Inkjet-printed morphogenesis of tumor-stroma interface using bi-cellular bioinks of collagen-poly(N-isopropyl acrylamide-co-methyl methacrylate) mixture.

Recent advances in biomaterials and 3D printing/culture methods enable various tissue-engineered tumor models. However, it is still challenging to achieve native tumor-like characteristics due to lower cell density than native tissues and prolonged culture duration for maturation. Here, we report a new method to create tumoroids with a mechanically active tumor-stroma interface at extremely high cell density. This method, named "inkjet-printed morphogenesis" (iPM) of the tumor-stroma interface, is based on a hypothesis that cellular contractile force can significantly remodel the cell-laden polymer matrix to form densely-packed tissue-like constructs. Thus, differential cell-derived compaction of tumor cells and cancer-associated fibroblasts (CAFs) can be used to build a mechanically active tumor-stroma interface. In this methods, two kinds of bioinks are prepared, in which tumor cells and CAFs are suspended respectively in the mixture of collagen and poly (N-isopropyl acrylamide-co-methyl methacrylate) solution. These two cellular inks are inkjet-printed in multi-line or multi-layer patterns. As a result of cell-derived compaction, the resulting structure forms tumoroids with mechanically active tumor-stroma interface at extremely high cell density. We further test our working hypothesis that the morphogenesis can be controlled by manipulating the force balance between cellular contractile force and matrix stiffness. Furthermore, this new concept of "morphogenetic printing" is demonstrated to create more complex structures beyond current 3D bioprinting techniques.

Designing Interfacial Reactions for Nanometer-Scale Surface Patterning of PDMS with Controlled Elastic Modulus.

Control over the surface chemistry of elastomers such as polydimethylsiloxane (PDMS) is important for many applications. However, achieving nanostructured chemical control on amorphous material interfaces below the length scale of substrate heterogeneity is not straightforward, and can be particularly difficult to decouple from changes in network structure that are required for certain applications (e.g., variation of elastic modulus for cell culture). We have recently reported a new method for precisely structured surface functionalization of PDMS and other soft materials, which displays high densities of ligands directly on the material surface, maximizing steric accessibility. Here, we systematically examine structural factors in the PDMS components (e.g., base and cross-linker structures) that impact efficiency of the interfacial reaction that leads to surface functionalization. Applying this understanding, we demonstrate routes for generating equivalent nanometer-scale functional patterns on PDMS with elastic moduli from 0.013 to 1.4 MPa, establishing a foundation for use in applications such as cell culture.

Self-Assembled Asperities for Pressure-Tunable Adhesion.

Control of adhesion is important in a host of applications including soft robotics, pick-and-place manufacturing, wearable devices, and transfer printing. While there are adhesive systems with discrete switchability between states of high and low adhesion, achieving continuously variable adhesion strength remains a challenge. In this work, a pressure-tunable adhesive (PTA) that is based on the self-assembly of stiff microscale asperities on an elastomeric substrate is presented. It is demonstrated that the adhesion strength of the PTA increases with the applied compressive preload due to the unique contact formation mechanism caused by the asperities. Additionally, a contact mechanics model is developed to explain the resulting trends. For a specific PTA design, the critical pull-off force can be increased from 0.4 to 30 mN by increasing the applied preload from 1 to 30 mN. Finally, the applicability of precision control of adhesion strength is demonstrated by utilizing the PTA for pick-and-place material handling. The approach in pressure-tunable adhesive design based on self-assembly of asperities presents a scalable and versatile approach that is applicable to a variety of material systems having different mechanical or surface properties.

Systemic Corticosteroids in the Management of Pediatric Cystic Fibrosis Pulmonary Exacerbations.

