Results of an interlaboratory study on the working curve in vat photopolymerization

The working curve informs resin properties and print parameters for stereolithography, digital light processing, and other photopolymer additive manufacturing (PAM) technologies. First demonstrated in 1992, the working curve measurement of cure depth vs radiant exposure of light is now a foundational measurement in the field of PAM. Despite its widespread use in industry and academia, there is no formal method or procedure for performing the working curve measurement, raising questions about the utility of reported working curve parameters. Here, an interlaboratory study (ILS) is described in which 24 individual laboratories performed a working curve measurement on an aliquot from a single batch of PAM resin. The ILS reveals that there is enormous scatter in the working curve data and the key fit parameters derived from it. The measured depth of light penetration Dp varied by as much as 7x between participants, while the critical radiant exposure for gelation Ec varied by as much as 70x. This significant scatter is attributed to a lack of common procedure, variation in light engines, epistemic uncertainties from the Jacobs equation, and the use of measurement tools with insufficient precision. The ILS findings highlight an urgent need for procedural standardization and better hardware characterization in this rapidly growing field.

A Data-Driven Approach to Complex Voxel Predictions in Grayscale Digital Light Processing Additive Manufacturing Using U-Nets and Generative Adversarial Networks

Data-driven U-net machine learning (ML) models, including the pix2pix conditional generative adversarial network (cGAN), are shown to predict 3D printed voxel geometry in digital light processing (DLP) additive manufacturing. A confocal microscopy-based workflow allows for the high-throughput acquisition of data on thousands of voxel interactions arising from randomly gray-scaled digital photomasks. Validation between prints and predictions shows accurate predictions with sub-pixel scale resolution. The trained cGAN performs virtual DLP experiments such as feature size-dependent cure depth, anti-aliasing, and sub-pixel geometry control. The pix2pix model is also applicable to larger masks than it is trained on. To this end, the model can qualitatively inform layer-scale and voxel-scale print failures in real 3D-printed parts. Overall, machine learning models and the data-driven methodology, exemplified by U-nets and cGANs, show considerable promise for predicting and correcting photomasks to achieve increased precision in DLP additive manufacturing.

Influence of fluorescent dopants on the vat photopolymerization of acrylate-based plastic scintillators for application in neutron/gamma pulse shape discrimination

Plastic scintillators, a class of solid-state materials used for radiation detection, were additively manufactured with vat photopolymerization. The photopolymer resins consisted of a primary dopant and a secondary dopant dissolved in a bisphenol A ethoxylate diacrylate-based matrix. The absorptive dopants significantly influence important print parameters, for example, secondary dopants decrease the light penetration depth by a factor > 12 ×. The primary dopant 2,5-diphenyloxazole had minimal impact on the printing process even when loaded at 25 % by mass of the resin. Working curve measurements, which relate energy dose to cure depth, were performed as a function of feature size to further assess the influence of dopants. Photopatterns smaller than 150 μm width had apparent increases in critical energy dose compared to larger photopatterns, while all resins maintained printed features in line gratings with 50 μm of separation. Printed scintillator monoliths were compared to scintillators cast by traditional molding, demonstrating that the layer-by-layer printing process does not decrease scintillation response. A maximum light output of 31 % of a benchmark plastic scintillator (EJ-200) and successful pulse shape discrimination were achieved with 20 % by mass 2,5-diphenyloxazole as the primary dopant and 0.1 % by mass 9,9-dimethyl-2,7-distyrylfluorene as the secondary dopant in printed scintillator samples.

Synergistic Fire Resistance of Nanobrick Wall Coated 3D Printed Photopolymer Lattices

Photopolymer additive manufacturing has become the subject of widespread interest in recent years due to its capacity to enable fabrication of difficult geometries that are impossible to build with traditional manufacturing methods. The flammability of photopolymer resin materials and the lattice structures enabled by 3D printing is a barrier to widespread adoption that has not yet been adequately addressed. Here, a water-based nanobrick wall coating is deposited on 3D printed parts with simple (i.e., dense solid) or complex (i.e., lattice) geometries. When subject to flammability testing, the printed parts exhibit no melt dripping and a propensity toward failure at the print layer interfaces. Moving from a simple solid geometry to a latticed geometry leads to reduced time to failure during flammability testing. For nonlatticed parts, the coating provides negligible improvement in fire resistance, but coating of the latticed structures significantly increases time to failure by up to ≈340% compared to the uncoated lattice. The synergistic effect of coating and latticing is attributed to the lattice structures' increased surface area to volume ratio, allowing for an increased coating:photopolymer ratio and the ability of the lattice to better accommodate thermal expansion strains. Overall, nanobrick wall coated lattices can serve as metamaterials to increase applications of polymer additive manufacturing in extreme environments.

