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Hydrogels with embedded functional particulates are widely used to create soft materials with innovative functionalities. In order to advance these soft materials to functional devices and machines, critical technical challenges are the precise positioning of particulates within the hydrogels and the construction of the hydrogels into a complex geometry. Inkjet printing is a promising method for addressing these challenges and ultimately achieving hydrogels with voxelized functionalities, so-called digital hydrogels. However, the development of the inkjet printing process primarily relies on empirical optimization of its printing and curing protocol. In this study, a general scaling law is proposed to predict the transport of particulates within the hydrogel during inkjet printing. This scaling law is based on a hypothesis that water-matrix interaction during the curing of inkjet-printed particle-laden polymeric drops determines the intra-drop particle distribution. Based on the hypothesis, a dimensionless similarity parameter of the water-matrix interaction is proposed, determined by the hydrogel's water evaporation coefficient, particle size, and mechanical properties. The hypothesis was tested by correlating the intra-drop particle distribution to the similarity parameter. The results confirmed the scaling law capable of guiding ink formulation and printing and curing protocol.

We studied the effect of current supply duration at final-step currents during the stepwise electrical sintering of silver (Ag) nanoparticles (NPs). Ag NPs ink was inkjet-printed onto Eagle-XG glass substrates. Constant final-step currents of 0.4 and 0.5 A with various time intervals were applied to the printed samples. The final-step current of 0.5 A damaged the line at a comparatively shorter time duration. On the other hand, the lower final-step current of 0.4 A prevented the line damage at longer time durations while producing comparatively lower Ag NPs specific resistance. The minimum specific resistances of the printed samples sintered at 0.4 and 0.5 A were 3.59 μΩ∙cm and 3.79 μΩ∙cm, respectively. Furthermore, numerical temperature estimation and scanning electron microscope (SEM) analysis were conducted to elaborate on the results. The numerical temperature estimation results implied that the lower estimated peak temperature at the final-step current of 0.4 A helped prevent Ag NP line damage. The SEM micrographs suggested that a high surface porosity—caused by higher sintering peak temperatures—in the case of the 0.5 A final-step current resulted in a comparatively higher Ag NP line-specific resistance. This contribution is a step forward in the development of Ag NP sintering for printed electronics applications.

Owing to its unique properties, silver (Ag) in the form of nanoparticle (NP) ink promises to play a vital role in the development of printed and flexible electronics. Once printed, metal NP inks require a thermal treatment process called sintering to render them conductive. Among the various methods, electrical sintering is a highly selective and rapid sintering method. Here, we studied the electrical sintering of inkjet-printed Ag NP lines via a stepwise current increment sintering (SCIS) technique. In the SCIS technique, the supplied electric current was gradually increased in multiple steps from low electric currents to higher electric currents to avoid thermal damage to the printed Ag NP ink lines. In less than 0.15 s, a line resistivity as low as 6.8 μΩcm was obtained which was comparable with furnace sintered line resistivity of 6.13 μΩcm obtained at 250 °C in 600 s. Furthermore, a numerical model was developed for the SCIS process temperature estimation. The results enabled us to elaborate on the relationship between the Ag NP line resistivity and the process temperature under various electric currents. Under the applied SCIS technique, a stable sintering process was carried out avoiding the conductive ink line and substrate damage.

Silver nanowires (Ag NWs) have been in the spotlight as a nanomaterials for roll-to-roll printed stretchable electrode because of excellent mechanical properties. Electrical conductivity of Ag NWs was affected by the nanowire orientation. Here, we printed Ag NWs ink with the drop-on-demand dispenser and stretched the Ag NWs electrode on silicone rubber and thermoplastic polyurethane (TPU) up to 140% in length. The averaged resistance at released state of 50 test samples of Ag NWs electrodes on silicone rubber film after 10% stretching test was approximately 14% lower than before stretching test. During the stretching test, both electrodes and film were stretched and strain was induced. However, the resistance at released state of Ag NWs electrodes on TPU film was increased as strain increased. The Ag NWs alignment to the stretching direction was observed on silicone rubber film by FESEM images, but alignment of Ag NWs on TPU film by stretching did not occur. Because the adhesion class (ASTM D 3359) of Ag NWs electrodes with the silicone rubber film is 1B and TPU film is 5B, Ag NWs could slide and rotate on silicone rubber film as applying the strain. We confirmed that alignment of Ag NWs on the silicone rubber film by stretching resulted in reduction of electrical resistance.

