Inkjet Printing of Soft, Stretchable Optical Waveguides through the Photopolymerization of High-Profile Linear Patterns

A nanograting-based flexible and stretchable waveguide for tactile sensing

Based on the related characteristics of optical waveguide and flexible optical materials, a flexible and stretchable optical waveguide structure oriented to tactile perception is proposed. The sensing principle of optical waveguide is based on mechanical deformation caused by output light loss. It overcomes the shortcomings of traditional optical waveguide devices, which are unable to conform to irregular surface. The flexible and stretchable optical waveguide is fabricated with nanoreplica molding method, and it has been applied to the measurement of pressure and strain in the field of tactile sensing. The flexible and stretchable optical waveguide had a strain detection range of 0 to 12.5%, and the external force detection range is from 0 to 23 × 10-3 N.

Heterogeneous sensing in a multifunctional soft sensor for human-robot interfaces

Soft sensors have been playing a crucial role in detecting different types of physical stimuli to part or the entire body of a robot, analogous to mechanoreceptors or proprioceptors in biology. Most of the currently available soft sensors with compact form factors can detect only a single deformation mode at a time due to the limitation in combining multiple sensing mechanisms in a limited space. However, realizing multiple modalities in a soft sensor without increasing its original form factor is beneficial, because even a single input stimulus to a robot may induce a combination of multiple modes of deformation. Here, we report a multifunctional soft sensor capable of decoupling combined deformation modes of stretching, bending, and compression, as well as detecting individual deformation modes, in a compact form factor. The key enabling design feature of the proposed sensor is a combination of heterogeneous sensing mechanisms: optical, microfluidic, and piezoresistive sensing. We characterize the performance on both detection and decoupling of deformation modes, by implementing both a simple algorithm of threshold evaluation and a machine learning technique based on an artificial neural network. The proposed soft sensor is able to estimate eight different deformation modes with accuracies higher than 95%. We lastly demonstrate the potential of the proposed sensor as a method of human-robot interfaces with several application examples highlighting its multifunctionality.

Optical Planar Waveguide Sensor with Integrated Digitally-Printed Light Coupling-in and Readout Elements

Optical planar waveguide sensors, able to detect and process information from the environment in a fast, cost-effective, and remote fashion, are of great interest currently in different application areas including security, metrology, automotive, aerospace, consumer electronics, energy, environment, or health. Integration of networks of these systems together with other optical elements, such as light sources, readout, or detection systems, in a planar waveguide geometry is greatly demanded towards more compact, portable, and versatile sensing platforms. Herein, we report an optical temperature sensor with a planar waveguide architecture integrating inkjet-printed luminescent light coupling-in and readout elements with matched emission and excitation. The first luminescent element, when illuminated with light in its absorption band, emits light that is partially coupled into the propagation modes of the planar waveguide. Remote excitation of this element can be performed without the need for special alignment of the light source. A thermoresponsive liquid crystal-based film regulates the amount of light coupled out from the planar waveguide at the sensing location. The second luminescent element partly absorbs the waveguided light that reaches its location and emits at longer wavelengths, serving as a temperature readout element through luminescence intensity measurements. Overall, the ability of inkjet technology to digitally print luminescent elements demonstrates great potential for the integration and miniaturization of light coupling-in and readout elements in optical planar waveguide sensing platforms.

Digital Luminescence Patterning via Inkjet Printing of a Photoacid Catalysed Organic-Inorganic Hybrid Formulation

Accurate positioning of luminescent materials at the microscale is essential for the further development of diverse application fields including optoelectronics, energy, biotechnology and anti-counterfeiting. In this respect, inkjet printing has recently attracted great interest due to its ability to precisely deposit with high throughput and no contact, functional materials on different types of substrates. Here, we present a novel photoacid catalysed organic-inorganic hybrid luminescent ink. The formulation, containing monomers bearing epoxy and silane functionalities, a photoacid generator and a small percentage of Rhodamine-B, shows good jetting properties and adequate wetting of the deposited droplets on the receiving substrates. Ultraviolet exposure of the deposited material triggers the cationic ring-opening polymerization reaction of the epoxy groups. Concomitantly, if atmospheric water is available, hydrolysis and condensation takes place, overall leading to a luminescent crosslinked hybrid organic-inorganic polymeric material obtained through a simple one-step curing process, without post baking steps. Advantageously, protection of the ink from actinic light delays the hydrolysis and condensation conferring long-term stability to the ink. Digital patterning leads to patterned emissive surfaces and elements with good adhesion to different substrates, mechanical and optical properties for the fabrication of optical and photonic elements and devices.

Heterogeneous Integration of Three-Primary-Color Photoluminescent Nanoparticle Arrays with Defined Interfaces

Minimized photoluminescent devices require both high-density fluorescent arrays and minimal cross-talk between neighboring pixels on the limited area. However, the challenges to achieve the overall integration of nanomaterial-based devices hinder the development of microscale full-color displays, including micro/nanoarray density, orientation control, multimaterial interface morphology, and uniform colors. Here, we report a heterogeneous integration approach to control the orientation, combination, and density of fluorescent micro/nanoarrays on flexible substrates. By controlling the defined interface and critical shrinkage width of liquid bridges, the width of three-primary-color micro/nanolines reached 100 nm. The interval between two parallel luminous lines is down to 40 μm, and the optical cross-talk effect is remarkably reduced. This work provides a facile approach to prepare high-performance micro-photoluminescent and imaging arrays for next-generation flexible display and lighting technology.

Non-contact printing of optical waveguides using capillary bridges

Non-contact printing methods such as inkjet, electro hydrodynamic, and aerosol printing have attracted attention for their precise deposition of functional materials that are needed in printed electronics, optoelectronics, photonics, biotechnology, and microfluidics. In this article, we demonstrate printing of tapered optical waveguides with losses of 0.61 ± 0.26 dB/cm, with the best performing structure achieving 0.19 dB/cm. Such continuous features are indispensable for successfully printing functional patterns, but they are often corrupted by capillary forces. The proposed inkjet printing method uses these forces to align liquid bridges into continuous features, enabling the printing of smooth lines on substrates with arbitrary contact angles.

Direct Writing of Microfluidic Footpaths by Pyro-EHD Printing

In this study, we report a direct writing method for the fabrication of microfluidic footpaths by pyro-electrohydrodynamic (EHD) jet printing. Here, we propose the use of a nozzle-free three-dimensional printing technique for the fabrication of printed structures that can be embedded in a variety of soft, transparent, flexible, and biocompatible polymers and thus easily integrated into lab-on-chip devices. We prove the advantage of the high resolution and flexibility of pyro-EHD printing for the realization of microfluidic channels well below the standard limit in dimension of conventional ink-jet printing technique and simply adaptable to the end-user desires in terms of geometry and materials. Starting from the description of the innovative approach proposed for the channel fabrication, we demonstrate the design, fabrication, and proof of a microfluidic matrix of interconnected channels. The method described here could be a breakthrough technology for the fabrication of in situ implantable, stretchable, and biocompatible devices, opening new routes in the field of biomedical engineering and wearable electronics.