Search

Crystallization of polyamide 12 (PA12) is an essential process requiring thorough investigation for evaluating the mechanical properties after the polymer parts are manufactured. The change in crystallization temperature results in different crystallization behaviors for PA12. Hence, the crystal morphology of PA12 achieved provides important information about crystallization behavior, especially for those produced through additive manufacturing due to its heterogenous cooling rate in a single print bed. Considering the need of investigating PA12 crystallization using phase-field modeling, this paper aims to simulate the spherulite morphology of PA12 undergoing isothermal crystallization. This model is compared with the spherulite morphologies obtained from the optical microscopy test. The model shows that PA12 spherulites have thicker dendrites when the isothermal temperature is higher. The present phase-field model can determine the spherulite morphologies of bulk printed PA12 based on the crystallization condition and be used to evaluate the properties of the printed part. Nanyang Technological University This research work is conducted in collaboration with HP Inc. and supported by Nanyang Technological University and the Singapore Government through the Industry Alignment Fund–Industry Collaboration Projects Grant (I1801E0028).

Photodetectors, which are capable of detecting light with varied wavelength, have nowadays been widely applied onto emerging fields such as security, entertainment, healthcare, environment, and so on. As the one with a two-dimensional layered structure, molybdenum disulfide (MoS2) possesses striking optical and electrical properties that can be used in photodetecting, yet the challenges remain in terms of material processing, device fabrication simplicity, and enhancement of overall photodetection performance. In this work, a photodetectable paper based on a mixture of double-phased MoS2 (1T and 2H) and MoO3 was successfully fabricated through a straightforward route, that is, chemical exfoliation and deposition of MoS2 powder on a flexible cellulose ester membrane, followed by inkjet-printed PEDOT:PSS as electrodes. The obtained device shows varied sensitivity to the light with different wavelengths. Compared with that under green and red lights, the prepared photodetector has the highest internal quantum efficiency (0.063%) and responsivity (0.134 mA W-1), while having longest response/recovery time (17.5/15.3 s) when illuminated with purple light (405 nm). The achieved responsivity is much higher than other reported liquid exfoliation- and solution-derived MoS2 photodetectors. This is ascribed to (1) the enhanced photoelectron generation caused by both MoS2 and MoO3 and (2) the good electric conductivity and efficient charge transport caused by the metallic 1T MoS2. This work demonstrates the feasibility of fabricating the MoS2-based photodetector with excellent performance through a simple exfoliation/filtration and inject printing route, and the detailed study on the response to light with different wavelengths unveils the interaction between the device and the incident light, further broadening the potential applications of such design.

Reconstructing of cell architecture plays a vital role in tissue engineering. Recent developments of self-assembling of cells into three-dimensional (3D) matrix pattern using surface acoustic waves have paved a way for a better tissue engineering platform thanks to its unique properties such as nature of noninvasive and noncontact, high biocompatibility, low-power consumption, automation capability, and fast actuation. This article discloses a method to manipulate the orientation and curvature of 3D matrix pattern by redesigning the top wall of microfluidic chamber and the technique to create a 3D longitudinal pattern along preinserted polydimethylsiloxane (PDMS) rods. Experimental results showed a good agreement with model predictions. This research can actively contribute to the development of better organs-on-chips platforms with capability of controlling cell architecture and density. Meanwhile, the 3D longitudinal pattern is suitable for self-assembling of microvasculatures. Ministry of Education (MOE) Nanyang Technological University The authors gratefully acknowledge the support of (i) Nanyang Technological University and the Ministry of Education of Singapore through a PhD Scholarship; and (ii) the Academic Research Fund (AcRF) Tier 1 Grants (RG 96/18).