Characterizing light engine uniformity and its influence on liquid crystal display based vat photopolymerization printing

Vat photopolymerization (VP) is a rapidly growing category of additive manufacturing. As VP methods mature the expectation is that the quality of printed parts will be highly reproducible. At present, detailed characterization of the light engines used in liquid crystal display (LCD)-based VP systems is lacking and so it is unclear if they are built to sufficiently tight tolerances to meet the current and/or future needs of additive manufacturing. Herein, we map the irradiance, spectral characteristics, and optical divergence of a nominally 405 nm LCD-based VP light engine. We find that there is notable variation in all of these properties as a function of position on the light engine that cause changes in extent of polymerization and surface texture. We further demonstrate through a derived photon absorption figure of merit and through printed test parts that the spatial heterogeneity observed in the light engine is significant enough to affect part fidelity. These findings help to explain several possible causes of variable part quality and also highlight the need for improved optical performance on LCD-based VP printers.

A 3D printed mimetic composite for the treatment of growth plate injuries in a rabbit model

Growth plate injuries affecting the pediatric population may cause unwanted bony repair tissue that leads to abnormal bone elongation. Clinical treatment involves bony bar resection and implantation of an interpositional material, but success is limited and the bony bar often reforms. No treatment attempts to regenerate the growth plate cartilage. Herein we develop a 3D printed growth plate mimetic composite as a potential regenerative medicine approach with the goal of preventing limb length discrepancies and inducing cartilage regeneration. A poly(ethylene glycol)-based resin was used with digital light processing to 3D print a mechanical support structure infilled with a soft cartilage-mimetic hydrogel containing chondrogenic cues. Our biomimetic composite has similar mechanical properties to native rabbit growth plate and induced chondrogenic differentiation of rabbit mesenchymal stromal cells in vitro. We evaluated its efficacy as a regenerative interpositional material applied after bony bar resection in a rabbit model of growth plate injury. Radiographic imaging was used to monitor limb length and tibial plateau angle, microcomputed tomography assessed bone morphology, and histology characterized the repair tissue that formed. Our 3D printed growth plate mimetic composite resulted in improved tibial lengthening compared to an untreated control, cartilage-mimetic hydrogel only condition, and a fat graft. However, in vivo the 3D printed growth plate mimetic composite did not show cartilage regeneration within the construct histologically. Nevertheless, this study demonstrates the feasibility of a 3D printed biomimetic composite to improve limb lengthening, a key functional outcome, supporting its further investigation as a treatment for growth plate injuries.

Electrostatically-blind quantitative piezoresponse force microscopy free of distributed-force artifacts

The presence of electrostatic forces and associated artifacts complicates the interpretation of piezoresponse force microscopy (PFM) and electrochemical strain microscopy (ESM). Eliminating these artifacts provides an opportunity for precisely mapping domain wall structures and dynamics, accurately quantifying local piezoelectric coupling coefficients, and reliably investigating hysteretic processes at the single nanometer scale to determine properties and mechanisms which underly important applications including computing, batteries and biology. Here we exploit the existence of an electrostatic blind spot (ESBS) along the length of the cantilever, due to the distributed nature of the electrostatic force, which can be universally used to separate unwanted long range electrostatic contributions from short range electromechanical responses of interest. The results of ESBS-PFM are compared to state-of-the-art interferometric displacement sensing PFM, showing excellent agreement above their respective noise floors. Ultimately, ESBS-PFM allows for absolute quantification of piezoelectric coupling coefficients independent of probe, lab or experimental conditions. As such, we expect the widespread adoption of EBSB-PFM to be a paradigm shift in the quantification of nanoscale electromechanics.

Photopatterning of two stage reactive polymer networks with CO2-philic thiol–acrylate chemistry: enhanced mechanical toughness and CO2/N2 selectivity

Two stage reactive polymer (TSRP) networks can be programmed with spatially varying heterogeneity, presenting a new way of designing material structure and controlling or enhancing properties.

