and theoretical simulation via empirical models, allow recognizing a significant contribution of dHA shape factor on scaffold elastic moduli variation as a function of the relative volume fraction. Secondly, the peculiar needle-like shape of dHA crystals also influences microscopic (i.e. crystallinity, adhesion forces) and macroscopic (i.e. roughness) properties with relevant effects on biological response of the composite scaffold: differential scanning calorimetry (DSC) analyses clearly indicate a reduction of crystallization heat-from 66.75 to 43.05 J g(-1)-while atomic force microscopy (AFM) ones show a significant increase of roughness-from (78.15  ±  32.71) to (136.13  ±  63.21) nm-and of pull-off forces-from 33.7% to 48.7%. Accordingly, experimental studies with MG-63 osteoblast-like cells show a more efficient in vitro secretion of alkaline phosphatase (ALP) and collagen I and a more copious in vivo formation of new bone trabeculae, thus suggesting a relevant role of dHA to support the main mechanisms involved in bone regeneration.

In the last decade, different technological approaches have been proposed for the fabrication of micro‐gels as cell carriers to investigate in vitro response. Among them, electro fluid dynamic atomization (EFDAs) is emerging as a highly versatile process that allows atomizing polymer solutions at the micrometric size scale by the application of high voltage electric field. Here, we propose to revisit the process configuration to optimize the fabrication of sodium alginate micro‐gels with different physical and mechanical properties and validate their use for in vitro culture with human mesenchymal stem cells (hMSCs). By optical microscopy, we explored the strict correlation between particle morphology and process parameters, that is, voltage, flow rate, electrode gap and needle diameter. Meanwhile, by scanning electron microscopy and atomic force microscopy, we investigated basic differences in terms of physical (i.e., swelling) and mechanical properties (i.e., stiffness), ascribable to alginates solutions with different concentrations. Lastly, in vitro tests confirmed the effect of physical properties of microgels on the biological response of hMSCs cultured onto the surface. In perspective, the integration of alginate with peculiar extracellular matrix (ECM) components (i.e., proteins, mineral phase) may contribute to assign specific biological functionalities to the microgels in order to reproduce 3D models to investigate in vivo‐like behavior of cells.

Electrospun polymeric fibers are currently used as 3D models for in vitro applications in biomedical areas, i.e., tissue engineering, cell and drug delivery. The high customization of the electrospinning process offers numerous opportunities to manipulate and control surface area, fiber diameter, and fiber density to evaluate the response of cells under different morphological and/or biochemical stimuli. The aim of this study was to investigate—via atomic force microscopy (AFM)—the chemical and morphological changes in bi-component electrospun fibers (BEFs) during the in vitro degradation process using a biological medium. BEFs were fabricated by electrospinning a mixture of synthetic-polycaprolactone (PCL)-and natural polymers (gelatin) into a binary solution. During the hydrolytic degradation of protein, no significant remarkable effects were recognized in terms of fiber integrity. However, increases in surface roughness as well as a decrease in fiber diameter as a function of the degradation conditions were detected. We suggest that morphological and chemical changes due to the local release of gelatin positively influence cell behavior in culture, in terms of cell adhesion and spreading, thus working to mimic the native microenvironment of natural tissues.