Warpage of printed parts, especially for multi-jet fusion (MJF) process, is believed to have strong correlation with crystallization shrinkage. Hence, this article investigates the shrinkage phenomenon of polyamide-12 (PA12) during crystallization process. Various cooling rates show that PA12 exhibits different crystallization behaviour, which affect the crystallinity, crystal phase and crystal shrinkage when performed in MJF. The investigation shows that crystallization shrinkage of PA12 coincides with the exothermic reaction of crystallization process. With cooling rate as slow as 0.2 °C/min, PA12 sample shrinks 1.64% during the crystallization process with crystallinity calculated at 24.9%. X-ray diffraction (XRD) suggests shrinkage are closely related to lamellar folding and growth of γ crystal phase within PA12 samples during crystallization. Spherulites are found to be smaller in diameter with faster cooling rate. These investigations provide detailed understanding of the crystallization behaviour of PA12 for MJF process. Submitted/Accepted version This research work is supported under the RIE2020 Industry Alignment Fund – Industry Collaboration Projects (IAF-ICP) Funding Initiative (I1801E0028), as well as cash and in-kind contribution from the industry partner, HP Inc.

Precise, automatic and reliable position control of micro-objects such as single particles, biological cells or bio-organisms is critical for applications in biotechnology and tissue engineering. However, conventional acoustofluidic techniques generally lack reliability and automation capability thus are often incapable of building an efficient and automated system where the biological cells need to be precisely manipulated in three dimensions (3D). To overcome these limitations, we developed an acoustofluidic closed-loop control system which is combined with computer vision techniques and standing surface acoustic waves (SSAWs) to implement selective, automatic and precise position control of an object, such as a single cell or microparticle in a microfluidic chamber. Position of the object is in situ extracted from living images that are captured from a video camera. By utilizing the closed-loop control strategy, the object is precisely moved to the desired location in 3D patterns or along designed trajectories by manipulating the phase angle and power signal of the SSAWs. Controlling of breast cancer cells has been conducted to verify the principle and biocompatibility of the control system. This system could be employed to build an automatic system for cell analysis, cell isolation, self-assembling of materials into complex microstructures, or lab-on-chip and organ-on-chip applications. Ministry of Education (MOE) Nanyang Technological University The authors gratefully acknowledge the support of (i) Nanyang Technological University and the Ministry of Education of Singapore through a PhD Scholarship; (ii) the UK Engineering and Physical Sciences Research Council (EPSRC) grants EP/P018998/1; (iii) Special Interesting Group of Acoustofluidics funded by UK Fluids Network (EP/N032861/1).

Photodetectors, which are capable of detecting light with varied wavelength, have nowadays been widely applied onto emerging fields such as security, entertainment, healthcare, environment, and so on. As the one with a two-dimensional layered structure, molybdenum disulfide (MoS

Spatial distribution of biological cells plays a key role in tissue engineering for reconstituting the cellular microenvironment, and recently, acoustofluidics are explored as a viable tool for controlling structures in tissue fabrication because of its good biocompatibility, low-power consumption, automation capability, nature of non-invasive, and non-contact. Herein, a reusable acoustofluidic device is developed using surface acoustic waves for manipulating microparticles/cells to form a 3D matrix pattern inside a scaffold-based hydrogel contained in a millimetric chamber. The 3D patterned and polymerized hydrogel construct can be easily and safely removed from the chamber using a proposed lifting technique, which prevent any physical damages or contaminations and promote the reusability of the chamber. The generated 3D patterns of microparticles and cells are numerically studied using a finite-element method, which is well validated by the experimental results. The proposed acoustofluidic device is a useful tool for in vitro engineering 3D scaffold-based artificial tissues for drug and toxicity testing and building organs-on-chip applications. Ministry of Education (MOE) Nanyang Technological University The authors gratefully acknowledge the support of 1) Nanyang Technological University and the Ministry of Education of Singapore through a Ph.D. Scholarship and AcRF Tier 1 research grant (RG 96/18); 2) the UK Engineering and Physical Sciences Research Council (EPSRC) grants (EP/P018998/1); and 3) Special Interesting Group of Acoustofluidics funded by UK Fluids Network (EP/N032861/1).