Microscale Photopatterning of Through-thickness Modulus in a Monolithic and Functionally Graded 3D Printed Part

3D printing is transforming traditional processing methods for applications ranging from tissue engineering to optics. To fulfill its maximum potential, 3D printing requires a robust technique for producing structures with precise three-dimensional (x, y and z) control of mechanical properties. Previous efforts to realize such spatial control of modulus within 3D printed parts have largely focused on low-resolution (mm to cm scale) multi-material processes and grayscale approaches that spatially vary the modulus in the x-y plane and energy dose-based (E = I 0 t exp) models that do not account for the resin's sub-linear response to irradiation intensity. Here, we demonstrate a novel approach for through-thickness (z) voxelated control of mechanical properties within a single-material, monolithic part. Control over the local modulus is enabled by a predictive model that incorporates the observed non-reciprocal dose response of the material. The model is validated by an application of atomic force microscopy to map the through-thickness modulus on multi-layered 3D parts. Overall, both smooth gradations (30 MPa change over ≈75 μm) and sharp step-changes (30 MPa change over ≈5 μm) in modulus are realized in poly(ethylene glycol) diacrylate based 3D constructs, paving the way for advancements in tissue engineering, stimuli-responsive 4D printing and graded metamaterials.

Digital light processing in a hybrid atomic force microscope: In Situ, nanoscale characterization of the printing process

Stereolithography (SLA) and digital light processing (DLP) are powerful additive manufacturing techniques that address a wide range of applications including regenerative medicine, prototyping, and manufacturing. Unfortunately, these printing processes introduce micrometer-scale anisotropic inhomogeneities due to the resin absorptivity, diffusivity, reaction kinetics, and swelling during the requisite photoexposure. Previously, it has not been possible to characterize high-resolution mechanical heterogeneity as it develops during the printing process. By combining DLP 3D printing with atomic force microscopy in a hybrid instrument, heterogeneity of a single, in situ printed voxel is characterized. Here, we describe the instrument and demonstrate three modalities for characterizing voxels during and after printing. Sensing Modality I maps the mechanical properties of just-printed, resin-immersed voxels, providing the framework to study the relationships between voxel sizes, print exposure parameters, and voxel-voxel interactions. Modality II captures the nanometric, in situ working curve and is the first demonstration of in situ cure depth measurement. Modality III dynamically senses local rheological changes in the resin by monitoring the viscoelastic damping coefficient of the resin during patterning. Overall, this instrument equips researchers with a tool to develop rich insight into resin development, process optimization, and fundamental printing limits.

Permanent and reversibly programmable shapes in liquid crystal elastomer microparticles capable of shape switching

Reversibly programmable liquid crystal elastomer microparticles (LCEMPs), formed as a covalent adaptable network (CAN), with an average diameter of 7 μm ± 2 μm, were synthesized via a thiol-Michael dispersion polymerization. The particles were programmed to a prolate shape via a photoinitiated addition-fragmentation chain-transfer (AFT) exchange reaction by activating the AFT after undergoing compression. Due to the thermotropic nature of the AFT-LCEMPs, shape switching was driven by heating the particles above their nematic-isotropic phase transition temperature (T_NI). The programmed particles subsequently displayed cyclable two-way shape switching from prolate to spherical when at low or high temperatures, respectively. Furthermore, the shape programming is reversible, and a second programming step was done to erase the prolate shape by initiating AFT at high temperature while the particles were in their spherical shape. Upon cooling, the particles remained spherical until additional programming steps were taken. Particles were also programmed to maintain a permanent oblate shape. Additionally, the particle surface was programmed with a diffraction grating, demonstrating programmable complex surface topography via AFT activation.

Snakeskin-Inspired Elastomers with Extremely Low Coefficient of Friction under Dry Conditions

Soft elastomers are critical to a broad range of existing and emerging technologies. One major limitation of soft elastomers is the large friction of coefficient (COF) due to inherently large adhesion and internal loss. In applications where lubrication is not applicable, such as soft robotics, wearable electronics, and biomedical devices, elastomers with inherently low dry COF are required. Inspired by the low COF of snakeskins atop soft bodies, this study reports the development of elastomers with low dry COF by growing a hybrid skin layer with a strong interface with a large stiffness gradient. Using a solid-liquid interfacial polymerization (SLIP) process, hybrid skin layers are imparted onto elastomers, which reduces the COF of the elastomers from 1.6 to 0.1, without sacrificing the bulk compliance and ductility of elastomer. Compared with existing surface modification methods, the SLIP process offers spatial control and ability to modify flat, prepatterned, curved, and inner surfaces, which is essential to engineer multifunctional skin layers for emerging applications.

Defining the Cardiac Fibroblast Secretome in a Fibrotic Microenvironment

Background

Cardiac fibroblasts (CFs) have the ability to sense stiffness changes and respond to biochemical cues to modulate their states as either quiescent or activated myofibroblasts. Given the potential for secretion of bioactive molecules to modulate the cardiac microenvironment, we sought to determine how the CF secretome changes with matrix stiffness and biochemical cues and how this affects cardiac myocytes via paracrine signaling.