In printed electronics, laser ablation is used to repair defective patterns on transparent, flexible, and thin films, using high-power lasers. The distance between the film surface and laser focus is sensitive to changes as the narrow focus depth of the lens is the range of tens of microns. However, a film fixed on a conductive vacuum chuck (CVC) is always curved, owing to chucking bending, thus, laser focusing must be locally performed before ablation. Therefore, this study proposes a non-contact measurement method for the surface flatness of a transparent and thin film, to compensate for laser defocusing in a large area. The surface flatness was obtained using camera-focus points on the porous surface of the CVC. The focus points were interpolated to achieve a smooth and continuous surface flatness for chucking bending. A laser distance sensor was used to verify the surface flatness from the proposed method. The surface flatness was used to inspect the printed patterns, and to perform laser ablation on the film. The proposed method is advantageous for large-area laser ablation and is expected to become indispensable for repairing machines in printed electronics.

The temperature effect on the electrical characteristics and surface morphology of printed silver ink was investigated for laser irradiation with various laser scanning speeds and intensities. Inkjet-printed silver lines onto a glass substrate were irradiated with a 532 nm continuous wave. The electrical resistance was measured after laser sintering with various scanning speeds and intensities. The temperature field was invoked in the printed silver ink line by laser scanning sintering and this was predicted by numerical analysis. This temperature field show a quasi-steady state behavior since laser irradiation can be considered as a moving heat source. Field emission scanning electron microscope images were then examined with calculated temperatures of printed silver ink. The relation of surface morphology with temperature field were discussed. The effect of a laser scanning speed on the specific resistance of the conductive ink line was intensively investigated.

Hydrogel-based soft materials have been used in numerous applications in healthcare, food, pharmaceutical, and cosmetic industries. Manufacturing hydrogels whose functional properties and compositions are voxelized at superior spatial resolutions can significantly improve current applications as well as will enable a new generation of soft materials. However, it remains challenging to control the structure and composition of soft materials reliably. In this context, the drop-on-demand (DOD) printing of hydrogels shows excellent potential to address this manufacturing challenge. Despite this potential, a lack of mechanistic understanding of the behavior of printed hydrogel drops makes it challenging to design and optimize DOD printing protocols for a wide variety of hydrogels. In particular, the curing of hydrogel drops, which requires dehydration of printed hydrogel drops, is poorly understood. In this study, thus, a hypothesis was postulated and tested that water-matrix interaction at drop-drop interfaces during curing processes determine the quality of hydrogels printed. Both computational and experimental studies were performed to establish a mechanism of the water-matrix interaction within printed hydrogel drops. The results were further discussed to establish a dimensionless similarity parameter that can characterize water transports during the hydrogel dehydration process.

In this study, the sintering behavior of silver (Ag) nanoparticle inks was investigated when different particle sizes and sintering methods were employed. Ag nanoparticle inks with two different particle sizes were investigated with average particle diameters of 12 nm and 50 nm. The two kinds of Ag nanoparticle inks were inkjet-printed onto glass substrates. The printed inks were sintered by a furnace and a continuous wave (CW) laser at a wavelength of 532 nm. The specific resistance and surface morphology of the Ag nanoparticle (Ag NP) inks were investigated under various furnace temperatures, laser intensities, and time durations. The ex situ specific resistance of each Ag NP ink was measured by a multimeter to determine the effects of various sintering conditions such as the furnace temperature, laser intensity, and sintering duration. To investigate the correlation between the specific resistance and surface morphology of the Ag NP inks, field emission scanning electron microscopy images were obtained. In addition, the effect of the particle size on the specific resistances of the inks annealed by the furnace and CW laser was evaluated.