Rationale: Pulmonary exacerbation (PEx) events contribute to lung function decline in people with cystic fibrosis (CF). CF Foundation PEx guidelines note that a short course of systemic corticosteroids may offer benefit without contributing to long-term adverse effects. However, insufficient evidence exists to recommend systemic corticosteroids for PEx treatment. Objectives: To determine if systemic corticosteroids for the treatment of in-hospital pediatric PEx are associated with improved clinical outcomes compared with treatment without systemic corticosteroids. Methods: We conducted a retrospective cohort study using the CF Foundation Patient Registry-Pediatric Health Information System linked database. People with CF were included if hospitalized for a PEx between 2006 and 2018 and were 6-21 years of age. Time to next PEx was assessed by Cox proportional hazards regression. Lung function outcomes were assessed by linear mixed-effect modeling and generalized estimating equations. To address confounding by indication, inverse probability treatment weighting was used. Results: A total of 3,471 people with CF contributed 9,787 PEx for analysis. Systemic corticosteroids were used in 15% of all PEx. In our primary analysis, systemic corticosteroids were not associated with better pre- to post-PEx percent predicted forced expiratory volume in 1 second responses (mean difference, -0.36; 95% confidence interval [CI], -1.14, 0.42; P = 0.4) or a higher odds of returning to lung function baseline (odds ratio, 0.97; 95% CI, 0.84-1.12; P = 0.7) but were associated with a reduced chance of future PEx requiring intravenous antibiotics (hazard ratio, 0.91; 95% CI, 0.85-0.96; P = 0.002). When restricting the analysis to one PEx per person, lung function outcomes remained no different among PEx treated with or without systemic corticosteroids, but, in contrast to our primary analysis, the use of systemic corticosteroids was no longer associated with a reduced chance of having a future PEx requiring intravenous antibiotics (hazard ratio, 0.96; 95% CI, 0.86, 1.07; P = 0.42). Conclusions: Systemic corticosteroid treatment for in-hospital pediatric PEx was not associated with improved lung function outcomes. Prospective trials are needed to better evaluate the risks and benefits of systemic corticosteroid use for PEx treatment in children with CF.

Activation of Mechanophores in a Thermoset Matrix by Instrumented Scratch.

Scratches in polymer coatings and barrier layers negatively impact optical properties (haze, light transmission, etc.), initiate routes of degradation or corrosion (moisture permeability), and nucleate delamination of the coating. Detecting scratches in coatings on advanced materials systems is an important component of structural health monitoring but can be difficult if the defects are too small to be detected by the naked eye. The primary focus of the present work is to investigate scratch damage using fluorescence lifetime imaging microscopy (FLIM) and mechanical activation of a mechanophore (MP)-containing transparent epoxy coating. The approach utilizes a Berkovich tip to scratch MP-epoxy coatings under a linearly increasing normal load. The goal is to utilize the fluorescent behavior of activated MPs to enable the detection of microscale scratches and molecular scale changes in polymeric systems. Taking advantage of the amine functionality present in a polyetheramine/bisphenol A epoxy network, a modified rhodamine dye is covalently bonded into a transparent, thermoset polymer network. Following instrumented scratch application, subsequent fluorescence imaging of the scratched MP-epoxy reveals the extent of fluorescence activation induced by the mechanical deformation. In this work, the rhodamine-based mechanophore is used to identify both ductile and fracture-dominated processes during the scratch application. The fluorescence intensity increases linearly with the applied normal load and is sensitive to fracture dominated processes. Fluorescence lifetime and hyperspectral imaging of damage zones provide additional insight into the local (nanoscopic) environment and molecular structure of the MP around the fracture process zone, respectively. The mechanophore/scratch deformation approach allows a fluorescence microscope to probe local yielding and fracture events in a powerful way that enhances the optical characterization of damage zones formed by standard scratch test methods and leads to novel defect detection strategies.

Design and validation of a modular micro-robotic system for the mechanical characterization of soft tissues.

The mechanical properties of tissues are critical design parameters for biomaterials and regenerative therapies seeking to restore functionality after disease or injury. Characterizing the mechanical properties of native tissues and extracellular matrix throughout embryonic development helps us understand the microenvironments that promote growth and remodeling, activities critical for biomaterials to support. The mechanical characterization of small, soft materials like the embryonic tissues of the mouse, an established mammalian model for development, is challenging due to difficulties in handling minute geometries and resolving forces of low magnitude. While uniaxial tensile testing is the physiologically relevant modality to characterize tissues that are loaded in tension in vivo, there are no commercially available instruments that can simultaneously measure sufficiently low tensile force magnitudes, directly measure sample deformation, keep samples hydrated throughout testing, and effectively grip minute geometries to test small tissues. To address this gap, we developed a micromanipulator and spring system that can mechanically characterize small, soft materials under tension. We demonstrate the capability of this system to measure the force contribution of soft materials, silicone, fibronectin sheets, and fibrin gels with a 5 nN - 50 µN force resolution and perform a variety of mechanical tests. Additionally, we investigated murine embryonic tendon mechanics, demonstrating the instrument can measure differences in mechanics of small, soft tissues as a function of developmental stage. This system can be further utilized to mechanically characterize soft biomaterials and small tissues and provide physiologically relevant parameters for designing scaffolds that seek to emulate native tissue mechanics. STATEMENT OF SIGNIFICANCE: The mechanical properties of cellular microenvironments are critical parameters that contribute to the modulation of tissue growth and remodeling. The field of tissue engineering endeavors to recapitulate these microenvironments in order to construct tissues de novo. Therefore, it is crucial to uncover the mechanical properties of the cellular microenvironment during tissue formation. Here, we present a system capable of acquiring microscale forces and optically measuring sample deformation to calculate the stress-strain response of soft, embryonic tissues under tension, and easily adaptable to accommodate biomaterials of various sizes and stiffnesses. Altogether, this modular system enables researchers to probe the unknown mechanical properties of soft tissues throughout development to inform the engineering of physiologically relevant microenvironments.