Methods and Results

Myofibroblast activation was modulated in vitro by combining stiffness cues with TGFβ1 (transforming growth factor β 1) treatment using engineered poly (ethylene glycol) hydrogels, and in vivo with isoproterenol treatment. Stiffness, TGFβ1, and isoproterenol treatment increased AKT (protein kinase B) phosphorylation, indicating that this pathway may be central to myofibroblast activation regardless of the treatment. Although activation of AKT was shared, different activating cues had distinct effects on downstream cytokine secretion, indicating that not all activated myofibroblasts share the same secretome. To test the effect of cytokines present in the CF secretome on paracrine signaling, neonatal rat ventricular cardiomyocytes were treated with CF conditioned media. Conditioned media from myofibroblasts cultured on stiff substrates and activated by TGFβ1 caused hypertrophy, and one of the cytokines in that media was insulin growth factor 1, which is a known mediator of cardiac myocyte hypertrophy.

Conclusions

Culturing CFs on stiff substrates, treating with TGFβ1, and in vivo treatment with isoproterenol all caused myofibroblast activation. Each cue had distinct effects on the secretome or genes encoding the secretome, but only the secretome of activated myofibroblasts on stiff substrates treated with TGFβ1 caused myocyte hypertrophy, most likely through insulin growth factor 1.

Photo-tunable hydrogel mechanical heterogeneity informed by predictive transport kinetics model

Understanding the three-dimensional (3D) mechanical and chemical properties of distinctly different, adjacent biological tissues is crucial to mimicking their complex properties with materials. 3D printing is a technique often employed to spatially control the distribution of the biomaterials, such as hydrogels, of interest, but it is difficult to print both mechanically robust (high modulus and toughness) and biocompatible (low modulus) hydrogels in a single structure. Moreover, due to the fast diffusion of mobile species during printing and nonequilibrium swelling conditions of low-solids-content hydrogels, it is challenging to form the high-fidelity structures required to mimic tissues. Here a predictive transport and swelling model is presented to model these effects and then is used to compensate for these effects during printing. This model is validated experimentally by photopatterning spatially distinct hydrogel elastic moduli using a single photo-tunable poly(ethylene glycol) (PEG) pre-polymer solution by sequentially patterning and in-diffusing fresh pre-polymer for further polymerization.

Controlled Growth of Polyamide Films atop Homogenous and Heterogeneous Hydrogels using Gel-Liquid Interfacial Polymerization

Controlled growth of crosslinked polyamide (PA) thin films is demonstrated at the interface of a monomer-soaked hydrogel and an organic solution of the complementary monomer. Termed gel-liquid interfacial polymerization (GLIP), the resulting PA films are measured to be chemically and mechanically analogous to the active layer in thin film composite membranes. PA thin films are prepared using the GLIP process on both a morphologically homogeneous hydrogel prepared from poly(2-hydroxyethylmethacrylate) (PHEMA) and a phase-separated, heterogeneous hydrogel prepared from poly(acrylamide) (PAAm). Two monomer systems are examined: trimesoyl chloride (TMC) reacting with m-phenylene diamine (MPD) and TMC reacting with piperazine (PIP). Unlike the self-limiting growth behavior in TFC membrane fabrication, diffusion-limited, continuous growth of the PA films is observed, where both the thickness and roughness of the PA layers increase with reaction time. A key morphological difference is found between the two monomer systems using the GLIP process: TMC/MPD produces a ridge-and-valley surface morphology whereas TMC/PIP produces nodule/granular structures. The GLIP process represents a unique opportunity to not only explore the pore characteristics (size, spacing, and continuity) on the resulting structure and morphology of interfacially polymerized thin films, but also a method to modify the surface of (or encapsulate) hydrogels.

Decoupling subsurface inhomogeneities: a 3D finite element approach for contact nanomechanical measurements