In printed electronics applications, specific resistances of conductive lines are critical to the performance of the devices. The specific resistance of a silver (Ag) nanoparticle electrode is affected by surface morphology of the layered nanoparticles which were sintered by the heat treatment after printing. In this work, the relationship between surface morphology and specific resistance was investigated with various sintering temperatures and various layer thicknesses of Ag nanoparticle ink. Ag nanoparticles with an average size of approximately 50 nm were spin-coated on Eagle XG glass substrates with various spin speed to change the layer thickness of Ag nanoparticles from 200 nm to 900 nm. Coated Ag nanoparticle layers were heated from 150°C to 450°C for 30 min in a furnace. The result showed that higher sintering temperature produces larger grains in an Ag layer and decreases specific resistance of the layer, but that the maximum allowable heating temperature is limited by the thickness of the layer. When grain size exceeded the thickness of the layer, the morphology of the Ag nanoparticles changed to submicron-sized islands and the Ag layers did not have electrical conductivity any more.

Steady-state thermal analysis was performed on a thermal equivalent circuit to determine the heat generation during operation of an interior permanent magnet synchronous motor (IPMSM). New machines must be compact and light and produce high torque density under extreme environmental conditions. Thermal analysis of an IPMSM is particularly important because excessive heat generated from the core and magnet reduces the IPMSM's output and has adverse effects on the durability. Therefore, steady-state thermal analysis of an IPMSM was performed for changes in the design variables using a thermal equivalent circuit. The changed variables were the axis length and thickness of the housing. The results of this method were compared with those of the finite element method to verify the accuracy and reliability. § 이 논문은 2014년도 대한기계학회 열공학부문 춘계학술 대회(2014. 4. 23.-26., 제주대) 발표논문임 † Corresponding Author, smoon@hanyang.ac.kr C 2014 The Korean Society of Mechanical Engineers 김태현 · 유영범 · 나종승 · 유경태 · 문윤재 · 이재헌 · 이 주 · 박찬배 · 문승재 952 소형 경량화가 가능하다는 장점을 가지고 있다. 하지만 IPMSM 의 영구자석은 재료의 희소성으 로 인한 비용문제와 온도에 민감하여 물질의 온도 상승으로 영구감자가 발생할 수 있으며, 결과적으 로 전동기 효율 감소로 이어질 수 있는 단점을 가 지고 있다. 또한 소형 경량화가 될수록 방열면적 이 줄어들어 고정자 권선의 높은 전류밀도로 인해 발생한 열로 인하여 전동기 내부의 온도가 높아진 다. 이로 인한 출력 감소뿐만 아니라 전동기의 수 명에 악영향을 미칠 수 있다. 이와 같은 문제를 해결하기 위해서는 운행 조건 중 발생하는 열 발 생량을 근거로 하여, 정확한 열 해석이 뒷받침 되 어야 한다. 전동기의 열 해석 방법에는 열등가회로법과 유 한요소법이 있다. 먼저 열등가회로법은 전동기의 국부적인 평균 온도와 전반적인 온도 분포 및 열 의 흐름을 쉽게 파악 할 수 있지만, 온도 구배가 큰 시스템의 경우 열등가회로망을 사용해서 구한 평균 온도 값의 오차가 커지는 한계를 가지고 있 다. 반면에 유한요소법은 전동기의 내부의 얇은 공극 부분에서 발생하는 많은 격자수로 인해 열 해석 시 시간과 노력이 많이 필요하다는 단점이 있지만, 정확한 온도 예측이 가능하다는 장점을 가지고 있다. 본 논문에서는 설계단계에서 다양한 설계 변수 의 변경에 따른 열 특성 영향을 빠르게 해석할 수 있는 열등가회로법을 이용하여 정상상태의 열해석 기법을 수행하였다. 설계단계에서 변경된 설계변 수는 하우징의 축방향 길이와 두께를 변경하였다. 또한 해석 결과의 정확성과 신뢰성을 확보하기 위 해 유한요소법을 이용하여 얻은 결과와 비교하였 다.

