Integrated optical devices based on liquid crystals embedded in polydimethylsiloxane flexible substrates

The contribution of this thesis is to find possible solutions for the creation of interconnections and optical switches to be used in microoptofluidic systems in the frame of the research activities of the Optoelectronic laboratory of the Department of Information Engineering, Electronics and Telecommunications (DIET). The main goal is to explore a new technology for integrated optic based on a low cost technology to produce low driving power devices. Optofluidics is the science which links the field of photonics with microfluidics, for the creation of innovative and state-of-the-art devices. Liquid crystals (LC) can be used for optofluidic applications because they have the possibility to change without external mechanical actions, the average direction of the molecules through the application of electric fields, reorienting the crystal molecules in such a way as to alter their optical properties [1-2]. The research on LC is more than a century old, but only since the ‘80s of the past century these materials were employed in various fields, from flat panel displays used for televisions, tablets, and smartphones, to biomedical and telecommunication applications [3-5]. The results reported in this thesis include simulation, design and preliminary fabrication of optofluidic prototypes based on LC embedded in polydimethylsiloxane (PDMS) channels, defined as LC:PDMS, with co-planar electrodes to control LC molecular orientation and light propagation. Fabrication techniques which were used include microelectronic processes such as lithography, sputtering, evaporation, and electroplating. The simulations were performed through the combined use of COMSOL Multiphysics® and BeamPROP®. I used COMSOL Multiphysics® to determine the positioning of the molecules in a LC:PDMS waveguide. The LC are the core through which light propagates in a PDMS structure. In addition to these simulations, I used COMSOL Multiphysics® to determine the orientation of the LC under the effect of an electric field [6-7] to create low-power optofluidic devices [8], [11]. I used BeamPROP® to explore the optical propagation of various optical devices such as: optical couplers, the zero gap optical coupler, and a multimodal interferometer. All these devices have been simulated through various combinations of geometries which will be extensively explained in the following chapters. The fabrication of prototypes was made in the Microelectronic Technologies laboratory of DIET. The optofluidic prototypes that I designed could be used in interconnection systems on biosensing devices for chemical or biological applications [10-11], wearable [12], or lab on chips [13], which are increasingly being applied in many research fields [14]. Many of these devices need to interface with electronics for processing signals coming from the interaction between the device with molecules, liquids or other biological substances. Moreover it is necessary to create flexible and biocompatible interfaces, whose features are not guaranteed in classic metal tracks. As it will be clear in the first chapter, metal interconnections must be designed with spatial, energy and throughput restrictions. To develop the optofluidic prototypes, I chose to use a combination of two materials for their commercial availability and ease of use: E7 and 5CB LC produced by Merck® as the transmissive medium and PDMS Sylgard 184 produced by Dow Corning® for the cladding [15-16]. The molecules of the LC are anisotropic, whose shape is elongated like that of a cigar. Under appropriate temperature conditions these molecules retain a state of aggregation in which, while retaining some mechanical properties of the fluids, they have the characteristics of crystals such as birefringence or x-ray reflection. These properties are due to two factors that characterize the various phases of LC: the orientational and positional order that vary according to the temperature. E7 was used in its nematic mesophase. The material used for the cladding of my prototypes was PDMS, a thermosetting polymer, flexible, biocompatible, economical, easy to work, and suitable for the creation of optical and optofluidic devices due to its transparency. The thesis is organized in six chapters whose contents are briefly outlined below: • In the first chapter there is a brief description of optofluidics and the transport phenomena of the liquids in the microchannels. The essential parameters for a correct interpretation of the behavior of the materials in the devices will be defined. Some examples of microfluidic devices, Optofluidic Optical Components (OOC) will be mentioned. • In the second chapter, LC’s will be presented, along with their general characteristics and their behavior in the presence of electric fields. An overview of integrated optic devices based on LC will be reported. • In the third chapter the experimental results will be presented concerning the fabrications and the technologies used to obtain electro-optical LC:PDMS waveguides. • The fourth chapter will be dedicated to a brief description of COMSOL Multiphysics® and BeamPROP® simulators, and the implementation of the model of LC channels in PDMS both in 2D and 3D. Also a brief description of Monte Carlo simulations based on Lebwohl-Lasher potential will be mentioned. • In the fifth chapter an LC:PDMS optical directional coupler and the most significant results will be described. • The sixth chapter is dedicated to the multimodal interferometer and its field of application, the theory behind this device and the results obtained from the simulations using the BeamPROP® • In the conclusion, a brief recap of the results obtained in this thesis and future developments will be presented

First-line systemic therapy for metastatic castration-sensitive prostate cancer: An updated systematic review with novel findings