Novel material properties can be attained when embedding three-dimensional (3D) nanoparticles (NPs) in a variety of polymeric matrices. These inhomogeneities influence the bulk mechanical response due to the local high modulus mismatch between the particles and the matrix. The degree of the mechanical mismatch that is seen near a composite surface depends on the geometry/shape and spatial location and orientation of the particle with respect to the external contact loading. Isolating each particle's contribution to the surrounding elastic field can be numerically discerned but is experimentally complex, as there are limited direct characterization approaches available at the nanoscale. Atomic force microscopy (AFM) instrumentation is one such method that can quantify subsurface particle stiffness effects on nanocomposites with a resolution of a few nanometers. This work studies the spatial and geometrical effects of subsurface silver NPs on the local composite stiffness of a polystyrene matrix using 3D finite element (FE) models to interpret contact resonance (CR) AFM measurements. The present FE-AFM findings suggest both particle shape and particle orientation have a significant role in the degree of uniformity of the stiffness distribution in the embedding matrix. The applied CR-AFM technique shows that the NP geometry can be clearly distinguished when such inhomogeneities are relatively close, 17 nm, to a free surface whereas material-interface measurements at deeper subsurfaces are obscured by experimental noise. This work demonstrates that (i) numerical solutions can assist in qualitatively elucidating nanoinstrumentation stiffness profiles in terms of particle shape and orientation and (ii) CR-AFM measurements can quantify the influence of particle geometry and orientation on the surface nanomechanics of nanocomposite materials.

Monitoring Fast, Voxel-Scale Cure Kinetics via Sample-Coupled-Resonance Photorheology

Photopolymerizable materials are the focus of extensive research across a variety of fields ranging from additive manufacturing to regenerative medicine. However, poorly understood material mechanical and rheological properties during polymerization at the relevant exposure powers and single-voxel length-scales limit advancements in part performance and throughput. Here, a novel atomic force microscopy (AFM) technique, sample-coupled-resonance photorheology (SCRPR), to locally characterize the mechano-rheological properties of photopolymerized materials on the relevant reaction kinetic timescales, is demonstrated. By coupling an AFM tip to a photopolymer and exposing the coupled region to a laser, two fundamental photopolymerization phenomena: (1) timescales of photopolymerization at high laser power and (2) reciprocity between photodose and material properties are studied. The ability to capture rapid kinetic changes occurring during polymerization with SCRPR is demonstrated. It is found that reciprocity is only valid for a finite range of exposure powers in the verification material and polymerization is highly localized in a low-diffusion system. After polymerization, in situ imaging of a single polymerized voxel is performed using material-appropriate topographic and nanomechanical modalities of the AFM while still in the as-printed environment.

Scanning speed phenomenon in contact-resonance atomic force microscopy

This work presents data confirming the existence of a scan speed related phenomenon in contact-mode atomic force microscopy (AFM). Specifically, contact-resonance spectroscopy is used to interrogate this phenomenon. Above a critical scan speed, a monotonic decrease in the recorded contact-resonance frequency is observed with increasing scan speed. Proper characterization and understanding of this phenomenon is necessary to conduct accurate quantitative imaging using contact-resonance AFM, and other contact-mode AFM techniques, at higher scan speeds. A squeeze film hydrodynamic theory is proposed to explain this phenomenon, and model predictions are compared against the experimental data.

Influence of support-layer deformation on the intrinsic resistance of thin film composite membranes

It is commonly believed that the overall permeation resistance of thin film composite (TFC) membranes is dictated by the crosslinked, ultrathin polyamide barrier layer, while the porous support merely serves as the mechanical support. Although this assumption might be the case under low transmembrane pressure, it becomes questionable under high transmembrane pressure. A highly porous support normally yields under a pressure of a few MPa, which can result in a significant level of compressive strain that may significantly increase the resistance to permeation. However, quantifying the influence of porous support deformation on the overall resistance of the TFC membrane is challenging. In particular, it is difficult to determine the deformation/strain of the membrane during active separation. In this study, we use nanoimprint lithography (NIL) to achieve precise compressive deformation in commercial TFC membranes. By adjusting the NIL conditions, membranes were compressed to strain levels up to 60%. SEM and AFM measurements showed that the compression had minimal impact on the barrier-layer surface morphology and total surface area with most of the deformation occurring in the support layer. DI water permeation measurements revealed that the water flux reduction decreases with an increase of strain level. Most significantly, the intrinsic membrane resistance showed negligible changes at strain levels lower than 30%-40%, but increased exponentially at higher strain levels, reaching 250%-500% of pristine (unstrained) membrane values. Using a resistance-in-series model, the strain dependency of the TFC membrane resistance can be described.

Determination of the True Lateral Grain Size in Organic-Inorganic Halide Perovskite Thin Films

In this letter, methylammonium lead iodide (MAPbI₃) thin films were examined via piezoresponse force microscopy (PFM) and nanoindentation (NI) to determine if long-range atomic order existed across the full width and depth of the apparent grains. From the PFM, the piezoelectric response of the films was strongly correlated with low-index planes of the crystal structure and ferroelastic domains in macroscale solution-grown MAPbI₃ crystals, which implied long-range order near the top surface. From the NI, it was found that the induced cracks were straight and extended across the full width of the apparent grains, which indicated that the long-range order was not limited to the near-surface region, but extended through the film thickness. Interestingly, the two MAPbI₃ processes examined resulted in subtle differences in the extracted electro-mechanical and fracture properties, but exhibited similar power conversion efficiencies of >17% in completed devices.