In this work, the transient variation of a cross-sectional area of inkjet-printed silver nanoparticle ink was monitored as a function of temperature. The ink was composed of 34 wt% silver nanoparticles with an average grain size of 50 nm. In order to determine the relationship between shrinkage and electrical properties, the cross-sectional area and specific resistance change were measured with various sintering temperatures and times. The samples were sintered at 150, 200, and 250 °C for 60, 120, 300, 600, 1200, and 3000 s. The cross-sectional profile was investigated in order to calculate the cross-sectional area of the ink using a surface profiler. To examine the mass transfer in a conductive line with various sintering temperatures and times, the surface morphology was investigated via field-emission scanning electron microscopy. As the temperature increased, the shrinkage ratio of the ink's cross-sectional area increased. By investigating the relationship between specific resistance and shrinkage, the sintering mechanisms can be determined.

The morphologies of inkjet-printed narrow silver lines on fluorocarbon film-coated glass substrates were measured with varying contact angles and drop spacing to study the actual stability of line printing by using a practical inkjet system. From a practical stability point of view, three types of the lines were observed: stable, unstable, and meta-stable. The stable lines were free from any bulging or breaking; the unstable lines had repetitive and periodic instabilities; and the meta-stable lines had no repetitive instability but had irregular bulges that appear sparsely. Unstable line printing resulted from either the dynamic or static instability of bead flow, which arose when the pressure-driven bead flow was too large or too small compared with droplet deposition rate, respectively. Whether the printing would be stable or meta-stable was determined by the anti-bulging stability of the flow against other disturbances. The anti-bulging stability increased when the bead flow rate was balanced with the printing rate, whereas it decreased for the present system when the flow-balance became sensitive to drop spacing.

To determine the effect of temperature on the specific electrical conductance of inkjet-printed ink during continuous wave laser sintering, the temperature of the sintered ink was estimated. The ink, which contained 34 wt.% silver nanoparticles with an average size of approximately 50 nm, was inkjet-printed onto a liquid crystal display glass substrate. The printed ink was irradiated with a 532 nm continuous wave laser for 60 s with various laser intensities. During laser irradiation, the in-situ electrical conductance of the sintered ink was measured to estimate the transient thermal conductivity of the ink. The electrical conductance and thermal conductivity of the ink was coupled to obtain the transient temperature by applying the Wiedemann–Franz law to a two-dimensional transient heat conduction equation. The electrical conductance of laser-sintered ink was highly dependent on the sintering temperature of the ink.

본 연구에서는 전도성 잉크의 레이저 열경화 공정시 은 나노입자 잉크의 레이저 열경화 공정 온도를 수치해석하였다. 유리기판 위에 잉크젯 프린팅을 이용하여 인쇄한 은 나노 입자 잉크를 532 nm 파장의 CW 레이저를 각기 다른 세기로 60 초 동안 조사하여 가열하였다. 온도계산을 위해서, 열생성항에 들어가는 반사율을 구하였고, 레이저 조사 중 실시간 은 나노입자 잉크의 비저항을 측정하였다. 온도 계산은 2 차원 열전도 방정식에 Wiedemann-Franz law 를 적용하였다. 그 결과, 레이저 조사로 인해 인쇄된 잉크의 온도가 상승할수록 비저항이 떨어지는 결과를 확인하였다.

