Advancements in transfer printing techniques for flexible electronics:adjusting interfaces and promoting versatility

The burgeoning interest in flexible electronics necessitates the creation of patterning technology specifically tailored for flexible substrates and complex surface morphologies.Among a variety of patterning techniques,transfer printing emerges as one of the most efficient,cost-effective,and scalable methods.It boasts the ability for high-throughput fabrication of 0–3D micro-and nano-structures on flexible substrates,working in tandem with traditional lithography methods.This review highlights the critical issue of transfer printing:the flawless transfer of devices during the pick-up and printing process.We encapsulate recent advancements in numerous transfer printing techniques,with a particular emphasis on strategies to control adhesion forces at the substrate/device/stamp interfaces.These strategies are employed to meet the requirements of competing fractures for successful pick-up and print processes.The mechanism,advantages,disadvantages,and typical applications of each transfer printing technique will be thoroughly discussed.The conclusion section provides design guidelines and probes potential directions for future advancements.

Near-zero-adhesion-enabled intact wafer-scale resist-transfer printing for high-fidelity nanofabrication on arbitrary substrates

There is an urgent need for novel processes that can integrate different functional nanostructures onto specific substrates,so as to meet the fast-growing need for broad applications in nanoelectronics,nanophotonics,and fexible optoelectronics.Existing direct-lithography methods are difficult to use on fexible,nonplanar,and biocompatible surfaces.Therefore,this fabrication is usually accomplished by nanotransfer printing.However,large-scale integration of multiscale nanostructures with unconventional substrates remains challenging because fabrication yields and quality are often limited by the resolution,uniformity,adhesivity,and integrity of the nanostructures formed by direct transfer.Here,we proposed a resist-based transfer strategy enabled by near-zero adhesion,which was achieved by molecular modification to attain a critical surface energy interval.This approach enabled the intact transfer of wafer-scale,ultrathin-resist nanofilms onto arbitrary substrates with mitigated cracking and wrinkling,thereby facilitating the in situ fabrication of nanostructures for functional devices.Applying this approach,fabrication of three-dimensional-stacked multilayer structures with enhanced functionalities,nanoplasmonic structures with~10 nm resolution,and MoS2-based devices with excellent performance was demonstrated on specific substrates.These results collectively demonstrated the high stability,reliability,and throughput of our strategy for optical and electronic device applications.

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems,including flexible electronics,stretchable electronics,and micro light-emitting diode(LED)displays.Here we report a design of switchable dry adhesive based on shape memory polymer(SMP)with hemispherical indenters,which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design,fabrication,and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects(e.g.,silicon wafers),three-dimensional(3D)objects(e.g.,stainless steel balls),and rough objects(e.g.,frosted glasses)in two-dimensional(2D)or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

Properties of Novel Reactive Transfer Printing of Silk

The novel reactive transfer printing of silk was carried out through a hot-press adhesion and steaming. The special transfer paper was prepared by coating the paste mainly containing hot-melt adhesive hlgh-substituted hydroxypropyl cellulose （H-HPC） and printing thickener earboxymethyl cellulose （CMC）. The effects of each ingredient in the paste on color yield of the prints and dye penetration were investigated. The major results indicate that, color yield is chiefly governed by the adhesion extent imparted by H-HPC, the type of fixing alkaline agent, and the content of urea. Trichloroacetic acid （TCAA） as the fixing alkaline agent and adding 5% urea can enhance the color depth obviously. Dye penetration depends on the coating quantity on the transfer paper, the contents of urea and dicyandiamide. The printed silk possesses a higher color yield, color fastness of grade 3 or above, clear sharpness, and good handle when the paste contains 3 % H-HPC, 0. 7 % CMC, 3 % TCAA, 5 % urea, 3 % dicyandiamide, and 0. 5 % physical sorbent nano-silica.

