This review presents the recent development of printed gas sensors based on functional inks.

and intelligent applications including personalized feedback therapy, fast speech, and visual recognition. [1][2][3][4][5] In particular in the non-conventional space of smart applications requiring conformal attachment on non-flat surfaces such as on-body wearables, the notion of system on plastics (SOP) incorporating neuromorphic computing provides a potential solution. [1,2] To build such a flexible neuromorphic system, the fabrication of memristors equipped with synaptic functions is a key step to forming the artificial neural network. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] However, current memristor manufacturing technologies such as chemical vapor deposition (CVD), [7,8,[11][12][13] spin-coating, [14,15] or entire transfer [16] impose enormous challenges on flexible substrates as they suffer from high temperature, low yield and complex sacrificial layer removal. Efforts in finding low temperature fabrication technique and robust resistive switching (RS) material are essential to equip the SOP with the data storage and processing capability demanded by target applications. The printing technique-a forefront 3D monolithic integration approach-is suitable for high-volume, low-temperature manufacturing on non-conformal surfaces. [22][23][24][25][26][27][28] The printing technique is shown to offer more freedom in the design of Realization of memristors capable of storing and processing data on flexible substrates is a key enabling technology toward "system-on-plastics". Recent advancements in printing techniques show enormous potential to overcome the major challenges of the current manufacturing processes that require high temperature and planar topography, which may radically change the system integration approach on flexible substrates. However, fully printed memristors are yet to be successfully demonstrated due to the lack of a robust printable switching medium and a reliable printing process. An aerosol-jet-printed Ag/MoS 2 /Ag memristor is realized in a cross-bar structure by developing a scalable and low temperature printing technique utilizing a functional molybdenum disulfide (MoS 2 ) ink platform. The fully printed devices exhibit an ultra-low switching voltage (0.18 V), a high switching ratio (10 7 ), a wide range of tuneable resistance states (10-10 10 Ω) for multi-bit data storage, and a low standby power consumption of 1 fW and a switching energy of 4.5 fJ per transition set. Moreover, the MoS 2 memristor exhibits both volatile and non-volatile resistive switching behavior by controlling the current compliance levels, which efficiently mimic the short-term and longterm plasticity of biological synapses, demonstrating its potential to enable energy-efficient artificial neuromorphic computing.

In article number 1900740, Tawfique Hasan, Aaron Thean, Yong‐Wei Zhang, Kah‐Wee Ang, and co‐workers report the demonstration of flexible MoS2 memristive artificial synapses via scalable, low‐temperature aerosol‐jet printing. Fully printed memristors in a cross‐bar structure enable efficient emulation of synaptic plasticity functions with femtojoule energy consumption. This work paves the way toward realizing system‐on‐plastics for brain‐inspired neuromorphic computing.

Solution-processable thin-film dielectrics represent an important material family for large-area, fully-printed electronics. Yet, in recent years, it has seen only limited development, and has mostly remained confined to pure polymers. Although it is possible to achieve excellent printability, these polymers have low (≈2-5) dielectric constants (ε r). There have been recent attempts to use solution-processed 2D hexagonal boron nitride (h-BN) as an alternative. However, the deposited h-BN flakes create porous thin-films, compromising their mechanical integrity, substrate adhesion, and susceptibility to moisture. These challenges are addressed by developing a "one-pot" formulation of polyurethane (PU)-based inks with h-BN nano-fillers. The approach enables coating of pinhole-free, flexible PU+h-BN dielectric thin-films. The h-BN dispersion concentration is optimized with respect to exfoliation yield, optical transparency, and thin-film uniformity. A maximum ε r ≈ 7.57 is achieved, a twofold increase over pure PU, with only 0.7 vol% h-BN in the dielectric thinfilm. A high optical transparency of ≈78.0% (≈0.65% variation) is measured across a 25 cm 2 area for a 10 μm thick dielectric. The dielectric property of the composite is also consistent, with a measured areal capacitance variation of <8% across 64 printed capacitors. The formulation represents an optically transparent, flexible thin-film, with enhanced dielectric constant for printed electronics.

Printing has drawn a lot of attention as a means of low per-unit cost and high throughput patterning of graphene inks for scaled-up thin-form factor device manufacturing. However, traditional printing processes require a flat surface and are incapable of achieving patterning onto 3D objects. Here, a conformal printing method is presented to achieve functional graphene-based patterns onto arbitrarily shaped surfaces. Using experimental design, a water-insoluble graphene ink with optimum conductivity is formulated. Then single-and multilayered electrically functional structures are printed onto a sacrificial layer using conventional screen printing. The print is then floated on water, allowing the dissolution of the sacrificial layer, while retaining the functional patterns. The single-and multilayer patterns can then be directly transferred onto arbitrarily shaped 3D objects without requiring any postdeposition processing. Using this technique, conformal printing of single-and multilayer functional devices that include joule heaters, resistive deformation sensors, and proximity sensors on hard, flexible, and soft substrates, such as glass, latex, thermoplastics, textiles, and even candies and marshmallows, is demonstrated. This simple strategy promises to add new device and sensing functionalities to previously inert 3D surfaces.