There is little information on how the change in serum aminotransferase affects mortality. We investigated the association between changes in serum aminotransferase levels and mortality from all causes, cardiovascular disease (CVD), and liver disease. Three percent of men from the Korean National Health Insurance database were sampled randomly at the end of 2002. After excluding patients with cancer, CVD, CVD risk factors, or liver disease, those who participated in 2 consecutive rounds of the national health screening examination were included (n = 68,431). The primary outcome was CVD mortality. Secondary outcomes were liver disease mortality and all-cause mortality. Change in metabolic profiles was analyzed based on changes in liver enzyme levels. Elevated levels of serum aminotransferase were associated with CVD, liver disease, and all-cause mortality. Men who had sustained elevation of serum aminotransferase during 2 subsequent liver enzyme tests showed a significantly higher risk of CVD mortality (adjusted hazard ratio [aHR] 1.95; 95% confidence interval [CI] 1.07–3.56, 2.29; 1.27–4.12) than the sustained normal group. In contrast, the normalization group (aHR 1.52, 95% CI 0.82–2.81 for aspartate aminotransferase [AST]; aHR 1.35, 95% CI 0.70–2.61 for alanine aminotransferase [ALT]) and the new elevation group (aHR 1.27, 0.66–2.44 for AST; aHR 0.99, 95% CI 0.49–2.20 for ALT) were not different from the sustained normal group in CVD mortality. Individuals with serum aminotransferase elevation, particularly when sustained, are at higher risk of mortality, and should receive appropriate medical attention.

The variation in electric conductivity was examined for laser irradiation with various beam intensities. A 532-nm continuous wave laser was irradiated onto inkjet-printed silver lines on a glass substrate and the electrical resistance was measured in situ during the irradiation. The results demonstrate that electrical conductivity varies nonlinearly with laser intensity, and has a minimum specific resistance of 3.1 x 10(-8) Ωm at 4 kW/cm2 irradiation. These results are interesting because the specific resistance achieved by the present laser irradiation was approximately 1.9 times lower than the best value obtainable by oven heating, even though it was still higher by 1.9 times than that of bulk silver. It is also demonstrated that the irradiation time required to complete the sintering process decreases with laser intensity. The numerical simulation of laser heating shows that the heating temperature could be as high as 250 degrees C for laser sintering, while it is limited to 250 degrees C for oven sintering. The characteristics of sintering with laser intensity based on the results of field emission scanning electron microscope images are discussed.

In this work, the in-situ properties of silver nanoparticle ink were estimated during laser sintering process. The silver nanoparticle ink was composed of 34 wt% silver nanoparticles with an average size of approximately 50 nm, and was deposited on a glass substrate via inkjet printing technology. A 532 nm continuous-wave laser was irradiated to the printed ink for 60 s under various laser intensities. During the laser irradiation, the in-situ electrical conductance of the sintered ink was measured to obtain the transient thermal conductivity of the silver nanoparticle ink using the Wiedemann Franz law. The 2-dimensional, transient heat-conduction equation was calculated to obtain the transient temperature of the silver nanoparticle ink. By coupling the calculated temperature with the measured, transient electrical conductance, the transient thermal conductivity of the ink during the laser sintering process was derived in the calculation. The calculated thermal conductivity of the ink sintered at a laser intensity of 467.9 W/cm2 with 598 K is 355.5 W/mK, which is 86.4% of the thermal conductivity of bulk silver, 411.4 W/mK, at that tempearture. The difference resulting from the porosity of the sintered ink has an effect on the thermal conductivity of the sintered ink.

Industrial solar cell fabrication generally adopts printing process to deposit the front electrodes, which needs additional heat treatment after printing to enhance electrical conductivity. As a heating method, laser irradiation draws attention not only because of its special selectivity, but also because of its intense heating to achieve high electric conductivity which is essential to reduce ohmic loss of solar cells. In this study, variation of electric conductivity was examined with laser irradiation having various beam intensity. 532 nm continuous wave (CW) laser was irradiated on inkjet-printed silver lines on glass substrate and electrical resistance was measured in situ during the irradiation. The results demonstrate that electric conductivity varies nonlinearly with laser intensity, having minimum specific resistance of 4.1 x 10 -8 Ωm at 529 W/cm 2 irradiation. The results is interesting because the specific resistance achieved by the present laser irradiation was about 1.8 times lower than the best value obtainable by oven heating, even though it was still higher by 2.5 times than that of bulk silver. It is also demonstrated that the irradiation time, needed to finish sintering process, decreases with laser intensity. The numerical simulation of laser heating showed that the optimal heating temperature could be as high as 300 oC for laser sintering, while it was limited to 250 oC for oven sintering. The nonlinear response of sintering with heating intensity was discussed, based on the results of FESEM images and XRD analysis.