Although both docetaxel and androgen-receptor-axis-targeted (ARAT) agents have yielded survival improvements in combination with androgen deprivation therapy (ADT) compared to ADT alone in metastatic castration-sensitive prostate cancer (mCSPC) patients, the optimal therapeutic choice remains to be established. We analyzed estimates of the hazard ratios for death (OS-HRs) in patients treated in the first-line setting enrolled in the GETUG-AFU15, CHAARTED, STAMPEDE, LATITUDE, ENZAMET, and TITAN trials. Overall, men with mCSPC receiving ADT with vs. without either an ARAT agent or docetaxel as first-line systemic therapy showed a pooled OS-HR of 0.69 (95 % CI: 0.61-0.78), with significant heterogeneity (p = 0.045, I2 = 52.5 %). Network meta-analysis showed an OS-HR in patients receiving an ARAT agent vs. docetaxel of 0.78 (95 %CI: 0.67-0.91). In conclusion, the evidence analysed indicates that an ARAT agent may provide improved OS outcomes compared to docetaxel. Prospective randomized trials are warranted

ECMO for COVID-19 patients in Europe and Israel

Since March 15th, 2020, 177 centres from Europe and Israel have joined the study, routinely reporting on the ECMO support they provide to COVID-19 patients. The mean annual number of cases treated with ECMO in the participating centres before the pandemic (2019) was 55. The number of COVID-19 patients has increased rapidly each week reaching 1531 treated patients as of September 14th. The greatest number of cases has been reported from France (n = 385), UK (n = 193), Germany (n = 176), Spain (n = 166), and Italy (n = 136) .The mean age of treated patients was 52.6 years (range 16–80), 79% were male. The ECMO configuration used was VV in 91% of cases, VA in 5% and other in 4%. The mean PaO2 before ECMO implantation was 65 mmHg. The mean duration of ECMO support thus far has been 18 days and the mean ICU length of stay of these patients was 33 days. As of the 14th September, overall 841 patients have been weaned from ECMO support, 601 died during ECMO support, 71 died after withdrawal of ECMO, 79 are still receiving ECMO support and for 10 patients status n.a. . Our preliminary data suggest that patients placed on ECMO with severe refractory respiratory or cardiac failure secondary to COVID-19 have a reasonable (55%) chance of survival. Further extensive data analysis is expected to provide invaluable information on the demographics, severity of illness, indications and different ECMO management strategies in these patients

Design of optical directional couplers made of polydimethysiloxane liquid crystal channel waveguides

We present numerical simulations of a directional coupler based on three-dimensional waveguides made of a nematic liquid crystal, acting as the waveguide core, infiltrated in polydimethysiloxane channels. Modeling is based on the combination of minimization of Oseen-Frank energy of the liquid crystal molecules with a beam propagation algorithm. Design of the coupler waveguides is optimized to minimize coupling lengths and maximise efficiencies. Such components can be made at low cost on flexible plastic substrates and can be also integrated with optofluidic devices for biomedical applications

Simulation of Optofluidic LC:PDMS Directional Couplers for Photonic Switching

Integrated optofluidics is a new technology field which combines photonics with microfluidic techniques to fabricate devices for applications in many fields such as datacom, biosensing and lab-on-chip [1]. Such devices are inexpensive to fabricate, and can be flexible because polydimethylsiloxane (PDMS) can be used as substrate material. It is possible to create switchable and reconfigurable devices by combining PDMS channels filled with nematic liquid crystals (NLC) used as core material, referred as LC:PDMS optical waveguides. The process to fabricate well defined PDMS channels for simple waveguides or directional couplers in which a liquid crystal can be infiltrated by capillarity, is very simple [2]. The mold to create a micro-channel is obtained through soft photolithography with SU-8 2005, which is a negative photoresist, on a 2.5 cm x 2.5 cm silicon crystalline wafer. The wafer is preliminarily cleaned in a solution of 2% of HF to remove the native oxide and then SU-8, is spinned on the treated surface so that a thickness of 5 μm is obtained. PDMS (Sylgard 184 by Dow Corning) is used for the fabrication of waveguide cladding. In our simulations we have studied the directional coupler structure sketched in Fig. 1 with a square cross section of 2 μm x 2 μm as a basic structure to make an optical switch [3]. The orientation of the NLC molecules in the PDMS channels has been studied through the Oseen-Frank free energy minimization in a model built by using Comsol Multiphysics® in order to derive the related distribution of the refractive index of the NLC. The results have been imported in a BeamPROP’s device simulator to observe light propagation at the wavelength of 1550 nm. The NLC infiltrated in the waveguides has an extraordinary refractive index of 1.689 and a ordinary refractive index of 1.5, the PDMS used for the waveguides has a refractive index of 1.3997. We simulated three directional couplers with three different gaps between the waveguides of 1 μm, 0.75 μm and 0.5 μm which provide a complete exchange of optical power between the two waveguides at the lengths of 500 μm, 300 μm and 125 μm respectively (Fig 2). Furthermore we calculated the extinction ratio (ER), as a figure of merit, for every directional coupler and plotted in Fig. 3. The ER is defined as the ratio between the power levels at the output of the directional coupler and we found that an ER over 20 dB can be obtained

