Fig. 1. Laser hybrid fabrication based on laser additive, formative, and subtractive manufacturing and its applications in flexible micro-nano sensors^[32,35,51]

Fig. 2. Technical characteristics and typical target materials of laser additive manufacturing. (a) Schematic diagram of laser sintering process^[83]; (b) schematic diagram of changes in metal nano ink during laser sintering process^[83]; (c) Cu electrodes on PI film (inset: microscopic image)^[54]; (d) Ni conductive patterns on PET substrate (inset: magnified SEM image)^[55]; (e) flexible and conformally adhered Ag NP-based strain sensor (inset: optical image of conductive traces)^[23]; (f) optical micrograph of laser-sintered Au NP film (inset: magnified SEM image)^[84]; (g) Zn NP-based transient device array and grid pattern by laser digital printing and sintering (inset: optical image of Zn traces)^[85]; (h) photograph of a flexible and stretchable electronic device composed of a laser-sintered LM NP film deposited onto a flexible printed circuit board, where the laser-sintered patterns (light gray) are used to create conductive traces, resistors and capacitors between Cu traces and integrated circuits (inset: SEM images of representative areas of the as-printed and laser-sintered LM NP film)^[74]

Fig. 3. Technical characteristics and typical target materials of laser formative manufacturing. (a) Schematic of fabrication of amorphous carbon (AC) and LIG with different wavelengths of laser^[119]; (b) schematic of fabrication of LIG using various laser sources and carbon precursors; (c) morphological evolution of carbon precursor particles during laser carbonization of PI to LIG^[120]; (d) generation of LIG and its derivatives from carbon-based cloth precursors^[114,135]; (e) generation of LIG and its derivatives from carbon-based paper precursors^[139-141]; (f) generation of LIG and its derivatives from carbon-based food precursors^[114]; (g) generation of LIG and its derivatives from carbon-based polymer precursors^{[137,142-145]}; (h) generation of LIG and its derivatives from carbon-based wood precursors^[146-148]; (i) generation of LIG and its derivatives from carbon-based leaf precursors^[11,149]

Fig. 4. Laser subtractive manufacturing. Pyrolysis: (a) high-aspect-ratio 3D PDMS array structures fabricated by laser back surface scanning^[168]; (b) schematic diagram of a soft-robotic hand consisting of PDMS pneumatic fingers grabbing an object without gravitational resistance^[168]. Ablation: (c) femtosecond laser-processed hybrid superhydrophobic/-philic microporous SERS substrate for ultratrace molecular Raman detection^[170]; (d) UV laser engraving of PDMS textured surface^[32]. LIPSS: (e) femtosecond laser-induced formation of highly uniform and controllable photonic structures on an SOI surface^[191]; (f) 3D topography image of highly uniform subwavelength grating on an SOI device (inset: SEM image)^[191]. Cutting: (g) stretchable transparent Kirigami electrode by UV laser digital cutting^[199]; (h) Kirigami-structured stretchable strain sensor by laser-assisted fabrication^[200]

Fig. 5. Flexible physical sensors. (a) Schematic diagram of fabrication of highly sensitive skin sensor by laser-patterning and laser-induced crack generation^[205]; (b) illustration of measuring the epicentral motions of fingers, where the upper left image depicts the measurement of the topographical change of the wrist caused by the finger motions, and the lower right image shows the SEM image of the cracked region of the sensor, with a scale bar of 40 μm^[205]; (c) schematic of the strain sensors fabrication processes, including screen printing of Ag, laser direct writing to form graphene films and Kirigami structures, and Ecoflex passivation^[200]; (d) schematic of an integrated device with printed electrodes^[200]; (e) a device photo under bending^[200]; (f) upper image shows respiration monitoring during office work of a volunteer (insets: a photo of the device attached to the abdominal area and zoom-in of the resistance change results corresponding to breathing), and lower image shows the real-time averaged breathing period extracted from the upper image (inset: histogram of the breathing period for 2 h)^[200]; (g) laser fabrication process of the piezoresistive sensor material array with the serpentine interconnect^[206]; (h) top view of the active layer, including sensing pixels and insulating serpentine paths^[206]; (i) picture of the material array^[206]; (j) deformation distribution on active layers from the bottom view of the serpentine-interconnect model^[206]

