In situ oxidation and reduction of cerium dioxide nanoparticles studied by scanning transmission electron microscopy

Reducing electron beam damage through alternative STEM scanning strategies, Part II: Attempt towards an empirical model describing the damage process

In this second part of a series we attempt to construct an empirical model that can mimick all experimental observations made regarding the role of an alternative interleaved scan pattern in STEM imaging on the beam damage in a specific zeolite sample. We make use of a 2D diffusion model that describes the dissipation of the deposited beam energy in the sequence of probe positions that are visited during the scan pattern. The diffusion process allows for the concept of trying to 'outrun' the beam damage by carefully tuning the dwell time and distance between consecutively visited probe positions. We add a non linear function to include a threshold effect and evaluate the accumulated damage in each part of the image as a function of scan pattern details. Together, these ingredients are able to describe qualitatively all aspects of the experimental data and provide us with a model that could guide a further optimisation towards even lower beam damage without lowering the applied electron dose. We deliberately remain vague on what is diffusing here which avoids introducing too many sample specific details. This provides hope that the model can be applied also in sample classes that were not yet studied in such great detail by adjusting higher level parameters: a sample dependent diffusion constant and damage threshold.

Acute Skin Wounds Treated with Mesenchymal Stem Cells and Biopolymer Compositions Alone and in Combination: Evaluation of Agent Efficacy and Analysis of Healing Mechanisms

We studied the efficacy of using mesenchymal stem cells (MSC) and a polymeric compound (based on chitosan and cellulose with integrated cerium dioxide nanoparticles (PCCD)) in wound healing, and to compare the effects with various invasive and external drugs used for the same purpose. Two wounds were made on the backs of each of 112 Wistar rats, removing the skin. Eight groups were studied: Control_0—intact wounds; Control_ss—0.9% NaCl injections; MSC injections; Control_msc—intact wounds on the opposite side of the body from the MSC group; external application of the PCCD; external application of a combination of the drugs PCCD + MSC; DCh –ointment Dioxomethyltetrahydropyrimidine + Chloramphenicol; and DHCB—injections of a deproteinized hemoderivative of calf blood. After 14 days, we evaluated the state and size of the wounds, studied the level of microcirculation, performed a histological study, and identified and counted the different types of cells. The most effective remedy was combination PCCD + MSC. The treatments in the PCCD and MSC groups were more effective than in the DHCB and DCh groups. Invasive drugs and DCh slowed the regeneration process. DHCB did not affect the rate of healing for acute wounds without ischemia during the first week. The proven efficacy of developed polymeric compounds demonstrates the feasibility of further studies in clinical practice.

Minimising damage in high resolution scanning transmission electron microscope images of nanoscale structures and processes

Beam damage caused during acquisition of the highest resolution images is the current limitation in the vast majority of experiments performed in a scanning transmission electron microscope (STEM). While the principles behind the processes of knock-on and radiolysis damage are well-known (as are other contributing effects, such as heat and electric fields), understanding how and especially when beam damage is distributed across the entire sample volume during an experiment has not been examined in detail. Here we use standard models for damage and diffusion to elucidate how beam damage spreads across the sample as a function of the microscope conditions to determine an "optimum" sampling approach that maximises the high-resolution information in any image acquisition. We find that the standard STEM approach of scanning an image sequentially accelerates damage because of increased overlap of diffusion processes. These regions of accelerated damage can be significantly decelerated by increasing the distance between the acquired pixels in the scan, forming a "spotscan" mode of acquisition. The optimum distance between these pixels can be broadly defined by the fundamental properties of each material, allowing experiments to be designed for specific beam sensitive materials. As an added bonus, if we use inpainting to reconstruct the sparse distribution of pixels in the image we can significantly increase the speed of the STEM process, allowing dynamic phenomena, and the onset of damage, to be studied directly.

Efficacy of A Novel Smart Polymeric Nanodrug in the Treatment of Experimental Wounds in Rats

High-quality and aesthetic wound healing, as well as effective medical support of this process, continue to be relevant. This study aims to evaluate the medical efficacy of a novel smart polymeric nanodrug (SPN) on the rate and mechanism of wound healing in experimental animals. The study was carried out in male Wistar rats (aged 8-9 months). In these animals, identical square wounds down to the fascia were made in non-sterile conditions on the back on both sides of the vertebra. SPN was used for the treatment of one wound, and the other wound was left without treatment (control group). Biocompatible citrate-stabilized cerium oxide nanoparticles integrated into a polysaccharide hydrogel matrix containing natural and synthetic polysaccharide polymers (pectin, alginate, chitosan, agar-agar, water-soluble cellulose derivatives) were used as the therapeutic agent. Changes in the wound sizes (area, volume) over time and wound temperature were assessed on Days 0, 1, 3, 5, 7, and 14. Histological examination of the wounds was performed on Days 3, 7, and 14. The study showed that the use of SPN accelerated wound healing in comparison with control wounds by inhibiting the inflammatory response, which was measured by a decreased number of white blood cells in SPN-treated wounds. It also accelerated the development of fibroblasts, with an early onset of new collagen synthesis, which eventually led to the formation of more tender postoperative scars. Thus, the study demonstrated that the use of SPN for the treatment of wounds was effective and promising.

Sliding History-Dependent Adhesion of Nanoscale Silicon Contacts Revealed by in Situ Transmission Electron Microscopy

Nanoscale asperity-on-asperity sliding experiments were conducted using a nanoindentation apparatus inside a transmission electron microscope, allowing for atomic-scale resolution of contact formation, sliding, and adhesive separation of two silicon nanoasperities in real time. The formation and separation of the contacts without sliding revealed adhesion forces often below detectable limits (ca. 5 nN) or at most equal to values expected from van der Waals forces. Lateral sliding during contact by distances ranging from 3.7 μm down to as little as 20 nm resulted in an average 19× increase in the adhesive pull-off force, with increases as large as 32× seen. Adhesion after sliding increased with both the sliding speed and the applied normal contact stress. Unlike cold welding, where irreversible material changes like flow occur, these effects were repeatable and reversible multiple times, for multiple pairs of asperities. We hypothesize that sliding removes passivating surface terminal species, most likely hydrogen or hydroxyl groups, making sites available to form strong covalent bonds across the interface that increase adhesion. Upon separation, repassivation occurs within the experimentally limited lower bound time frame of 5 s, with full recovery of low adhesion. The results demonstrate the strong sliding history-dependence of adhesion, which hinges on the interplay between tribologically induced removal of adsorbed species and repassivation of unsaturated bonds on freshly separated surfaces by dissociative chemisorption.