Bipolar electrochemistry deals with the exposure of an isolated conducting substrate that has no direct connection with a power supply except via an electric field. Therefore it can be considered as a "wireless technique". The polarization of the substrate with respect to the surrounding medium generates a potential difference between its opposite ends which can support localized electrochemical oxidation reduction reactions and break the surface symmetry of the substrate. The method was applied in the present thesis to materials science and biological systems. In the frame of designing asymmetric particles, also called "Janus" particles, bipolar electrochemistry was adapted for the bulk preparation of these objects. Conductive substrates with different nature, sizes and shapes have been modified with various materials such as metals, ionic and inorganic compounds using this approach. Moreover, a control over the deposit topology could be achieved for substrates at different length scales. Bipolar electrodeposition is also a good tool for investigating the generation of different metal morphologies. Further developments in the bipolar setup allowed us to use the technology for microstructuration of conductive objects. Furthermore the concept has shown to be very useful in the field of the induced motion of particles. The asymmetric objects that have been prepared by bipolar electrodeposition were employed as microswimmers which could show both translational and rotational motion. The application of electric fields in the bipolar setup can be used for the direct generation of motion of isotropic objects through bubble generation. A levitation motion of objects combined with light emission was possible using this concept. Finally, bipolar electrochemistry was also used for studying the intrinsic conductivity of biological molecules (DNA), which is of great importance in the nanotechnology.

No description available.

Swimming is a fundamental feature in many living systems. Biological microorganisms move in the search of food, appropriate pH, temperature, mate and for many other elements crucial for life. A classic example is sperm cell, which travels thousands of its body length in the complex genital tract of females to reach the egg. Inspired by such unique character and diversified motion abilities of the biological world, researchers have always been intrigued to create small artificial microbots which could swim and perform complex tasks. In his famous talk ’There is plenty of room at the bottom’ in 1960, Richard Feynman suggested designing swallowable doctors which could travel in the blood vessels and perform the surgery. Although seemingly exquisite and far-fetched, this idea laid the foundation stone to pave the path towards building autonomously propelled artificial machines with applications ranging from targeted drug delivery to environmental remediation. However, considerable challenges are yet to be addressed before developing fully functional artificial machines, especially in the biomedical applications. For instance, directed transport in vivo, using man-made artificial machines face many obstacles starting from their fabrication, fuels for powering them and their interactions with the surroundings. Rapid changes in the environment in vivo, would make it difficult in selecting the ideal material and shape design of the microswimmer and would most probably require a flexible structure which could potentially squeeze itself and easily pass through small cavities. With most of the swimmers, in the past, being designed from inorganic materials, leave them unsuitable for biological applications. In addition, the environments inside an animal body is dominated by various complexity such as flows of bodily fluids, cavities and soft tissues. In laboratory settings, often these peculiarities are ignored as mostly the motion behavior is tested in stagnant conditions on solid substrates and it is unclear how would an artificial machine will behave in such complex environments. In this thesis, we combined the advances in microfluidics to benefit the microswimmer research manifold. In the last few years, microfluidics and micromotors have been used together in various instances because of their co-sharing regime of low Reynolds number and excellent fluid manipulation abilities at the microscale. In addition, microfluidics offer unique opportunities in designing structures with well-engineered shapes. With these points in mind, in this thesis, we used microfluidics to fabricate microswimmers and design custom made environments to mimic the complexity present in vivo, and to study the feedback of artificial swimmers in them. Specifically, in the first part of the thesis, two microfluidic strategies namely droplet microfluidics and stop-flow lithography were investigated to design hydrogel-based micromotors. Besides, in the next part, we developed complex environments and studied the motion behavior of conventional microswimmers in them. In the first subpart of the thesis, using droplet microfluidics, we designed polyacrylamide and poly (ethylene glycol) diacrylate (PEGDA) based Janus droplets using co-flowing phases with enzyme immobilized in one of the phases to confer asymmetry. The droplets were polymerized on-chip using UV polymerization. We found that the polyacrylamide and PEGDA 565 particles did not result into efficient bubble production when suspended in H2O2 solution and we explain this behaviour using the analogy of smaller pore size and possible poisoning of the enzyme by acrylamide. But, when a 10 v/v% PEGDA 700 was selected as the polymer material, it