Optofluidic LC:PDMS Directional Couplers for Low Power Switches

We demonstrate directional couplers made of polydimethylsiloxane channels filled with nematic liquid crystals in view of low power flexible optical switches, which are polarization insensitive. An extinction ratio of over 20 dB is obtained at the wavelength of 1550 nm

Microoptofluidics using PDMS and liquid crystals: fabrication technology and devices

Recently polydimethylsiloxane (PDMS) gained a lot of interest to fabricate microfluidic flexible channels to make inexpensive, flexible and reconfigurable devices for many applications ranging from datacom to sensing and biomedical lab on a chip applications. Optical waveguides made of PDMS channels filled with nematic liquid crystals (LC), referred as LC:PDMS waveguides, were demonstrated showing polarization independent transmission of light at both visible and near infrared wavelengths. LC molecules are homeotropically aligned to the PDMS surface, without using any alignment layer as usually required in LC standard electro-optic devices. This is due to the interface hydrophobic interaction between the PDMS inner surface and the nematic LC molecules. Such optical waveguides can be made through a standard casting and molding technique, combined with filling procedure by capillarity to infiltrate the LC in its isotropic phase at 80 °C under vacuum in the PDMS channels. Such solution allows the design and fabrication of switchable and tunable devices by exploiting the efficient electro-optic and nonlinear optical effects in LC. One advantage of such approach with respect of classical integrated devices is a strong reduction of the power budget in terms of both energy dissipation and driving power. We show fabrication techniques and characterization of LC:PDMS waveguides, including fabrication of ITO electrodes deposited on PDMS surfaces in order to obtain low consuming power integrated optic devices. We report characteristics of directional couplers based on LC:PDMS waveguides with an extinction ratio of the output power level of over 20 dB, as a basis for low power optical switches. Directional couplers with a length of 45 μm and a waveguide gap as short as 300 nm are also feasible in view of compact and low power flexible optical switches operating at the fiber optic communication wavelength of 1550 nm

Integrated Optics Based on Liquid Crystals Embedded in PDMS Microfluidic Channels

The combination of photonics with microfluidic techniques has been discovered to be a viable new technology to make inexpensive, flexible and reconfigurable devices for many applications ranging from datacom to sensing and biomedical lab on a chip applications [1]. Recently polydimethylsiloxane (PDMS) gained a lot of interest to fabricate microfluidic flexible channels. Optical waveguides made of PDMS channels filled with nematic liquid crystals (LC), referred as LC:PDMS waveguides, were demonstrated showing polarization independent transmission of light at both visible and near infrared wavelengths [2]. LC molecules are homeotropically aligned to the PDMS surface, without using any alignment layer as usually required in LC standard electro-optic devices. This is due to the interface hydrophobic interaction between the PDMS inner surface and the nematic LC molecules. Such optical waveguides can be made through a standard casting and molding technique, combined with filling procedure by capillarity to infiltrate the LC in its isotropic phase at 80 °C under vacuum in the PDMS channels. Such solution allows the design and fabrication of switchable and tunable devices by exploiting the efficient electro-optic and nonlinear optical effects in LC. One advantage of such approach with respect of classical integrated devices is a strong reduction of the power budget in terms of both energy dissipation and driving power. In this paper we show fabrication techniques and characterization of LC:PDMS waveguides and the potentialities to obtain low consuming power integrated electro-optic devices. As an example we report short directional couplers in view of low power flexible optical switches, which are polarization insensitive with an extinction ratio of over 20 dB at the wavelength of 1550 nm

Short optofluidic directional couplers for low power switches

We demonstrate optical directional couplers made of polydimethylsiloxane channels filled with nematic liquid crystals in view of low power flexible optical switches. Transmission of light through such optical channels is polarization insensitive. Coupling of light can be performed with an extinction ratio of over 20 dB at the wavelength of 1550 nm for any input polarization. Directional couplers as short as 45 μm can also be obtained with a gap between the waveguides of just 0.3 μm. Preliminary fabrication of ITO electrodes deposited on PDMS is described to change the coupler state. By applying a voltage a reorientation of the liquid crystal molecules parallel to the applied electric field raises the refractive index of liquid crystal therefore light injected in one waveguide remains in the same waveguide without coupling determining a bar-state of the directional coupler. Images at a scanning electrode microscope show that ITO electrodes with good uniformity can be obtained with a thickness of about 27 nm. Photolithographic masks have been also fabricated for final switch fabrication