High-Precision Photoacoustic Nitrogen Dioxide Gas Analyzer for Fast Dynamic Measurement.

A high-precision photoacoustic (PA) gas analyzer for fast dynamic measurement of ambient nitrogen dioxide (NO2) was developed. The PA analyzer used a differential PA cell combined with two mufflers to achieve rapid gas flow gas detection. A high-power laser diode (LD) with a center wavelength of 450 nm was used as the PA signal excitation source. To reduce the saturated absorption effect of NO2, ambient air was pumped into the analyzer at a flow rate of 900 sccm. Two mufflers were combined with the differential PA cell to reduce the noise caused by the airflow and pump. The parameters of the mufflers were optimized by using a finite element method. The experimental results showed that the gas flow noise was suppressed by 95%. The response time of the PAS analyzer was 34 s. The detection limits of the analyzer were 0.64 and 0.17 ppb when the integration times were 1 and 15 s, respectively. A 120 h continuous monitoring result was compared with the data from the National Environmental Monitoring Station to demonstrate the high reliability of the analyzer.

Ultrahigh Sensitive Trace Gas Sensing System with Dual Fiber-Optic Cantilever Multiplexing-Based Differential Photoacoustic Detection.

An ultrahigh sensitive trace gas sensing system was presented with dual cantilever-based differential photoacoustic detection. By combining the double enhancement of multipass absorption and optical differential detection, the gas detection sensitivity was significantly improved. The dual-channel synchronous photoacoustic detection was realized by fiber-optic Fabry-Perot interference spectrum multiplexing. The photoacoustic signals detected by two fiber-optic cantilever microphones installed in a differential photoacoustic cell (DPAC) were out of phase, while the detected gas flow noises were in phase. The optical differential detection method achieved both highly sensitive optical interference measurement and differential noise suppression. In the multipass configuration, the interaction path between excitation light and target gas achieved 4.1 m, which improved the photoacoustic signal by an order of magnitude compared with a single reflection. The maximum gas flow allowed by the system based on the DPAC was 250 sccm, which realized the dynamic monitoring of H2S in the SF6 background. The detection limit for H2S in SF6 background was 5.1 ppb, which corresponds to the normalized noise equivalent absorption coefficient of 9 × 10-10 cm-1 W Hz-1/2.

Integrated near-infrared fiber-optic photoacoustic sensing demodulator for ultra-high sensitivity gas detection.

An integrated near-infrared fiber-optic photoacoustic sensing demodulator was established for ultra-high sensitivity gas detection. The demodulator has capacities of interference spectrum acquisition and calculation, laser modulation control as well as digital lock-in amplification. FPGA was utilized to realize all the control and signal processing functions, which immensely improved the integration and stability of the system. The photoacoustic signal detection based on fiber-optic Fabry-Perot (F-P) acoustic sensor was realized by applying ultra-high resolution spectral demodulation technique. The detectable frequency of photoacoustic signal achieved 10 kHz. The system integrated lock-in amplification technology, which made the noise sound pressure and dynamic response range of sound pressure detection reached 3.7 µPa/√Hz @1 kHz and 142 dB, respectively. The trace C2H2 gas was tested with a multi-pass resonant photoacoustic cell. Ultra-high sensitivity gas detection was accomplished, which was based on high acoustic detection sensitivity and the matching digital lock-in amplification. The system detection limit and normalized noise equivalent absorption (NNEA) coefficient were reached 3.5 ppb and 6.7 × 10-10 cm-1WHz-1/2, respectively. The devised demodulator can be applied for long-distance gas measurement, which depends on the fact that both the near-infrared photoacoustic excitation light and the probe light employ optical fiber as transmission medium.

Fiber-Optic Photoacoustic Gas Sensor with Multiplexed Fabry-Pérot Interferometric Cantilevers.