resulted in very efficient bubble evolution, although the Janus geometry was compromised which restricted swimming for these particles. The second subpart dealt with applying stop-flow lithography technique for designing hydrogel micromotors with different shapes and these shapes corresponded to different swimming modes. Exploiting laminar flow in the low Reynolds number region in the microfluidics channels, we fabricated micromotors with variable composition, shape and controlled active regions. Furthermore, we studied the different trajectories resulting from the complex interactions between swimmer body and fluid dynamics around it and connected them to the theoretical findings. We found close agreement between the experimental results and the theoretical outcomes: I-shaped structure behaved as a pump, U-shaped as a propeller and S-shaped as a rotor. Post fabrication, during real applications, the micromotors will be exposed to complex environments for instance interfaces and flows. To evaluate the feedback of microswimmers in these situations, in the next two sections, we designed custom made environments using microfluidics and we studied the response of well-studied Janus microswimmers in them. It should be noted that in the following two sections we used Janus particles rather than the bubble driven swimmers (fabricated in the first section) for simplicity. In this section, we designed an oil-water interface using a special microfluidic trap design and explored the motion behaviour of a very well-studied Pt@SiO2 Janus micromotors on them. The chip geometry facilitated on-demand merging of a droplet of particles and the ‘fuel’ (H2O2) inside the trap. Additionally, the large surface of the trap resulted in high surface energy which was compensated by partial wetting of the glass substrate. This partial wetting created patches of oil on the glass which we refer to as ‘oil dimples’. The dimples gave us the unique opportunity to directly compare the propulsion and performance of Janus motors at both interfaces (oil-water and solid-water) within the same setup and under similar experimental conditions. The swimming pattern and the speed values were found to be similar at the two interfaces and we conjecture an interplay of various factors such as microscale friction, lubrication, surface locking by the surfactant, reaction product absorption by oil and potential Marangoni influences for this similarity. In the next section, we designed a laminar flowing system using a square glass capillary and studied the response of a spherically symmetrical Janus micromotor in the conditions of flow. Previously, in the literature the response of Pt@SiO2, which is a model pusher-type micromotor, has been studied and they have been demonstrated to migrate cross-stream when the flow is imposed. In this thesis, we introduce a Cu@SiO2 colloid which we hypothesize to resemble a puller-type configuration based on theoretical flow field calculations. Additionally, in the literature, it has been predicted that pullers would exhibit upstream migration when placed under the conditions of flow. Indeed, when placed under flow, these particles migrate upstream, resembling many of the swimmers from biological world. These experimental findings are recovered theoretically using a simple squirmer model in puller configuration. The model also predicted a unique jumping behaviour for these particles, at very high flow rate. When increasing the flow rate in the experiments, we actually capture this characteristics. Finally, based on the theoretical flow field calculations and particularly their upstream response in the imposed flow, we conjecture a puller configuration for Cu@SiO2 micromotors. To sum up, this thesis made important advances by creating a number of different shapes of microswimmers and designing complex environments using microfluidics in which microswimmers can be placed and their response can be studied. Although, in this thesis we emphasized on Janus particles, in future, these custom-made environments can be used to assess the behaviour of other microswimmers including biological ones. While still many engineering and medical problems need to be solved before fully functional applications of artificial microswimmers are realized, manifestations of various shape designs and understanding their behaviours in complex surroundings are the first crucial steps.:Contents: Acknowledgements List of Abbreviations 1. Introduction 2. Fundamentals of active matter and microfluidics 2.1. Active matter 2.1.1. Physical fundamentals of motion at microscale 2.1.2. Biological microswimmers 2.2. Review Paper: Microfluidics for microswimmers 3. Aims and Motivation 4. Results and Discussion 4.1. Microfluidics for fabrication of microswimmers 4.1.1. Introduction 4.1.2. Droplet microfluidics 4.1.3. Stop-flow lithography 4.1.4. Paper - Fundamental Modes of Swimming Correspond to Fundamental Modes of Shape: Engineering I–, U–, and S– Shaped Swimmers 4.2. Microfluidics for specific environments: Interfaces 4.2.1. Introduction 4.2.2. Paper - Study of Active Janus Particles in the Presence of an Engineered Oil–Water Interface 4.3. Microfluidics for specific environments: Flow 4.3.1. Introduction 4.3.2. Paper - Upstream rheotaxis of catalytic Janus spheres 5. Summary and Final Remarks 6. Experimental Details 6.1. Fabrication of hydrogel particles using droplet microfluidics 6.2. Characterization of the hydrogel particles 6.3. Motion studies of the hydrogel particles A. Appendix A.1. Droplet microfluidics A.2. Stop-flow lithography A.3. Microfluidics for specific environments: Interfaces A.4. Microfluidics for specific environments: Flow B. List of publications Bibliography C. Erklärung