Pressure tunable adhesion of rough elastomers.

The ability to control adhesion is critical in various technologies including wearable electronics, pressure sensitive adhesives, and robotic systems. Biomimetic fibrillar structures, random surface roughness, and chemical surface treatments have been employed to modify the adhesion energy of materials used in these applications. However, polymer thin film dewetting has not been investigated as a surface modification tool to control adhesion. In this work, polystyrene thin films are thermally annealed on a polydimethylsiloxane substrate, causing them to dewet and form stiff, microscopic asperities on the soft substrate. The size of the asperities increases with increasing pre-annealing film thickness. Adhesion is quantified by flat-punch normal indentation testing. The largest asperities exhibited a decrease in adhesion to below the sensitivity of the instrument. More interestingly, the surfaces covered with the smallest asperities displayed a pressure-dependent adhesive response. By increasing the normal compressive stress applied prior to separation, the total debonding energy increased monotonically on the smallest asperity-covered surfaces.

An engineer's introduction to mechanophores.

Mechanophores (MPs) are a class of stimuli-responsive materials that are of increasing interest to engineers due to their potential applications as stress sensors. These mechanically responsive molecules change color or become fluorescent upon application of a mechanical stimulus as they undergo a chemical reaction when a load is applied. By incorporating MPs such as spirolactam, spiropyran, or dianthracene into a material system, the real-time stress distribution of the matrix can be directly observed through a visual response, ideal for damage and failure sensing applications. A wide array of applications that require continuous structural health monitoring could benefit from MPs including flexible electronics, protective coatings, and polymer matrix composites. However, there are significant technical challenges preventing MP implementation in industry. Effective strategies to quantitatively calibrate the photo response of the MP with applied stress magnitudes must be developed. Additionally, environmental conditions, including temperature, humidity, and ultraviolet light exposure can potentially impact the performance of MPs. By addressing these limitations, engineers can work to move MPs from the synthetic chemistry bench to the field. This review aims to highlight recent progress in MP research, discuss barriers to implementation, and provide an outlook on the future of MPs, specifically focused on polymeric material systems. Although the focus is on engineering MPs for bulk materials, a brief overview of mechanochemistry will be discussed followed by methods for activation and quantification of MP photo response (concentrating specifically on fluorescently active species). Finally, current challenges and future directions in MP research will be addressed.

Polymer thin film adhesion utilizing the transition from surface wrinkling to delamination.

Understanding the adhesion of rigid thin films to compliant substrates is critical for the development and implementation of flexible electronic devices and wearable sensor technologies. Quantifying the strength of a film-substrate interface can be challenging due to the brittleness of glassy films which can greatly complicate sample preparation, handling, and testing. Here, a method for measuring the adhesion of glassy thin films to soft elastomeric substrates is explored that exploits an understanding of surface buckling instabilities, specifically the transition from wrinkling to delamination. The adhesion (given by the critical strain energy release rate (Gc)) for two model materials' interfaces is quantified by determining the critical delamination strain for thin glassy polymer films (polystyrene (PS) and poly(methyl methacrylate) (PMMA)) from an elastomeric substrate (poly(dimethyl siloxane) (PDMS)). By accounting for edge defects that greatly reduce the critical strain for delamination, reasonable adjusted Gc values of 21.0 ± 5.1 mJ m-2 and 32.2 ± 4.9 mJ m-2 are found for PS-PDMS and PMMA-PDMS interfaces, respectively. The utilization of this method to characterize film modulus and adhesion could be used as a facile measurement technique for more applied polymer thin film systems.