A fiber-optic photoacoustic (PA) gas sensor with multiplexed Fabry-Pérot (F-P) interferometric cantilevers is demonstrated. A compact cylindrical nonresonant PA tube with a volume of only 0.45 mL is designed. The PA signal is measured by two symmetrically installed fiber-optic interferometric cantilever microphones (FOICMs) to improve the signal-to-noise ratio (SNR). For multiplexing the two cantilevers by a single demodulation system, a dual cavity length synchronous measurement method based on total-phase demodulation algorithm with ultrahigh resolution is developed. The PA signal detection is realized by the second-harmonic wavelength modulation spectroscopy (2f-WMS) technique. The sensor performance is verified by conducting the detection of trace acetylene (C2H2). The normalized noise equivalent absorption (NNEA) coefficient is 2.5 × 10-9 cm-1·W·Hz-1/2, and the minimum detection limit (MDL) downs to about 0.2 ppm with an averaging time of 1 s. The fiber-optic PA gas sensor has characteristics of high resolution and immunity to electromagnetic and vibration interference. Furthermore, the technical scheme of the multiplexed cantilever demodulation shows great potential for remote multipoint monitoring of gases in harsh environments.

Molecular Ferroelectric Crystals with Superior Pyroelectricity, Plasticity, and Recyclability.

The pyroelectric effect is used in a wide range of applications such as infrared (IR) detection and thermal energy harvesting, which require the pyroelectric materials to simultaneously have a high pyroelectric coefficient and a low dielectric constant for high figures of merit. However, in conventional proper ferroelectrics, the positive correlation between the pyroelectric coefficient and the dielectric constant imposes an insurmountable challenge in upgrading the figures of merit. Here, we explored superior pyroelectricity in [(CH3)4N][FeCl4] (TMA-FC) and [(CH3)4N][FeCl3Br] (TMA-FCB) molecular ferroelectric plastic crystals, which could decouple this positive correlation due to the nature of improper polarization behavior. Therefore, TMA-FC and TMA-FCB derive a high pyroelectric coefficient and a low dielectric constant simultaneously, yielding record-high figures of merit around room temperature. Furthermore, the favorable plasticity enables ferroelectric crystals to attach surfaces with different shapes for device design and integration. More interestingly, the molecular ferroelectrics could be softened and reshaped at elevated temperatures without decay in pyroelectricity, making them recyclable for cost savings and e-waste reduction. Combined with the facile fabrication process, the findings of this work would open avenues for employing molecular ferroelectric plastic crystals in the manufacture of high-performance pyroelectric devices.

Digital Mechanical Metasurfaces for Reprogrammable Structural Display.

Stimuli-responsive surfaces with reversible surface topography and controllable physical and mechanical properties are highly desirable for various engineering applications: e.g., information encoding, anticounterfeiting, micromanipulations, displays, etc. Here we present a digital type of stimuli-responsive surface composed of discrete silicon scales supported on individual core-shell magnetic micropillars (MMP) and realize precise control over the local topography of the surfaces. The individual MMP can be reversibly modulated between two contrasting bending states (state 0, hard to bend; state 1, easy to bend) by controlling the spatial distribution of the magnetic nanoparticles inside the pillar shells. These two different states of the micropillars induce a completely different topography of the supporting scales that can be utilized as mechanical pixels upon applying actuation magnetic fields. We further build a three-dimensional (3D) microcontrolling platform for digital modulation of the micropillar states. With this platform, a large array of 50 × 50 MMP surfaces can be programmed and reprogrammed into any combination of the local states by simply reading a matrix of binary digits. Such a digital modulation process facilitates the practical application of the MMP surfaces for fast and reprogrammable display of various structural patterns, as demonstrated for microscale letters, millimeter-scale QR code, and complex Chinese characters. The digital modulation and on-demand reprogrammability of the MMP surfaces reported here are expected to advance the development of various other forms of digital mechanical metasurfaces that can easily transform digital information into encoded mechanical responses.

A Comparative Study on the Microscale and Macroscale Mechanical Properties of Dental Resin Composites.