A thermal actuated switchable dry adhesive with high reversibility for transfer printing

Transfer printing based on switchable adhesive that heterogeneously integrates materials is essential to develop novel electronic systems,such as flexible electronics and micro LED displays.Here,we report a robust design of a thermal actuated switchable dry adhesive,which features a stiff sphere embedded in a thermally responsive shape memory polymer(SMP)substrate and encapsulated by an elastomeric membrane.This construct bypasses the unfavorable micro-and nano-fabrication processes and yields an adhesion switchability of over1000 by combining the peel-rate dependent effect of the elastomeric membrane and the thermal actuation of the sub-surface embedded stiff sphere.Experimental and numerical studies reveal the underlying thermal actuated mechanism and provide insights into the design and operation of the switchable adhesive.Demonstrations of this concept in stamps for transfer printing of fragile objects,such as silicon wafers,silicon chips,and inorganic micro-LED chips,onto challenging non-adhesive surfaces illustrate its potential in heterogeneous material integration applications,such as flexible electronics manufacturing and deterministic assembly.

Efficient nonfullerene organic solar cells with active layers fabricated by water transfer printing

Preparation of high-quality films plays an important role to achieve high-performance nonfullerene (NF) organic solar cells. NF active layer films are typically fabricated by spin coating. Novel fabrication methods to process the NF active layer are desirable to be compatible with large-area production. Herein, we report on the fabrication of NF active layer films via a water transfer printing method.This method delivers a uniform film with controllable film thicknesses. NF active layers of PDBD-T:ITIC and PBDB-T-2F:IT-4F were fabricated via the method to validate its effectiveness. Solar cells with the water transfer-printed active layers show comparable performance (up to 11.7%) to the cells with spin-coated active layers. Furthermore, NF solar modules containing 4-sub cells with the active area of 3.2 cm2 are also fabricated via the method. The module shows VOC of up to 3.4 V and a power conversion efficiency of 8.1% with the PBDB-T-2F:IT-4F active layer.

Regulatable interfacial adhesion between stamp and ink for transfer printing

As an emerging processing technology,transfer printing enables the assembly of functional material arrays(called inks)on various substrates with micro/nanoscale resolution and has been widely used in the fabrication of flexible electronics and display systems.The critical steps in transfer printing are the ink pick-up and printing processes governed by the switching of adhesion states at the stamp/ink interface.In this review,we first introduce the history of transfer printing in terms of the transfer methods,transferred materials,and applications.Then,the fundamental characteristics of the transfer printing system and typical strategies for regulating the stamp/ink interfacial adhesion strength are summarized and exemplified.Finally,future challenges and opportunities for developing the novel stamps,inks,and substrates with intelligent adhesion capability are discussed,aiming to inspire the innovation in the design of transfer printing systems.

Highly durable machine-learned waterproof electronic glove based on low-cost thermal transfer printing for amphibious wearable applications

Gesture recording,modeling,and understanding based on a robust electronic glove(E-glove)are of great significance for efficient human-machine cooperation in harsh environments.However,such robust edge-intelligence interfaces remain challenging as existing E-gloves are limited in terms of integration,waterproofness,scalability,and interface stability between different components.Here,we report on the design,manufacturing,and application scenarios for a waterproof E-glove,which is of low cost,lightweight,and scalable for mass production,as well as environmental robustness,waterproofness,and washability.An improved neural network architecture is proposed to implement environment-adaptive learning and inference for hand gestures,which achieves an amphibious recognition accuracy of 100%in 26 categories by analyzing 2,600 hand gesture patterns.We demonstrate that the E-glove can be used for amphibious remote vehicle navigation via hand gestures,potentially opening the way for efficient human-human and human-machine cooperation in harsh environments.