Lab-on-a-chip systems for point-of-care testing demonstrate a promising development towards more accurate diagnostic tests that are of extreme importance for the future global health. This work presents an agglutination assay performed in micrometer sized well using Janus PS/Ag/AgCl micromotors to enhance the interactions between goat anti-human IgM functionalized particles and Human IgM. The fabricated microwell chips are a suitable platform to analyze the interaction between different particles and to perform the agglutination assays. The interaction between active Janus particles and passive and functionalized particles is studied, as well as the influence of ions on the motion of the Janus particles. Agglutination assays are performed with and without the presence of Janus particles, and in different PBS concentrations. Once illuminated with blue light, passive SiO2 particles were effectively excluded from Janus particles, while SiO2 NH2 particles revealed attraction. In contrast, functionalized SiO2 NH2 Ab particles suspended in PBS did not show any interaction. It was found that the optimal working conditions for antibodies and Janus particles differed and, as a result, the Janus particles did not reveal a desirable interaction between the functionalized particles and IgM. Further experiments should be performed to find the proper conditions in which the antibodies and the Janus particles maintain their activities. It is believed that an effective interaction between the functionalized and Janus particles could be achieved by modifying the parameters that affect their interaction such as the zeta potential and the medium in which the assay is being performed. This preliminary work provides the first steps towards the development of a fully integrated lab on a chip system for point of care testing.:Abstract ........................................................................................................................ iii Acknowledgments.......................................................................................................... v Table of Contents .......................................................................................................... vi List of Tables ............................................................................................................. viii List of Figures ............................................................................................................... ix Abbreviations ................................................................................................................. x 1. Introduction ............................................................................................................ 1 1.1 In vitro diagnostic tests ........................................................................................ 1 1.1.1 Point-of-care tests ......................................................................................... 2 1.2 Agglutination assay .............................................................................................. 2 1.3 Lab-on-a-chip ....................................................................................................... 5 1.4 Self-propelled particles ........................................................................................ 6 1.4.1 Light-driven Ag/AgCl micromotors ............................................................. 6 1.5 Aim ...................................................................................................................... 9 2. Materials and Methods ......................................................................................... 11 2.1 Microwell fabrication .................................................................................... 11 2.2 Microswimmers fabrication .......................................................................... 12 2.3 Functionalization of particles ........................................................................ 12 2.4.1 Scanning electron microscope ............................................................... 14 2.4.2 UV-vis spectroscopy .............................................................................. 14 2.4.3 Zeta potential ......................................................................................... 14 2.4.4 Optical microscopy ................................................................................ 15 2.5 Motion Experiments ...................................................................................... 15 2.6 Agglutination assay ....................................................................................... 16 2.7 Effect of PBS ................................................................................................. 16 2.7.1 Janus particles ........................................................................................ 16 2.7.2 Agglutination assay ................................................................................ 17 2.7.3 Exclusion of functionalized particles ..................................................... 17 3. Results and Discussion ........................................................................................ 18 3.1 Microwell chip with integrated Janus particles ................................................. 18 3.2 Characterization of particles .............................................................................. 19 3.2.1 UV-vis spectroscopy ................................................................................... 