Dental resin composites are universal restorative materials, and various kinds of fillers are used to reinforce their mechanical properties. However, a combined study on the microscale and macroscale mechanical properties of dental resin composites is missing, and the reinforcing mechanism of the composites is still not fully clarified. In this work, the effects of the nano-silica particle on the mechanical properties of dental resin composites were studied by combined dynamic nanoindentation tests and macroscale tensile tests. The reinforcing mechanism of the composites was explored by combining near-infrared spectroscopy, scanning electron microscope, and atomic force microscope characterizations. It was found that the tensile modulus increased from 2.47 GPa to 3.17 GPa, and the ultimate tensile strength increased from 36.22 MPa to 51.75 MPa, with the particle contents increasing from 0% to 10%. From the nanoindentation tests, the storage modulus and hardness of the composites increased by 36.27% and 40.90%, respectively. The storage modulus and hardness were also found to increase by 44.11% and 46.46% when the testing frequency increased from 1 Hz to 210 Hz. Moreover, based on a modulus mapping technique, we found a boundary layer in which the modulus gradually decreased from the edge of the nanoparticle to the resin matrix. Finite element modeling was adopted to illustrate the role of this gradient boundary layer in alleviating the shear stress concentration on the filler-matrix interface. The present study validates mechanical reinforcement and provides a potential new insight for understanding the reinforcing mechanism of dental resin composites.

Organic Persistent RTP Crystals: From Brittle to Flexible by Tunable Self-Partitioned Molecular Packing.

Aiming to solve the trade-off of "room-temperature phosphorescence (RTP)-flexibility" in principle, organic RTP crystals with elastic/plastic deformation are realized. These properties are mainly due to the divisional aggregation structures of aromatics and alkoxy chains, and can be modulated by the controllable molecular configurations. The longest RTP lifetime of 972.3 ms is achieved as the highest record for organic flexible crystals. Plastic crystals with persistent RTP are realized, which can be applied into biomedical optical technologies by afterglow delivery. Moreover, the relationship among elastic/plastic deformation, RTP property, and aggregated structures is established. The elastic/plastic deformation is mainly determined by the difference of interaction energies from the aromatics and the alkoxy chains. For the BP-OR series with twisted configurations, the alkoxy chain with the middle length is favorable for the RTP property, while the strength of the π-π coupling is the cruical factor to the RTP property of the Xan-OR series with planar skeletons. A new way to promote the development of flexible RTP crystals, by modulation of aggregated structures as well as rational distribution of intermolecular interactions, is explored.

Aligned Magnetic Nanocomposites for Modularized and Recyclable Soft Microrobots.

Creating reconfigurable and recyclable soft microrobots that can execute multimodal locomotion has been a challenge due to the difficulties in material processing and structure engineering at a small scale. Here, we propose a facile technique to manufacture diverse soft microrobots (∼100 µm in all dimensions) by mechanically assembling modular magnetic microactuators into different three-dimensional (3D) configurations. The module is composed of a cubic micropillar supported on a square substrate, both made of elastomer matrix embedded with prealigned magnetic nanoparticle chains. By directionally bonding the sides or backs of identical modules together, we demonstrate that assemblies from only two and four modules can execute a wide range of locomotion, including gripping microscale objects, crawling and crossing solid obstacles, swimming within narrow and tortuous microchannels, and rolling along flat and inclined surfaces, upon applying proper magnetic fields. The assembled microrobots can additionally perform pick-transfer-place and cargo-release tasks at the microscale. More importantly, like the game of block-building, the microrobots can be disassembled back to separate modules and then reassembled to other configurations as demanded. The present study not only provides a versatile and economic manufacturing technique for reconfigurable and recyclable soft microrobots, enabling unlimited design space for diverse robotic locomotion from limited materials and module structures, but also extends the functionality and dexterity of existing soft robots to microscale that should facilitate practical applications at such small scale.

Optimal resin monomer ratios for light-cured dental resins.