Formation of a Crack-Free, Hybrid Skin Layer with Tunable Surface Topography and Improved Gas Permeation Selectivity on Elastomers Using Gel-Liquid Infiltration Polymerization

Surface modifications of elastomers and gels are crucial for emerging applications such as soft robotics and flexible electronics, in large part because they provide a platform to control wettability, adhesion, and permeability. Current surface modification methods via ultraviolet-ozone (UVO) and/or O₂ plasma, atomic layer deposition (ALD), plasmas deposition, and chemical treatment impart a dense polymer or inorganic layer on the surface that is brittle and easy to fracture at low strain levels. This paper presents a new method, based on gel-liquid infiltration polymerization, to form hybrid skin layers atop elastomers. The method is unique in that it allows for control of the skin layer topography, with tunable feature sizes and aspect ratios as high as 1.8 without fracture. Unlike previous techniques, the skin layer formed here dramatically improves the barrier properties of the elastomer, while preserving skin layer flexibility. Moreover, the method is versatile and likely applicable to most interfacial polymerization systems and network polymers on flat and patterned surfaces.

Light-Stimulated Permanent Shape Reconfiguration in Cross-Linked Polymer Microparticles

Reconfiguring the permanent shape of elastomeric microparticles has been impossible due to the incapability of plastic deformation in these materials. To address this limitation, we synthesize the first instance of microparticles comprising a covalent adaptable network (CAN). CANs are cross-linked polymer networks capable of reconfiguring their network topology, enabling stress relaxation and shape changing behaviors, and reversible addition-fragmentation chain transfer (RAFT) is the corresponding dynamic chemistry used in this work to enable CAN-based microparticles. Using nanoimprint lithography to apply controllable deformations we demonstrate that upon light stimulation microparticles are able to reconfigure their shape to permanently fix large aspect ratios and nanoscale surface topographies.

Detection of atomic force microscopy cantilever displacement with a transmitted electron beam

The response time of an atomic force microscopy (AFM) cantilever can be decreased by reducing cantilever size; however, the fastest AFM cantilevers are currently nearing the smallest size that can be detected with the conventional optical lever approach. Here, we demonstrate an electron beam detection scheme for measuring AFM cantilever oscillations. The oscillating AFM tip is positioned perpendicular to and in the path of a stationary focused nanometer sized electron beam. As the tip oscillates, the thickness of the material under the electron beam changes, causing a fluctuation in the number of scattered transmitted electrons that are detected. We demonstrate detection of sub-nanometer vibration amplitudes with an electron beam, providing a pathway for dynamic AFM with cantilevers that are orders of magnitude smaller and faster than the current state of the art.

Influences of Substrate Adhesion and Particle Size on the Shape Memory Effect of Polystyrene Particles

Formulations and applications of micro- and nanoscale polymer particles have proliferated rapidly in recent years, yet knowledge of their mechanical behavior has not grown accordingly. In this study, we examine the ways that compressive strain, substrate surface energy, and particle size influence the shape memory cycle of polystyrene particles. Using nanoimprint lithography, differently sized particles are programmed into highly deformed, temporary shapes in contact with substrates of differing surface energies. Atomic force microscopy is used to obtain in situ measurements of particle shape recovery kinetics, and scanning electron microscopy is employed to assess differences in the profiles of particles at the conclusion of the shape memory cycle. Finally, finite element models are used to investigate the growing impact of surface energies at smaller length scales. Results reveal that the influence of substrate adhesion on particle recovery is size-dependent and can become dominating at submicron length scales.

Photothermally excited force modulation microscopy for broadband nanomechanical property measurements

We demonstrate photothermally excited force modulation microscopy (PTE FMM) for mechanical property characterization across a broad frequency range with an atomic force microscope (AFM). Photothermal excitation allows for an AFM cantilever driving force that varies smoothly as a function of drive frequency, thus avoiding the problem of spurious resonant vibrations that hinder piezoelectric excitation schemes. A complication of PTE FMM is that the sub-resonance cantilever vibration shape is fundamentally different compared to piezoelectric excitation. By directly measuring the vibrational shape of the cantilever, we show that PTE FMM is an accurate nanomechanical characterization method. PTE FMM is a pathway towards the characterization of frequency sensitive specimens such as polymers and biomaterials with frequency range limited only by the resonance frequency of the cantilever and the low frequency limit of the AFM.