Additive manufacturing, which is commonly known as three-dimensional (3D) printing, is a potential technology that possesses many advantages comparing with traditional fabrication technologies. The creative idea of additive manufacturing, which converts digital designs to physical objects via depositing material layer-by-layer, introduces many benefits such as capable of making complex structures, obtaining satisfactory material properties, prototyping rapidly and saving materials. Printing functional devices is an even more promising approach to utilize the benefits of additive manufacturing for micro-fabrication. Zinc oxide (ZnO) is a well-known wide bandgap semiconductor which has numerous applications in electronics, optoelectronics, piezoelectric, and piezotronics. Hence, inkjet printing of ZnO thin film is of significant interest for producing functional devices using additive manufacturing. In addition, the electrical properties of ZnO are essential to the performance of devices, such as responsivity of UV photodetector. Thus, it is important to develop an inkjet printing manufacturing process for producing ZnO thin film of desired electric properties. This thesis aims to develop and investigate the additive manufacturing process of ZnO thin film for electronic applications, and specifically to engineer the electrical properties of printed ZnO film to enhance the performance of printed UV photodetector. Owing to its fascinating features such as non-contact printing, high resolution, low cost and low-temperature process, inkjet printing is adopted as the main additive manufacturing method in this thesis. Inkjet printing process of a zinc precursor solution was successfully developed for the additive manufacturing of ZnO thin film. It was also found that inkjet-printed ZnO possesses a polycrystalline structure. To investigate the practicality of the proposed process, micro-sized flexible ZnO UV photodetectors were thoroughly prepared by a facile inkjet printing scheme. The fabricated devices can be potentially used for real-time monitoring of the UV intensity and warning users for health and safety purpose. Moreover, this additive manufacturing method can be applied to fabricate ZnO thin film on various flexible substrates, which makes the device more suitable for applications that require flexibility such as wearable devices. Electrical properties of printed UV photodetectors were characterized and studied under varied thermal annealing temperatures of up to 350 °C. It was found that both conductivity and responsivity of the printed ZnO film significantly increase along with the increase of annealing temperature. The enhancement of electrical conductivity and responsivity was attributed to band bending modification, which was reduced due to the fusing of grain boundaries under heat treatment. To explore and demonstrate the capability of integrating the inkjet printing of ZnO films with other 3D printing process, silver micro-heater was fabricated and employed to heat zinc precursor locally and form ZnO. Subsequently, electrical properties of printed ZnO films were further investigated by using localized Joule heating effect through the integrated fabrication of ZnO films and a silver microheater. The fabricated UV photodetector showed that the input power of microheater has a significant effect on the performance of the printed device. The successful inkjet printing of micro-sized ZnO thin films and the integrated photodetector has demonstrated the feasibility and great potentials of fabricating sophisticated semiconductor devices using additive manufacturing technology. Doctor of Philosophy

The coastal zone of the Mekong Delta is affected by the impacts of global climate change. Intensity and frequency of storm events and floods are expected to increase. Larger parts of the 245 km long shoreline of Ca Mau province are affected by severe erosion. It is thus necessary to adopt appropriate solutions to protect that shoreline. This study proposes a new type of breakwater structure - hollow cylinder breakwater, which is modified from the traditional semi-circular breakwater. The hollow cylinder breakwater is installed along the shoreline in order to dissipate the wave energy and protect coastal zone. A pilot project using 180 m hollow cylinder breakwater was constructed in Ca Mau province. Sliding and overturning safety factor of the breakwater are analyzed. Finite element method is used to analyze the settlement of the breakwater. It is found that the calculated settlement is in good agreement with monitoring data.

A method based on standing surface acoustic waves (SSAWs) is proposed to pattern and manipulate microparticles into a three-dimensional (3D) matrix inside a microchamber. An optical prism is used to observe the 3D alignment and patterning of the microparticles in the vertical and horizontal planes simultaneously. The acoustic radiation force effectively patterns the microparticles into lines of 3D space or crystal-lattice-like matrix patterns. A microparticle can be positioned precisely at a specified vertical location by balancing the forces of acoustic radiation, drag, buoyancy, and gravity acting on the microparticle. Experiments and finite-element numerical simulations both show that the acoustic radiation force increases gradually from the bottom of the chamber to the top, and microparticles can be moved up or down simply by adjusting the applied SSAW power. Our method has great potential for acoustofluidic applications, building the large-scale structures associated with biological objects and artificial neuron networks. MOE (Min. of Education, S’pore) Published version