Monomer ratios play a crucial role on the performances of dental resins, the optimal monomer ratios for dental resins are determined by combining the degree of conversion (DC), rate of polymerization (Rp), and mechanical properties, based on commonly-used Bis-GMA (bisphenol A-glycidyl methacrylate), UDMA (urethane dimethacrylate), and TEGDMA (triethyleneglycol dimethacrylate) resins. The DC and mechanical properties of the dental resins are examined by NIR (Near Infrared Ray) spectroscopy and nanoindentation tests, respectively. The results indicate that the Rp increases while the DC decreases with the loading content of Bis-GMA or UDMA in dental resins (i.e., Bis-GMA/TEGDMA and UDMA/TEGDMA). Meanwhile, both elastic modulus and hardness also present a tendency to increase. Various different monomers maybe create a strong polymer matrix in proper proportions, comprehensively comparing the performances of dental resins in different monomer ratios, the cured resins containing Bis-GMA (15-35 wt%), UDMA (37-60 wt%) and TEGDMA (20-35 wt%) show better material properties. The present study offers a quantitative analysis for Bis-GMA/UDMA/TEGDMA dental resins as well as provides guidance for the research of dental resins.

Cryo-EM structure of an amyloid fibril formed by full-length human SOD1 reveals its conformational conversion.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease. Misfolded Cu, Zn-superoxide dismutase (SOD1) has been linked to both familial and sporadic ALS. SOD1 fibrils formed in vitro share toxic properties with ALS inclusions. Here we produced cytotoxic amyloid fibrils from full-length apo human SOD1 under reducing conditions and determined the atomic structure using cryo-EM. The SOD1 fibril consists of a single protofilament with a left-handed helix. The fibril core exhibits a serpentine fold comprising N-terminal segment (residues 3-55) and C-terminal segment (residues 86-153) with an intrinsic disordered segment. The two segments are zipped up by three salt bridge pairs. By comparison with the structure of apo SOD1 dimer, we propose that eight ß-strands (to form a ß-barrel) and one α-helix in the subunit of apo SOD1 convert into thirteen ß-strands stabilized by five hydrophobic cavities in the SOD1 fibril. Our data provide insights into how SOD1 converts between structurally and functionally distinct states.

The residues 4 to 6 at the N-terminus in particular modulate fibril propagation of ß-microglobulin.

The ΔN6 truncation is the main posttranslational modification of ß-microglobulin (ßM) found in dialysis-related amyloid. Investigation of the interaction of wild-type (WT) ßM with N-terminally truncated variants is therefore of medical relevance. However, it is unclear which residues among the six residues at the N-terminus are crucial to the interactions and the modulation of amyloid fibril propagation of ßM. We herein analyzed homo- and heterotypic seeding of amyloid fibrils of WT human ßM and its N-terminally-truncated variants ΔN1 to ΔN6, lacking up to six residues at the N-terminus. At acidic pH 2.5, we produced amyloid fibrils from recombinant, WT ßM and its six truncated variants, and found that ΔN6 ßM fibrils exhibit a significantly lower conformational stability than WT ßM fibrils. Importantly, under more physiological conditions (pH 6.2), we assembled amyloid fibrils only from recombinant, ΔN4, ΔN5, and ΔN6 ßM but not from WT ßM and its three truncated variants ΔN1 to ΔN3. Notably, the removal of the six, five or four residues at the N-terminus leads to enhanced fibril formation, and homo- and heterotypic seeding of ΔN6 fibrils strongly promotes amyloid fibril formation of WT ßM and its six truncated variants, including at more physiological pH 6.2. Collectively, these results demonstrated that the residues 4 to 6 at the N-terminus particularly modulate amyloid fibril propagation of ßM and the interactions of WT ßM with N-terminally truncated variants, potentially indicating the direct relevance to the involvement of the protein's aggregation in dialysis-related amyloidosis.

Enhancing resin-dentin bond durability using a novel mussel-inspired monomer.