Fig. 6. Flexible physical sensors. (a) Schematic of assembly of the capacitive pressure sensor^[209]; (b) schematic of the LM on the laser-structured dielectric layer^[209]; (c) enhancement principle of signal intensity^[209]; (d) capacitance curve of the sensor attached on the wrist when pouring the water on the sensor^[209]; (e) wearable highly sensitive all-flexible tactile sensor for real-time monitoring of mechanical signals from human wrist pulse, palm movement, elbow bending, knee bending, finger bending, and walking and running movement^[209]; (f) schematic of the fully soft self-powered vibration sensor (SSVS) showing its structural details and laser-assisted fabrication^[32]; (g) cross-sectional schematic of SSVS^[32]; (h) photo of the assembled SSVS, with a scale bar of 1 cm^[32]; (i) photo of a wireless data acquisition system and SSVS worn on a human arm^[32]; (j) measured signals of SSVS during daily activities and emergencies, all waveforms are displayed at the same magnification showing various height-width ratios^[32]; (k) schematic illustration of roll-to-roll fabrication^[48]; (l) design concept of a battery-less inductive-capacitive (LC) sensor that measures the humidity level inside a sealed package^[48]; (m) fabricated 3 cm×3 cm humidity sensor^[48]; (n) change in sensors resonant frequency of different sensors as a function of relative humidity (RH)^[48]; (o) the resonant frequency of the sensor can be remotely measured using an external coil connected to an RF reader^[48]

Fig. 7. Flexible chemical sensors. (a) Laser fabrication schematic of the all in one open-microfluidics multiplex biosensing platform^[221]; (b) functionalization schematic of the biosensing platform for ion and pesticide sensing^[221]; (c)(d)(e) open circuit potential response of the potassium ion-selective electrode (K⁺ ISE), nitrate ion-selective electrode (NO3- ISE), and ammonium ion-selective electrode (NH4+ ISE) to KCl, KNO₃, and NH₄Cl solutions, respectively, within the concentration range from 10^-6 to 10^-2 mol/L^[221]; (f) current vs time response of an enzymatic pesticide sensor to different concentrations of parathion^[221]; (g) schematic illustration of fabrication procedure of flexible ZnO gas sensor^[222]; (h) photograph of the flexible ZnO gas sensor^[222]; (i)(j) schematics of structure and fabrication process of multiplexed electrochemical sensors for gout management and evaluation of the body’s hydration status^[37]; (k) optical images of the electrochemical sensor integrated with a microfluidic network before and after bending^[37]; (l) comparison in the human sweat uric acid level under low- and high-purine diets before extensive exercise^[37]

Fig. 8. Flexible electrophysiological sensors. (a) Schematic illustration of the nature-inspired Kiri-Spider serpentine structural design and laser fabrication for electrophysiological sensing^[237]; (b) electrode patterns exhibiting high levels of deformability placed over a spherical surface^[237]; (c) layout of layer-by-layer design of the sensor^[237]; (d) sensor attached to the sticky, breathable and stretchable patch^[237]; (e) schematic and graphical representation of both proposed and conventional electrodes for measuring EMG, ECG and EOG^[237]; (f) UV laser ablation to pattern Al films followed by cutting the stack layer^[50]; (g) CO₂ laser to drill the holes in the patch^[50]; (h) optical image of the patch-based sensor array exhibiting flexible, stretchable, sticky, and conformal adhesion to human skin^[50]; (i) cyclic measurements under a maximum strain of 30%^[50]; (j) side view of working principle of 2.5D laser cutting^[245]; (k) gesture recognition using developed sensor sleeve and employed convolutional neural network (CNN)-based intention recognition algorithm^[245]