The advantages of additive manufacturing for electronic devices have led to the demand of printing functional material in search of a replacement for the conventional subtractive fabrication process. Zinc oxide (ZnO), thanks to its interesting properties for the electronic and photonic applications, has gathered many attentions in the effort to fabricate functional devices additively. Although many potential methods have been proposed, most of them focus on the lowtemperature processing of the printed material to be compatible with the polymer substrate. These low-temperature fabrication processes could establish a high concentration of defects in printed ZnO which significantly affect the performance of the device. In this study, ZnO thin film for UV photodetector application was prepared by inkjet printing of zinc acetate dihydrate solution following by different heat treatment schemes. The effects of annealing to the intrinsic defect of printed ZnO and photoresponse characteristics under UV illumination were investigated. A longer response/decay time and higher photocurrent were observed after the annealing at 350°C for 30 minutes. X-ray photoelectron spectroscopy (XPS) analysis suggests that the reducing of defect concentration, such as oxygen vacancy, and excess oxygen species in printed ZnO is the main mechanism for the variation in photoresponse. The result provides a better understanding on the defect of inkjet-printed ZnO and could be applied in engineering the properties of the printed oxide-based semiconductor.

The increasing demand for wearable optoelectronics in biomedicine, prosthetics, and soft robotics calls for innovative and transformative technologies that permit facile fabrication of compact and flexible photodetectors with high performance. Herein, by developing a single‐step selective laser writing strategy that can finely tailor material properties through incident photon density control and lead to the formation of hierarchical hybrid nanocomposites, e.g., reduced graphene oxide (rGO)–zinc oxide (ZnO), a highly flexible and all rGO–ZnO hybrid‐based photodetector is successfully constructed. The device features 3D ultraporous hybrid films with high photoresponsivity as the active detection layer, and hybrid nanoflakes with superior electrical conductivity as interdigitated electrodes. Benefitting from enhanced photocarrier generation because of the ultraporous film morphology, efficient separation of electron–hole pairs at rGO–ZnO heterojunctions, and fast electron transport by highly conductive rGO nanosheets, the photodetector exhibits high, linear, and reproducible responsivities to a wide range of ultraviolet (UV) intensities. Furthermore, the excellent mechanical flexibility and robustness enable the photodetector to be conformally attached to skin, thus intimately monitoring the exposure dosage of human body to UV light for skin disease prevention. This study advances the fabrication of flexible optoelectronic devices with reduced complexity, facilitating the integration of wearable optoelectronics and epidermal systems. NRF (Natl Research Foundation, S’pore) Published version

A wearable and flexible pressure sensor is essential to the realization of personalized medicine through continuously monitoring an individual’s state of health and also the development of a highly intelligent robot. A flexible, wearable pressure sensor is fabricated based on novel single-wall carbon nanotube /tissue paper through a low-cost and scalable approach. The flexible, wearable sensor showed superior performance with concurrence of several merits, including high sensitivity for a broad pressure range and an ultralow energy consumption level of 10–6 W. Benefited from the excellent performance and the ultraconformal contact of the sensor with an uneven surface, vital human physiological signals (such as radial arterial pulse and muscle activity at various positions) can be monitored in real time and in situ. In addition, the pressure sensors could also be integrated onto robots as the artificial skin that could sense the force/pressure and also the distribution of force/pressure on the artificial skin. Accepted version