Numerous approaches have been developed to improve the resin-dentin bond performance, among which the bio-application of mussel-derived compounds have drawn great attention recently. To assess the performance of N-(3,4-dihydroxyphenethyl)methacrylamide (DMA), a mussel-derived compound, as a functional monomer in dental adhesive, its potential property to cross-link with dentin collagen and polymerize with adhesive will first be evaluated by transmission electron microscopy (TEM), attenuated total reflectance technique of Fourier transform infrared (ATR-FTIR), and atomic force microscopy (AFM) via Peakforce QNM mode. After validating the influence of DMA on collagen and adhesive separately, the overall performance of DMA/ethanol solution as a primer in dentin bonding was examined using micro-tensile bond strength (µTBS) testing, fracture pattern observation, and nanoleakage evaluation both immediately and after 10,000 times thermocycling aging. The inhibitory effect of DMA on endogenous metalloproteinases (MMPs) was evaluated by in situ zymography using confocal laser scanning microscopy (CLSM) and the cytotoxicity of DMA was evaluated using cell counting kit-8. Results demonstrated that DMA successfully cross-linked with dentin collagen via non-covalent bonds and had no influence on the polymerization and mechanical properties of the adhesive. Furthermore, even after 10,000 times thermocycling aging, the µTBS and nanoleakage expression of the DMA-treated groups showed no significant change compared with their immediate values. In situ zymography revealed reduced endogenous proteolytic activities after the application of DMA, and no cytotoxicity effect was observed for DMA concentration up to 25 â€‹µmol/L. Thus, DMA could be used as a novel, biocompatible functional monomer in dentin bonding.

Molecular Ferroelectric-Based Flexible Sensors Exhibiting Supersensitivity and Multimodal Capability for Detection.

Although excellent dielectric, piezoelectric, and pyroelectric properties matched with or even surpassing those of ferroelectric ceramics have been recently discovered in molecular ferroelectrics, their successful applications in devices are scarce. The fracture proneness of molecular ferroelectrics under mechanical loading precludes their applications as flexible sensors in bulk crystalline form. Here, self-powered flexible mechanical sensors prepared from the facile deposition of molecular ferroelectric [C(NH2 )3 ]ClO4 onto a porous polyurethane (PU) matrix are reported. [C(NH2 )3 ]ClO4 -PU is capable of detecting pressure of 3 Pa and strain of 1% that are hardly accessible by the state-of-the-art piezoelectric, triboelectric, and piezoresistive sensors, and presents the ability of sensing multimodal mechanical forces including compression, stretching, bending, shearing, and twisting with high cyclic stability. This scaling analysis corroborated with computational modeling provides detailed insights into the electro-mechanical coupling and establishes rules of engineering design and optimization for the hybrid sponges. Demonstrative applications of the [C(NH2 )3 ]ClO4 -PU array suggest potential uses in interactive electronics and robotic systems.

Knockdown circular RNA circGFRA1 inhibits glioma cell proliferation and migration by upregulating microRNA-99a.

Glioma is the most widespread and malignant brain tumor in the central nervous system of adult, causing multiple cancer-associated deaths worldwide. Here, we identified the impact of circGFRA1 on glioma, and aimed to uncover the underlying molecular mechanism. The expression of circGFRA1 of glioma specimens was evaluated by using quantitative reverse transcription PCR. Cell viability, proliferation, colony formation, apoptosis and migration were estimated utilizing CCK-8, EdU staining, colony formation assay, TUNEL staining and Transwell assay, respectively. Bioinformatics analysis, luciferase assay and RNA co-immunoprecipitation was utilized for verification of direct binding between circGFRA1 and miR-99a. Western blot was applied to investigate protein expression in U251 cells. The results showed that circGFRA1 expression was overexpressed in glioma specimens. Knockdown circGFRA1 declined viability, colony formation, proliferation and migrative potential, but enhanced U251 cell apoptosis. Moreover, circGFRA1 acts as a microRNA sponge for miR-99a. Furthermore, miR-99a was involved in the circGFRA1-regulated glioma cell behaviors. Silencing circGFRA1 reduced p/t-AKT, p/t-FOXO1 and p/t-mTOR expression levels via upregulating miR-99a expression. In conclusion, our study demonstrated that knockdown circGFRA1 inhibits glioma cell proliferation and migration by upregulating microRNA-99a.