Extrinsic and Dynamic Edge States of Two-Dimensional Lead Halide Perovskites.

Edges of two-dimensional (2D) halide perovskites are found to exhibit unusual properties such as enhanced photoluminescence lifetime and reduced photoluminescence emission energy. Here, we report the formation mechanism and the dynamic nature of edge states on exfoliated 2D halide perovskite thin crystals. In contrast to other 2D materials, the edge states in 2D perovskites are extrinsic and can be triggered by moisture with a concentration as low as ∼0.5 ppm. High-resolution atomic force microscopy and transmission electron microscopy characterizations reveal the width of the low-energy states is ∼40 nm wide. A temperature-dependent photoluminescence study suggests the edge states are a combination of several lower-energy states. Importantly, we demonstrate that the charge carriers on the dynamically formed edge states are not only long-lived but also highly mobile and can be conducted along the edges effectively with high mobilities of 5.4-7.0 cm2 V-1 s-1. This work provides significant insights on the origin of the edge states in 2D perovskites and provides routes to manipulate their optical and electrical properties through controlling their edges.

Mechanical strength of welding zones produced by material extrusion additive manufacturing.

As more manufacturing processes and research institutions adopt customized manufacturing as a key element in their design strategies and finished products, the resulting mechanical properties of parts produced through additive manufacturing (AM) must be characterized and understood. In material extrusion (MatEx), the most recently extruded polymer filament must bond to the previously extruded filament via polymer diffusion to form a "weld". The strength of the weld limits the performance of the manufactured part and is controlled through processing conditions. Under-standing the role of processing conditions, specifically extruder velocity and extruder temperature, on the overall strength of the weld will allow optimization of MatEx-AM parts. Here, the fracture toughness of a single weld is determined through a facile "trouser tear" Mode III fracture experiment. The actual weld thickness is observed directly by optical microscopy characterization of cross sections of MatEx-AM samples. Representative data of weld strength as a function of printing parameters on a commercial 3D printer demonstrates the robustness of the method.

Wrinkling-to-delamination transition in thin polymer films on compliant substrates.

Compressing a thin, stiff film attached to a thick, compliant substrate can lead to a number of different modes of mechanical deformation depending upon the material properties of the system. In this article we explore direct transitions from surface wrinkling to buckle delamination, and provide a theoretical framework for understanding the conditions under which such transitions take place, as well as the resulting dimensions of the wrinkling-induced delamination. A key conclusion of this work is that the width of the delamination blister formed from a wrinkled film is relatively strain-independent, suggesting that delaminations can be used in such systems to measure the adhesion energy at the film-substrate interface. In addition, we demonstrate how the length and width of delaminations can be tailored through straightforward control of the substrate and film properties in the system, illustrating how wrinkling delaminations can be used for both thin film metrology and patterning applications.

Weld formation during material extrusion additive manufacturing.

Material extrusion (ME) is a layer-by-layer additive manufacturing process that is now used in personal and commercial production where prototyping and customization are required. However, parts produced from ME frequently exhibit poor mechanical performance relative to those from traditional means; moreover, fundamental knowledge of the factors leading to development of inter-layer strength in this highly non-isothermal process is limited. In this work, we seek to understand the development of inter-layer weld strength from the perspective of polymer interdiffusion under conditions of rapidly changing mobility. Our framework centers around three interrelated components: in situ thermal measurements (via infrared imaging), temperature dependent molecular processes (via rheology), and mechanical testing (via mode III fracture). We develop the concept of an equivalent isothermal weld time and test its relationship to fracture energy. For the printing conditions studied the equivalent isothermal weld time for Tref = 230 °C ranged from 0.1 ms to 100 ms. The results of these analysis provide a basis for optimizing inter-layer strength, the limitations of the ME process, and guide development of new materials.

Observation of interfacial damage in a silk-epoxy composite, using a simple mechanoresponsive fluorescent probe.