The surge in popularity of lab-on-chip applications has set a new challenge for the fabrication of prototyping devices, such as electrokinetic devices. In such devices, a micro-electrode is the key component. Currently, microelectromechanical systems (MEMS) processes such as lift-off and etching techniques are employed to prepare the micro-sized conductive patterns. These processes are time-consuming, require a material removal step, clean-room facilities, and the utilisation of harmful chemicals. On the other hand, rapid fabrication is required by researchers designing such devices to test their functionality. Additive manufacturing technology such as the inkjet printing of conductive material is one potential solution to achieve that objective. In this study, we report the utilisation of inkjet printing for the rapid prototyping of alternating current (AC) electrokinetic devices on a rigid glass substrate. The non-lithographical and vacuum-free process for the fabrication of a microfluidic device was demonstrated. The smallest feature size of 60 μm was successfully printed. The crystalline structure of the printed material under different curing temperatures was characterised. It was found that these treatment conditions affect electrical conductivity. Although a low-temperature sintering process was applied, low resistivity was obtained. An AC electrokinetics device for the manipulation of microparticles has been prepared to illustrate such printed silver micro-patterns. The results strongly support the idea that inkjet printing is a powerful and cost-effective prototyping tool for researchers who work with electrokinetic devices. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Published version

Inkjet printing is a powerful and cost-effective technique for deposition of liquid inks with high accuracy, which is not only of great significance for graphic applications but also has enormous potential for the direct printing of optoelectronic devices. This review highlights a comprehensive overview of the progress that has been made in optoelectronics fabrication by the inkjet printing technique. The first part briefly covers the droplet-generation process in the nozzles of printheads and the physical properties affecting droplet formation and the profiles of the printed patterns. The second section outlines the recent activities related to applications of inkjet printing in optoelectronics fabrication including solar cells, light-emitting diodes, photodetectors and transparent electrodes. In each application field, the challenges with the inkjet printing process and the possible solutions are discussed before a few remarks. In the last section, a brief summary on the progress of inkjet printing fabrication of optoelectronics and an outlook for future research effort are presented.

Zinc oxide (ZnO) has gained much attention recently due to its excellent physical and chemical properties, and has been extensively studied in energy harvesting applications such as photovoltaic and piezoelectric devices. In recent years, its reversible wettability has also attracted increasing interest. The wettability of ZnO nanostructures with various morphologies has been studied. However, to the best of our knowledge, there is still a lack of investigations on further modifications on ZnO to provide more benefits than pristine ZnO. Comprehensive studies on the reversible wettability are still needed. In this study, a ZnO nanorod array was prepared via a hydrothermal process and subsequently coated with thin gold layers with varied thickness. The morphologies and structures, optical properties and wettability were investigated. It is revealed that the ZnO-Au system possesses recoverable wettability upon switching between visible-ultraviolet light and a dark environment, which is verified by the contact angle change. The introduction of the thin gold layer to the ZnO nanorod array effectively increases the recovery rate of the wettability. The improvements are attributed to the hierarchical structures, which are formed by depositing thin gold layers onto the ZnO nanorod array, the visible light sensitivity due to the plasmonic effect of the deposited gold, as well as the fast charge-induced surface status change upon light illumination or dark storage. The improvement is beneficial to applications in environmental purification, energy harvesting, micro-lenses, and smart devices.

Fabrication of small-sized patterns of inorganic semiconductor onto flexible substrates is a major concern when manufacturing wearable devices for measuring either biometric or environmental parameters. In this study, micro-sized flexible ZnO UV photodetectors have been thoroughly prepared by a facile inkjet printing technology and followed with heat treatments. A simple ink recipe of zinc acetate precursor solution was investigated. It is found that the substrate temperature during zinc precursor ink depositing has significant effects on ZnO pattern shape, film morphology, and crystallization. The device fabricated from the additive manufacturing approach has good bendability, Ohmic contact, short response time as low as 0.3 s, and high on/off ratio of 3525. We observed the sensor's dependence of response/decay time by the illuminating UV light intensity. The whole process is based on additive manufacturing which has many benefits such as rapid prototyping, saving material, being environmentally friendly, and being capable of creating high-resolution patterns. In addition, this method can be applied to flexible substrates, which makes the device more applicable for applications requiring flexibility such as wearable devices. The proposed all-inkjet-printing approach for a micro-sized ZnO UV photodetector would significantly simplify the fabrication process of micro-sized inorganic semiconductor-based devices. A potential application is real-time monitoring of UV light exposure to warn users about unsafe direct sunlight to implement suitable avoidance solutions.