Multi-scale design of the chela of the hermit crab Coenobita brevimanus.

The chela of the hermit crab protects its body against the attack from predators. Yet, a deep understanding of this mechanical defense is still lacking. Here, we investigate the chela of hermit crab, Coenobita brevimanus, and establish the relationships between the microstructures, chemical compositions and mechanical properties to gain insights into its biomechanical functions. We find that the chela is a multi-layered shell composed of five different layers with distinct features of the microstructures and chemical compositions, conferring different mechanical properties. Especially, an increase of the calcium carbonate content towards the layer furthest from the exterior, unlike the chemical gradients of many crustacean exoskeletons, provides a strong resistance to deformation. Nanoindentation measurements reveal that the overall gradient of the elastic modulus and hardness in the cross-section displays a sandwich profile, i.e., a soft core clamped by two stiff surface layers. Further mechanics modeling demonstrates that the high curvature and stiff innermost sublayer enhance the structural rigidity of the chela. In conjunction with the experimental observations, dynamic finite element analysis maps the time-spatial distribution of principal stress and indicates that fiber bridging might be the major mechanism against crack propagation at microscale. The lessons gained from the study of this multiphase biological composite could provide important insights into the design and fabrication of bioinspired materials for structural applications. STATEMENT OF SIGNIFICANCE: Multiple hierarchical structures have been discovered in a variety of exoskeletons. They are naturally designed to maintain the structural integrity and act as a protective layer for the animals. However, each kind of the hierarchical structures has its unique topology, chemical gradients as well as mechanical properties. We find that the chela is multi-layered shell composed of five different layers with distinct features of the microstructures and chemical compositions, conferring different mechanical properties. Especially, a large amount of helicoidal organic fibrils form highly organized 3D woven matrix in the innermost layer, providing a strong mechanical resistance to avoid catastrophic failure. The overall gradient of the elastic modulus and hardness in the cross-section display a sandwich profile, effectively minimizing the stress concentration and deformation. The lessons gained from the multiscale design strategy of the chela provide important insights into the design and fabrication of bioinspired materials.

Optimized Hierarchical Structure and Chemical Gradients Promote the Biomechanical Functions of the Spike of Mantis Shrimps.

The tail spike of the mantis shrimp is the appendage for counteracting the enemy from behind. Here, we investigate the correlations between the chemical compositions, the microstructures, and the mechanical properties of the spike. We find that the spike is a hollow beam with a varying cross section along the length. The cross section comprises four different layers with distinct features of microstructures and chemical compositions. The local mechanical properties of these layers correlate well with the microstructures and chemical compositions, a combination of which effectively restricts the crack propagation while maximizing the release of strain energy during deformation. Finite element analysis and mechanics modeling demonstrate that the optimized structure of the spike confines the mechanical damage in the region near the tip and prevents catastrophic breakage at the base. Furthermore, we use a 3D printing technique to fabricate multiple hollow cylindrical samples consisting of biomimetic microstructures of the spike and confirm that the combination of the Bouligand structure with radially oriented parallel sheets greatly improves the toughness and strength during compression tests. The multiscale design strategy of the spike revealed here is expected to be of great interest for the development of novel bioinspired materials.

Core-Shell Magnetic Micropillars for Reprogrammable Actuation.