Objectives The P300 is a useful psychophysiological index that reflects cognitive functions; however, the relationship between P300 indices and neuropsychological tests in Alzheimer's disease (AD) patients is unclear. Methods Thirty-one AD patients and 31 elderly normal control (NC) subjects were recruited. Age and education level were matched between the two groups. The relationship between the P300 and the Korean version of the Consortium to Establish a Registry for Alzheimer's disease (CERAD-K) assessment packet (including 11 neuropsychological tests) was examined in AD patients. Results Compared to the NC subjects, the AD patients exhibited significantly decreased P300 amplitudes; however, there was no significant difference between the two groups in terms of P300 latency. After a permutation-based correction for multiple tests, P300 amplitudes at the Cz and Pz electrodes were significantly correlated with performance on the word list recognition, constructional praxis, and word fluency neuropsychological tests in the AD patients. Additionally, P300 latencies at the Pz and C6 electrodes were also significantly correlated with performance on the Mini-Mental State Examination, CERAD-K version (MMSE-K), and Trail Making Test part A (TMT-A) neuropsychological tests in the AD patients. Conclusions The results suggest that the P300 is responsive to the deterioration of language, memory, and executive functions observed in AD patients. Although there was no significant difference between the AD patients and NC subjects in the P300 latency, P300 latency has been shown to reflect impaired global cognition and attention deficits associated with AD. Our results suggest that P300 indices could be used as biological markers that indicate impaired neuropsychological functions in AD patients.

Electrical sintering of the front electrode for crystalline silicon solar cells was performed applying a constant DC current to the printed lines. Conducting lines were printed on glass substrate by a drop-on-demand (DOD) inkjet printer and silver nanoparticle ink. Specific resistance and microstructure of sintered silver lines and were measured with varying DC current. To find the relation between temperatue increase with changing applied cuurent and specific resistance, temperature elevation was also calculated. Sintering process finished within a few milliseconds. Increasing applied DC current, specific resistance decreased and grain size increased after sintering. Achieved minimum specific resistance is approximately 1.7 times higher than specific resistance of the bulk silver.

Laser sintering of inkjet-printed silver nanoparicles was studied. Conducting lines were printed on glass and PET substrates of various thicknesses by a drop-n-demand (DOD) inkjet printer and silver nanoparticle ink. Selective sintering of the printed silver nanopartilces was then performed using laser beam to enhance electrical conductivity. Focused laser beam was irradiated normally to the silver layer printed on the transparent substrates with various beam intensity and various irradiation time. Conductivity and morphology of the laser sintered silver patterns were measured and compared with the results of oven-heated sintering. Temperature elevation by laser irradiation was also calculated based on laser beam intensity, irradiation time, surface reflectivity, and thermophysical property of a substrate to estimate enhancement of the electrical conductivity by laser sintering.

An innovative electrical current sintering technique is applied to joule-heat conductive ink by increasing current gradually through a stepwise-form. This stepwise electrical sintering technique is devised to overcome thermal damage of printed conductive ink line during electrical sintering due to its high initial resistance. To monitor a stepwise electrical sintering method, in-situ specific resistance was measured. Surface morphology of the sample was observed by field-emission scanning electron microscope. By increasing the current gradually, the conductive line can endure higher current because the specific resistance has dropped gradually during the process. Finally, enhanced final-step current produces lower specific resistance of the conductive line than that obtained from a constant current-supplying electrical sintering method without damaging printed conductive line.