Nanocellulose-based reusable liquid metal printed electronics fabricated by evaporation-induced transfer printing

Reusable electronics have received widespread attention and are urgently needed. Here, nanocellulosebased liquid metal(NC-LM) printed circuit has been fabricated by the evaporation-induced transfer printing technology. In this way, the liquid metal pattern is embedded into the nanocellulose membrane, which is beneficial for the stability of the circuit during use. Besides, the NC-LM circuit is ultrathin with just tens of microns. In particular, the finished product is environmentally friendly because it can be completely dissolved by water, and both the liquid metal ink and the nanocellulose membrane can be easily recollected and reused, thereby reducing waste and pollution to the environment. Several examples of flexible circuits have been designed to evaluate their performance. The mechanism of evaporation-induced transfer printing technology involves the deposition, aggregation, and coverage tightly of the nanosized cellulose fibrils as the water evaporated. This study provides an economical and environmentally friendly way for the fabrication of renewable flexible electronics.

Effect of Ink Molecular Weights and Annealing Conditions on Molecular Transfer Printing

The molecular transfer printing（MTP） technique has been invented to fabricate chemical patterns with high fidelity using homopolymer inks. In this work, we systematically studied the effects of the molecular weights of homopolymer inks and transfer conditions on the MTP process. We explored a large range of molecular weights（~3.5-56 kg·mol^（-1）） of hydroxyl-terminated polystyrene（PS-OH） and hydroxyl-terminated poly（methyl methacrylate）（PMMA-OH） in the MTP process, and found that the resulting chemical patterns on replicas from all five blends were functional and able to direct the assembly of films of the same blends. The transfer temperature and the film annealing sequences had an impact on the MTP process. MTP was sensitive to the transfer temperature and could only be performed within a certain temperature range, i.e. higher than the glass transition temperature（T_g） of copolymers and lower than the rearrangement temperature of the assembled domains. Pre-organization of the blend films was also necessary for MTP since the preferential wetting of PMMA domains at the replica surface might result in the formation of a PMMA wetting layer to prevent the presentation of underlying chemical patterns to the replica surface.

3D打印矩形粗糙通道内火箭煤油流动换热特性试验方法研究

为探究3D打印再生冷却通道在液体火箭发动机推力室中的替代应用特性,研制了具有不同内表面粗糙度的正弦波纹结构3D打印304不锈钢矩形通道。内截面名义尺寸为2.0 mm×2.0 mm,设计粗糙度分别为6.3、25.0、100.0μm,实际粗糙度Ra分别为11.88、12.70、17.53μm,通过将高温电阻率法和像素法相结合获得了3D打印通道的实际内径和壁厚,修正了火箭煤油流动换热的内壁温和热流密度,建立了3D打印粗糙通道内火箭煤油流动换热特性试验研究方法。试验参数如下:压力处于15~20 MPa范围、质量流速在12450~24900 kg·m^(-2)·s^(-1)之间、热流密度为5~15 MW·m^(-2)、流体温度为-150℃。研究结果表明:火箭煤油流动换热特性受到热流密度、流体温度和质量流速的影响;流体温度处于50~135℃范围内,换热系数增加约25%~33%;热流密度处于5.0~15.0 MW·m^(-2)范围内,换热系数增加了8.3%;质量流速为12450~24900 kg·m^(-2)·s^(-1)范围内,换热系数增加了60.2%。粗糙度增加对火箭煤油流动换热起到强化作用,粗糙度从11.88μm增加到17.53μm时,换热强化幅度超过20%以上。该研究可为3D打印通道在火箭发动机推力室中的替代应用提供参考。

Mass transfer techniques for large-scaleand high-density microLED arrays

Inorganic-based micro light-emitting diodes (microLEDs) offer more fascinating properties and unique demands in next-generation displays. However, the small size of the microLED chip (1–100 µm) makes it extremely challenging for high efficiency and low cost to accurately, selectively, integrate millions of microLED chips. Recent impressive technological advances have overcome the drawbacks of traditional pick-and-place techniques when they were utilized in the assembly of microLED display, including the most broadly recognized laser lift-off technique, contact micro-transfer printing (µTP) technique, laser non-contact µTP technique, and self-assembly technique. Herein, we firstly review the key developments in mass transfer technique and highlight their potential value, covering both the state-of-the-art devices and requirements for mass transfer in the assembly of the ultra-large-area display and virtual reality glasses. We begin with the significant challenges and the brief history of mass transfer technique, and expand that mass transfer technique is composed of two major techniques, namely, the epitaxial Lift-off technique and the pick-and-place technique. The basic concept and transfer effects for each representative epitaxial Lift-off and pick-and-place technique in mass transfer are then overviewed separately. Finally, the potential challenges and future research directions of mass transfer are discussed.