Polymer composite materials are found throughout the world both natural and artificial in origin. In the vast majority of applications in these arenas, composites serve as structural support or reinforcement. Demand for lightweight tough composites is growing in multiple application spaces such as aerospace, biomaterials, and infrastructure with physical properties as diverse as the applications. The unifying component in all composites is the presence of the interphase. Many measurement techniques and measurement tools have been developed for the study of this crucial region in composite materials. Many of these methods are great for the measurement and study of bulk properties or model systems. However, development of tools that permit the direct observation of interactions at the interphase during applied stress are needed. Here we employ fluorescence lifetime imaging and hyperspectral imaging to observe activation of a fluorogenic dye at the composite interface as a result of applied stress. The advantages of this system include commercial availability of the dye precursor, and simple one-pot functionalization. The attachment of the dye at the interface is easily monitored through emission wavelength shifts or fluorescence lifetime. Interfacial mechano-responsive dyes have potential for both fundamental studies as well as industrial use as a structural health monitoring tool.

Simultaneously Tailoring Surface Energies and Thermal Stabilities of Cellulose Nanocrystals Using Ion Exchange: Effects on Polymer Composite Properties for Transportation, Infrastructure, and Renewable Energy Applications.

Cellulose nanocrystals (CNCs) have great potential as sustainable reinforcing materials for polymers, but there are a number of obstacles to commercialization that must first be overcome. High levels of water absorption, low thermal stabilities, poor miscibility with nonpolar polymers, and irreversible aggregation of the dried CNCs are among the greatest challenges to producing cellulose nanocrystal-polymer nanocomposites. A simple, scalable technique to modify sulfated cellulose nanocrystals (Na-CNCs) has been developed to address all of these issues. By using an ion exchange process to replace Na+ with imidazolium or phosphonium cations, the surface energy is altered, the thermal stability is increased, and the miscibility of dried CNCs with a nonpolar polymer (epoxy and polystyrene) is enhanced. Characterization of the resulting ion exchanged CNCs (IE-CNCs) using potentiometry, inverse gas chromatography, dynamic vapor sorption, and laser scanning confocal microscopy reveals that the IE-CNCs have lower surface energies, adsorb less water, and have thermal stabilities of up to 100 °C higher than those of prepared protonated cellulose nanocrystals (H-CNCs) and 40 °C higher than that of neutralized Na-CNC. Methyl(triphenyl)phosphonium exchanged cellulose nanocrystals (MePh3P-CNC) adsorbed 30% less water than Na-CNC, retained less water during desorption, and were used to prepare well-dispersed epoxy composites without the aid of a solvent and well-dispersed polystyrene nanocomposites using a melt blending technique at 195 °C. Predictions of dispersion quality and glass transition temperatures from molecular modeling experiments match experimental observations. These fiber-reinforced polymers can be used as lightweight composites in transportation, infrastructure, and renewable energy applications.

Debonding mechanisms of soft materials at short contact times.

A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked poly(dimethylsiloxane) (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined Johnson-Kendall-Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMS-glass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 to 10,000 ms. This influence of dwell time is particularly pronounced for polymer-polymer (SBR-SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported.

Enhanced adhesion of elastic materials to small-scale wrinkles.

The adhesive properties of a material can be greatly affected simply by wrinkling its surface. We show the importance of selecting the wrinkle feature sizes (amplitude, b; and wavelength, λ) that complement the material-defined length scale related to the adhesion energy and modulus. A rigid circular cylindrical punch patterned with aligned wrinkles ranging in amplitude from 0.5 to 5.0 µm with a fixed aspect ratio of 0.1 is used to characterize the adhesion of elastic films of smooth poly(dimethyl siloxane) (PDMS). The cross-linker concentration used to form the PDMS layers is varied to determine the impact of material properties on wrinkled surface adhesion. The elastic films have an average thickness of 240 µm and the average probe radius is 1 mm, leading to a confined contact scenario. The separation stress and work of debonding are presented for each cross-linker concentration with testing rates ranging over 3 orders of magnitude. For stiffer films (10 wt % cross-linker, E' ≈ 3.00 MPa), small wrinkles (b ≈ 0.5 µm) increase the separation stress by nearly 200% relative to a smooth interface whereas large wrinkles (b ≈ 5.0 µm) are shown to reduce adhesion significantly. A substantial increase in the debonding energy is also observed for these small-amplitude wrinkles contacting stiff materials. No discernible impact of wrinkled surface topography on the adhesion of softer (2 and 4 wt % cross-linker, 0.05 MPa < E' < 0.30 MPa) films is measured.