Quantitative Contact Resonance Force Microscopy for Viscoelastic Measurement of Soft Materials at the Solid-Liquid Interface

Viscoelastic property measurements made at the solid-liquid interface are key to characterizing materials for a variety of biological and industrial applications. Further, nanostructured materials require nanoscale measurements. Here, material loss tangents (tan δ) were extracted from confounding liquid effects in nanoscale contact resonance force microscopy (CR-FM), an atomic force microscope based technique for observing mechanical properties of surfaces. Obtaining reliable CR-FM viscoelastic measurements in liquid is complicated by two effects. First, in liquid, spurious signals arise during cantilever excitation. Second, it is challenging to separate changes to cantilever behavior due to the sample from changes due to environmental damping and added mass effects. We overcame these challenges by applying photothermal cantilever excitation in multiple resonance modes and a predictive model for the hydrodynamic effects. We demonstrated quantitative, nanoscale viscoelastic CR-FM measurements of polymers at the solid-liquid interface. The technique is demonstrated on a point-by-point basis on polymer samples and while imaging in contact mode on a fixed plant cell wall. Values of tan δ for measurements made in water agreed with the values for measurements in air for some experimental conditions on polystyrene and for all examined conditions on polypropylene.

Vibrational shape tracking of atomic force microscopy cantilevers for improved sensitivity and accuracy of nanomechanical measurements

Contact resonance atomic force microscopy (CR-AFM) methods currently utilize the eigenvalues, or resonant frequencies, of an AFM cantilever in contact with a surface to quantify local mechanical properties. However, the cantilever eigenmodes, or vibrational shapes, also depend strongly on tip-sample contact stiffness. In this paper, we evaluate the potential of eigenmode measurements for improved accuracy and sensitivity of CR-AFM. We apply a recently developed, in situ laser scanning method to experimentally measure changes in cantilever eigenmodes as a function of tip-sample stiffness. Regions of maximum sensitivity for eigenvalues and eigenmodes are compared and found to occur at different values of contact stiffness. The results allow the development of practical guidelines for CR-AFM experiments, such as optimum laser spot positioning for different experimental conditions. These experiments provide insight into the complex system dynamics that can affect CR-AFM and lay a foundation for enhanced nanomechanical measurements with CR-AFM.

Characterizing the free and surface-coupled vibrations of heated-tip atomic force microscope cantilevers

Combining heated-tip atomic force microscopy (HT-AFM) with quantitative methods for determining surface mechanical properties, such as contact resonance force microscopy, creates an avenue for nanoscale thermomechanical property characterization. For nanomechanical methods that employ an atomic force microscope cantilever's vibrational modes, it is essential to understand how the vibrations of the U-shaped HT-AFM cantilever differ from those of a more traditional rectangular lever, for which analytical techniques are better developed. Here we show, with a combination of finite element analysis (FEA) and experiments, that the HT-AFM cantilever exhibits many more readily-excited vibrational modes over typical AFM frequencies compared to a rectangular cantilever. The arms of U-shaped HT-AFM cantilevers exhibit two distinct forms of flexural vibrations that differ depending on whether the two arms are vibrating in-phase or out-of-phase with one another. The in-phase vibrations are qualitatively similar to flexural vibrations in rectangular cantilevers and generally show larger sensitivity to surface stiffness changes than the out-of-phase vibrations. Vibration types can be identified from their frequency and by considering vibration amplitudes in the horizontal and vertical channels of the AFM at different laser spot positions on the cantilever. For identifying contact resonance vibrational modes, we also consider the sensitivity of the resonant frequencies to a change in applied force and hence to tip-sample contact stiffness. Finally, we assess how existing analytical models can be used to accurately predict contact stiffness from contact-resonance HT-AFM results. A simple two-parameter Euler-Bernoulli beam model provided good agreement with FEA for in-phase modes up to a contact stiffness 500 times the cantilever spring constant. By providing insight into cantilever vibrations and exploring the potential of current analysis techniques, our results lay the groundwork for future use of HT-AFM cantilevers for accurate nanomechanical property measurements.

Morphing metal-polymer janus particles

The direct deformation and shape recovery of micron-sized polystyrene particles via nanoimprint lithography is reported. The recovery of the programmed PS particles can be utilized to create a range of smart Janus particles with contrasting properties in conductivity and topography, by use of metal-layer constrained recovery.