Stimuli-responsive micro/nanostructures that exhibit not only programmable but also reprogrammable actuation behaviors are highly desirable for various advanced engineering applications (e.g., anticounterfeiting, information encoding, dynamic imaging and display, microrobotics, etc.) but yet to be realized with state-of-the-art technologies. Here we report a concept and a corresponding experimental technique for core-shell magnetic micropillars enabling simultaneously programmable and reprogrammable actuations using a simple magnetic field. The micropillars are composed of elastomeric hollow shells for shaping encapsulated with liquid magnetic nanocomposite resin cores for actuating. The spatial distribution of the magnetic nanoparticles inside the resin channels can be dynamically modulated within individual micropillars, which consequently regulates the magnetomechanical responses of the pillars upon actuation (bending deformation varied near 1 order of magnitude under the same actuation field). We demonstrate that the micropillars with contrasting bending responses can be configured in an arbitrary spatial pattern by direct magnetic writing, and the written pattern can then be easily magnetically erased to facilitate next-round rewriting and reconfiguration. This reprogrammable actuation capability of the micropillars is further demonstrated by their potential applications for rewritable paper and recyclable displays, where various microscale characteristics can be controlled to dynamically appear and disappear at the same or different locations of one single micropillar array. The core-shell magnetic micropillars reported here provide a universal prototype for reprogrammable responsive micro/nanostructures through rational design and facile fabrication from conventional materials.

Hybrid Magnetic Micropillar Arrays for Programmable Actuation.

Stimuli-responsive micro/nanostructures that can dynamically and reversibly adapt their configurations according to external stimuli have stimulated a wide scope of engineering applications, ranging from material surface engineering to micromanipulations. However, it remains a challenge to achieve a precise local control of the actuation to realize applications that require heterogeneous and on-demand responses. Here, a new experimental technique is developed for large arrays of hybrid magnetic micropillars and achieve precise local control of actuation using a simple magnetic field. By manipulating the spatial distribution of magnetic nanoparticles within individual elastomer micropillars, a wide range of the magnetomechanical responses from less than 5% to ≈50% for the ratio of the bending deflection to the original length of the pillars is realized. It is demonstrated that the micropillars with different degrees of bending deformation can be configured in any spatial pattern using a photomask-assisted template-casting technique to achieve heterogeneous, site-specific, and programmed bending actuations. This unprecedented local control of the micropillars offers exciting novel applications, as demonstrated here in encryptable surface printing and stamping, direction- and track-programmable microparticle/droplet transport, and smart magnetic micro-tweezers. The hybrid magnetic micropillars reported here provide a versatile prototype for heterogeneous and on-demand actuation using programmable stimuli-responsive micro/nanostructures.

Resin viscosity determines the condition for a valid exposure reciprocity law in dental composites.

OBJECTIVE: To provide conditions for the validity of the exposure reciprocity law as it pertains to the photopolymerization of dimethacrylate-based dental composites. METHODS: Composites made from different mass ratios of resin blends (Bis-GMA/TEGDMA and UDMA/TEGDMA) and silanized micro-sized glass fillers were used. All the composites used camphorquinone and ethyl 4-dimethylaminobenzoate as the photo initiator system. A cantilever beam-based instrument (NIST SRI 6005) coupled with NIR spectroscopy and a microprobe thermocouple was used to simultaneously measure the degree of conversion (DC), the polymerization stress (PS) due to the shrinkage, and the temperature change (TC) in real time during the photocuring process. The instrument has an integrated LED light curing unit providing irradiances ranging from 0.01W/cm2 to 4W/cm2 at a peak wavelength of 460nm (blue light). Vickers hardness of the composites was also measured. RESULTS: For every dental composite there exists a minimum radiant exposure required for an adequate polymerization (i.e., insignificant increase in polymerization with any further increase in the radiant exposure). This minimum predominantly depends on the resin viscosity of composite and can be predicted using an empirical equation established based on the test results. If the radiant exposure is above this minimum, the exposure reciprocity law is valid with respect to DC for high-fill composites (filler contents >50% by mass) while invalid for low-fill composites (that are clinically irrelevant). SIGNIFICANCE: The study promotes better understanding on the applicability of the exposure reciprocity law for dental composites. It also provides a guidance for altering the radiant exposure, with the clinically available curing light unit, needed to adequately cure the dental composite in question.