Fig. 9. Flexible multimodal sensors. (a) Enlarged schematic illustration of the wearable multifunctional system with double-sided device modules^[46]; (b) photograph of the multifunctional system in the gym, with a scale bar of 5 cm^[46]; (c) schematic illustration of the functions of the system during human sports^[46]; (d)‒(f) schematic illustration and optical images of the LIG-based self-powered, wireless, wearable sensing platform, with the energy harvested by TENGs from kinetic human motion to charge micro-supercapacitor arrays (MSCAs) as the sustained power supply for powering on-skin sensors, signal processing units, and wireless data transmission components^[47]; (g) optical image of the stretchable LIG-based top electrode with island-bridge layout in the TENG^[47]; (h) optical image of the stretchable LIG-based MSCAs attached on a human wrist^[47]; (i) conceptual illustration of the multifunctional wireless platform showing an exploded view of the different layers composing the platform containing the inertial, temperature, humidity, and breathing sensors^[247]; (j) optical photograph of the fabricated multifunctional platform (inset: the LM placed inside the curved patterned PDMS channel)^[247]; (k) real-time physical activity monitoring showing the raw output of resistance variation of the sensor when attached to the chest of a human subject performing tasks such as standing still, walking, jogging, laying down, and getting up^[247]; (l) real-time physical activity monitoring showing the raw output of resistance variation of the sensor when attached to the back of a legged robot performing movements such as walking in place, small movement, walking forward, and twisting^[247]; (m) the sensor with entirely laser-engraved components: the microfluidic module and the LEG-based chemical and physical sensors^[35]; (n) schematic of vector-mode laser cutting for microfluidic fabrication^[35]; (o) schematic of raster-mode laser for LEG-based chemical sensors fabrication^[35]; (p) photograph of a flexible lab-on-skin patch, with a scale bar of 1 cm^[35]; (q) layers of the sensor, from the bottom layer in contact with epidermis to the top layer^[35]; (r) photographs of a healthy subject wearing the sensor patch at different body parts^[35]

Fig. 10. Flexible multimodal sensors. (a) Schematic illustration of drawing of the first layer (stacking and vertical interconnect access (VIA) formation) and the second layer^[51]; (b) illustration of erasing the top layer electrode with accompanying optical images with a scale bar of 200 µm^[51]; (c) SEM images and corresponding illustration of the controlled VIA of four-layer device and the demonstration of the selective VIA control of the three-layer circuit system^[51]; (d) the device is actively adapted to the various user demands and corresponding attachment locations through impedance and sensor SOA, SOA is based on the laser rewriting of metal nanoparticles^[51]; (e) images of the developed skin electronic device including various sensing elements with on-skin power transferring ability, with scale bars of 15, 15, and 20 mm, respectively^[51]; (f) measuring of various factors that relates to skin conditions (real-time temperature, UV and humidity)^[51]; (g) measuring of ECG and EMG signals; (h) measuring of gyroscopic and EMG signals that relates to virtual hand control in real-time^[51]; (i) illustration of laser ablation strategy, using low power laser to pattern the metal traces and higher power laser to cut the PET film^[248]; (j) microscopic images of sensors and wires showing ablated Al sensors (red) and cut interconnect wires (cyan)^[248]; (k) schematic of corresponding sensor contacts of a soft finger approaching and gently touching a doll's forehead^[248]; (l) plots of corresponding proximity and temperature signals (calibrated data on the right vertical axis)^[248]

Fig. 11. Flexible multimodal sensors. (a) Schematic of the wearable sensor system attached onto a disposable diaper^[33]; (b) schematic of a wearable body condition sensor system integrated with tilt, breath, and moisture sensors, multiple channel signals are wirelessly transmitted to a smartphone interface via Bluetooth, the smartphone can generate an alarm under certain conditions^[33]; (c) schematic of the fabrication process for the non-adhesive LIG-based soft substrate to an LM droplet^[33]; (d) photo of the fabricated multimodal flexible sensor sheet^[33]; (e) photo of multimodal flexible sensors attached onto a diaper^[33]; (f) photos and results extracted from the smartphone of an artificial baby model wearing a wireless integrated tilt sensor for various body positions, including sleeping on the back (1), right side (2), left side (3), stomach (4), and standing posture (5)^[33]; (g) wireless monitoring results on a smartphone^[33]; (h) wireless real-time monitoring results measured from an adult lying on a mattress^[33]; (i) schematic of the flexible rain sensor system with reservoir computing (RC) processing^[262]; (j) photo of the flexible rain sensor and the sensor attached to an umbrella^[262]; (k) sensing mechanism of flexible rain sensor with and without water droplets^[262]; (l) resistance change of the sensor, estimated wind velocity, and estimated volume in an ambient environment with natural wind flow in the range of 1.5‒3.5, and 0‒1.0 m·s^-1[262]