Hydrodynamic corrections to contact resonance atomic force microscopy measurements of viscoelastic loss tangent

We present a method to improve accuracy in measurements of nanoscale viscoelastic material properties with contact resonance atomic force microscope methods. Through the use of the two-dimensional hydrodynamic function, we obtain a more precise estimate of the fluid damping experienced by the cantilever-sample system in contact resonance experiments, leading to more accurate values for the tip-sample damping and related material properties. Specifically, we consider the damping and added mass effects generated by both the proximity of the cantilever to the sample surface and the frequency dependence on the hydrodynamic loading of the system. The theoretical correction method is implemented on experimental contact resonance measurements. The measurements are taken on a thin polystyrene film and are used to determine the viscoelastic loss tangent, tan δ, of the material. The magnitude of the corrections become significant on materials with low tan δ (<0.1) and are especially important for measurements made with the first flexural mode of vibration.

Low-force AFM nanomechanics with higher-eigenmode contact resonance spectroscopy

Atomic force microscopy (AFM) methods for quantitative measurements of elastic modulus on stiff (>10 GPa) materials typically require tip-sample contact forces in the range from hundreds of nanonewtons to a few micronewtons. Such large forces can cause sample damage and preclude direct measurement of ultrathin films or nanofeatures. Here, we present a contact resonance spectroscopy AFM technique that utilizes a cantilever's higher flexural eigenmodes to enable modulus measurements with contact forces as low as 10 nN, even on stiff materials. Analysis with a simple analytical beam model of spectra for a compliant cantilever's fourth and fifth flexural eigenmodes in contact yielded good agreement with bulk measurements of modulus on glass samples in the 50-75 GPa range. In contrast, corresponding analysis of the conventionally used first and second eigenmode spectra gave poor agreement under the experimental conditions. We used finite element analysis to understand the dynamic contact response of a cantilever with a physically realistic geometry. Compared to lower eigenmodes, the results from higher modes are less affected by model parameters such as lateral stiffness that are either unknown or not considered in the analytical model. Overall, the technique enables local mechanical characterization of materials previously inaccessible to AFM-based nanomechanics methods.

Viscoelastic property mapping with contact resonance force microscopy

We demonstrate the accurate nanoscale mapping of near-surface loss and storage moduli on a polystyrene-polypropylene blend with contact resonance force microscopy (CR-FM). These viscoelastic properties are extracted from spatially resolved maps of the contact resonance frequency and quality factor of the AFM cantilever. We consider two methods of data acquisition: (i) discrete stepping between mapping points and (ii) continuous scanning. For point mapping and low-speed scanning, the values of the relative loss and storage modulus are in good agreement with the time-temperature superposition of low-frequency dynamic mechanical analysis measurements to the high frequencies probed by CR-FM.

Quantitative subsurface contact resonance force microscopy of model polymer nanocomposites

We present experimental results on the use of quantitative contact resonance force microscopy (CR-FM) for mapping the planar location and depth of 50 nm diameter silica nanoparticles buried beneath polystyrene films 30-165 nm thick. The presence of shallowly buried nanoparticles, with stiffness greater than that of the surrounding matrix, is shown to locally affect the surface contact stiffness of a material for all depths investigated. To achieve the necessary stiffness sensitivity, the CR-FM measurements are obtained utilizing the fifth contact eigenmode. Stiffness contrast is found to increase rapidly with initial increases in force, but plateaus at higher loads. Over the explored depth range, stiffness contrast spans roughly one order of magnitude, suggesting good depth differentiation. Scatter in the stiffness contrast for single images reveals nonuniformities in the model samples that can be explained by particle size dispersity. Finite element analysis is used to simulate the significant effect particle size can have on contact stiffness contrast. Finally, we show how measurements at a range of forces may be used to deconvolve particle size effects from depth effects.

Interfacial mobility and bonding strength in nanocomposite thin film membranes

The interfacial interaction strength and transition properties in a reverse selective thin film nanocomposite system, silica-poly[(trimethylsilyl)propyne] (SiO(x)-PTMSP), are investigated locally by heated tip atomic force microscopy. SiO(x)-PTMSP has recently been introduced as a new class of reverse selective membrane materials with extraordinarily high permeability and selectivity (reverse selectivity). Here, we examine the thermal transition properties of the polymer matrix and the debonding strength between PTMSP and silica. Transitions at 330 degrees C were identified as degradation processes. Criteria for debonding were found to include polymer viscoelastic responses, particle size, embedding depth, scan speed, and frequency of impact. Probe-particle impact forces revealed a debonding energy of 2.6 J/m(2) and an impact force transition that occurs 30 degrees C below the degradation temperature in the neat polymer, confirming the presence of enhanced polymer mobility at the SiO(x)-PTMSP interface.