Direct Patterning of Carbon Nanotube via Stamp Contact Printing Process for Stretchable and Sensitive Sensing Devices

Flexible and wearable sensing devices have broad application prospects in bio-monitoring such as pulse measurement,motion detection and voice recognition.In recent years,many significant improvements had been made to enhance the sensor’s performance including sensitivity,flexibility and repeatability.However,it is still extremely complicated and difficult to prepare a patterned sensor directly on a flexible substrate.Herein,inspired by typography,a lowcost,environmentally friendly stamping method for the mass production of transparent conductive carbon nanotube(CNT)film is proposed.In this dry transfer strategy,a porous CNT block was used as both the seal and the ink;and Ecoflex film was served as an object substrate.Welldesigned CNT patterns can be easily fabricated on the polymer substrate by engraving the target pattern on the CNT seal before the stamping process.Moreover,the CNT film can be directly used to fabricate ultrathin(300μm)strain sensor.This strain sensor possesses high sensitivity with a gauge factor(GF)up to 9960 at 85%strain,high stretchability(>200%)and repeatability(>5000 cycles).It has been used to measure pulse signals and detect joint motion,suggesting promising application prospects in flexible and wearable electronic devices.

A single micro-LED manipulation system based on micro-gripper

Micro-LEDs(μLEDs)have advantages in terms of brightness,power consumption,and response speed.In addition,they can also be used as micro-sensors implanted in the body via flexible electronic skin.One of the key techniques involved in the fabrication ofμLED-based devices is transfer printing.Although numerous methods have been proposed for transfer printing,improving the yield ofμLED arrays is still a formidable task.In this paper,we propose a novel method for improving the yield ofμLED arrays transferred by the stamping method,using an innovative design of piezoelectrically driven asymmetric micro-gripper.Traditional grippers are too large to manipulateμLEDs,and therefore two micro-sized cantilevers are added at the gripper tips.AμLED manipulation system is constructed based on the micro-gripper together with a three-dimensional positioning system.Experimental results using this system show that it can be used successfully to manipulateμLED arrays.

DTF热转印工艺特点及质量控制因素

DTF热转印工艺是近年来发展较快的新工艺。在对DTF工艺及产品长期实践研究的基础上,文章从机理上分析DTF工艺的工艺特点、控制条件,以及操作步骤对工艺质量的影响。

特斯拉阀通道结构毛细芯对环路热管启动性能的影响

受制于3D打印技术的制造精度,其制备的可用毛细芯孔径约为300μm。基于该毛细芯的环路热管(loop heat pipe,LHP)启动初期会产生蒸汽反向穿透毛细芯的现象,造成严重的热泄漏,对LHP的启动性能产生极大的消极影响。为了解决这个问题,本文开创性地将具有单向流动特性的特斯拉阀结构引入3D打印毛细芯中,设计制备了孔径为300μm的特斯拉阀通道结构毛细芯,并与相同孔径的圆柱直通孔毛细芯分别进行了渗透率以及在不同低功率(20W、40W、60W)下启动性能的对比。结果表明:3D打印特斯拉阀毛细芯具有明显的单向流动特征。其渗透率在流动方向上存在明显的差异,正向流动渗透率大于反向流动的渗透率;特斯拉阀毛细芯在不同功率下均能有效抑制蒸汽穿透,实现更快的启动;随着启动功率的增加,LHP的启动速度更快,特斯拉阀毛细芯对蒸汽穿透的抑制效果更明显,启动性能更好。

热转印系统色带传动过程张力分析与建模

为保证热转印过程中薄膜色带传动稳定进而实现高质量转印,建立正确的热转印色带传动张力模型十分关键。首先,研究了热转印色带传动系统组成及转印原理,并根据色带传动路径分辊间段、放卷段和收卷段3阶段对色带传动系统张力进行了建模与分析。然后,结合热转印色带卷材的黏弹性分析张力形成机制,改进了卷绕系统经典张力公式,建立了辊间段薄膜色带张力模型。针对传动系统的放卷区域,分析了摩擦对薄膜色带传动张力的影响,提出张力下降系数以评估张力损失。考虑色带张力非线性时变的特点,利用步进电动机负载模型求解收卷区域色带张力。最后,通过对比仿真与实验所得的色带张力变化曲线,验证了模型的准确性,为后续张力控制方案设计奠定了基础。

基于微转印技术的金刚石上硅材料制备与表征

硅基电子器件在高温、高功率、射频领域应用时面临热管理的挑战。为解决其散热和射频损耗问题,采用微转印技术在常温常压下将单晶硅薄膜转移至金刚石衬底上,制备出新型集成电路材料——金刚石上硅(SOD)。实验结果表明,转移的单晶硅薄膜表面平整、无明显损伤,保持了其原有的完整性。透射电子显微镜的观察结果进一步显示,金刚石与硅之间的界面结合紧密,不存在纳米级间隙。界面热阻测试结果显示,SOD样品的有效界面热阻(R_(EI))为30.30m^(2)·K/GW,明显优于SOI样品的R_(EI)(376.78m^(2)·K/GW)。该研究验证了SOD材料在热管理方面的潜力,也为未来新型硅基器件的设计与应用提供了参考。

基于胶膜转印的气动微阀及微流控芯片

微阀是微流控芯片的重要部件,然而现有的微阀存在加工复杂、不利于批量生产等缺点,限制了微流控芯片的应用范围。研发了基于胶膜转印的胶粘键合方法,采用聚甲基丙烯酸甲酯(PMMA)微流道和聚二甲硅氧烷(PDMS)柔性薄膜制备了带有气动微阀的微流控芯片。通过调节转印参数,在PDMS薄膜上可控地制备了厚度约为1μm胶膜,随后利用微米级的胶膜实现了PDMS薄膜与PMMA微流道层的胶粘键合,既避免了微流道的堵塞,又保证了微流道结构的精确性。设计并制备了由上层气体流道、中间柔性薄膜和下层液体流道构成的气动微阀,由上层气体流道内输入气压驱动PDMS薄膜向下层的液体流道变形以实现流体控制功能。构建了静力学模型分析微阀的关闭特性,根据薄膜变形的仿真结果优化微阀结构,确保微阀的有效截止。经测试,微阀具有优异的开关特性,关闭的响应时间为550ms,开启的响应时间为200ms。表征了不同气体和液体压强下的体积流量变化规律,验证了气动微阀可以实现对流体体积流量的精准调控。通过研发胶膜转印工艺实现了PMMA和PDMS的高效无损键合,开发了适合大规模制造的气动微阀,为微流控芯片进一步的实际应用开辟了新的途径。

植物蒸煮转印染在女装设计中的艺术表现与设计应用

如今,设计领域逐渐将关注点转向可持续发展,其中植物蒸煮转印染成为备受关注的传统手工艺。这一创新工艺不仅保留了植物染色的天然原料和生态理念,同时改良了传统染色方式。服装设计师运用该工艺的独特材质、鲜明色彩和图案,塑造出别具特色的服装。文章系统介绍了植物蒸煮转印染的方法和特点,结合实际案例深入探讨了其在女装设计中的应用,归纳总结了其在色彩、图案、面料和工艺等方面的表现。同时,运用现代设计手法将植物蒸煮转印染与服装设计相结合展示了其在服装设计中的多样性、实用性和审美价值,为相关时尚产品的开发提供了重要参考。