19 3.2.2 Zeta potential .............................................................................................. 21 3.2.3 Agglutination assay in PEG-covered glass slides ....................................... 22 3.3 Motion experiments ........................................................................................... 23 3.3.1 Exclusion time ............................................................................................ 23 3.3.2 On/off light cycles....................................................................................... 26 3.4 Agglutination assay ............................................................................................ 28 3.4.1 Assay performed in wells............................................................................ 28 3.4.2 Assay performed in wells with Janus particles ........................................... 29 3.5 Effect of PBS concentration............................................................................... 30 3.5.1 Janus particles ............................................................................................. 30 3.5.2 Agglutination assay ..................................................................................... 32 3.5.3 Exclusion of functionalized particles .......................................................... 33 4. Conclusions .......................................................................................................... 35 References .................................................................................................................... 37 Declaration of Research Integrity and Good Scientific Practice ................................. 42

We investigate coarse-grained models of suspended self-thermophoretic microswimmers. Upon heating, the Janus spheres, with hemispheres made of different materials, induce a heterogeneous local solvent temperature that causes the self-phoretic particle propulsion. Starting from molecular dynamics simulations that schematically resolve the molecular composition of the solvent and the microswimmer, we verify the coarse-grained description of the fluid in terms of a local molecular temperature field, and its role for the particle’s thermophoretic self-propulsion and hot Brownian motion. The latter is governed by effective nonequilibrium temperatures, which are measured from simulations by confining the particle position and orientation. They are theoretically shown to remain relevant for any further spatial coarse-graining towards a hydrodynamic description of the entire suspension as a homogeneous complex fluid.

Small devices capable of self-propulsion have potential application in areas of nanoscience where autonomous locomotion and programmability are needed. The specific base-pairing interactions that arise from DNA hybridisation permit the programmed assembly of matter and also the creation of controllable dynamical systems. The aim of this thesis is to use the tools of DNA nanotechnology to design synthetic active matter at the micro and nano scales. In the first section, DNA was used as an active medium capable of transporting information faster than diffusion in the form of chemical waves. DNA waves were generated experimentally using a DNA autocatalytic reaction in a microfluidic channel. The propagation velocity of DNA chemical waves was slowed down by creating concentration gradients that changed the reaction kinetics in space. The second section details the synthesis of chemically-propelled particles and the use of DNA as a 'programmable glue' to mediate their interactions. Janus micromotors were fabricated by physical vapour deposition and a wet-chemical approach was demonstrated to synthesise asymmetrical catalytic Pt-Au nanoparticles that function as nanomotors. Dynamic light scattering measurements showed nanomotor activity that depends on H₂O₂ concentration, consistent with chemical propulsion. Gold nanoparticles/Origami hybrids were assembled in 2D lattices of different symmetries arranged by DNA linkers. The third section details the design process and synthesis of nanomotors using DNA as a structural scaffold. 3D DNA Origami rectangular prisms were functionalised site-specifically with bioconjugated catalysts, i.e. Pt nanoparticles and catalase. Enzymatic nanomotors were also conjugated to various cargoes and their motor activity was demonstrated by Fluorescence Correlation Spectroscopy. In the final section, control mechanisms for autonomous nanomotors are studied, which includes the conformational change of DNA aptamers in response to chemical signals, as well as a design for an adaptive dynamical system based on DNA/enzyme reaction networks.

L’électrochimie bipolaire est possible quand un substrat conducteur qui n’est pas directement connecté à un générateur est exposé à un champ électrique. Il s’agit donc d’une technique « sans fil ». La polarisation du substrat par rapport à la solution génère une différence de potentiel entre les extrémités du substrat qui peuvent devenir le siège de réactions rédox et briser ainsi la symétrie à la surface du substrat. Dans cette thèse, cette méthode a été appliquée à l’élaboration de matériaux ainsi qu’à l’étude de systèmes biologiques. L’électrochimie bipolaire a été adaptée pour la préparation « bulk » de particules asymétriques également appelées particules « Janus ».Des substrats conducteurs de différentes natures, tailles et formes ont été modifiées avec des dépôts métalliques, ioniques ou inorganiques. De plus, un contrôle de la morphologie du dépôt a été possible sur des substrats d’échelle variée. L’électrodéposition bipolaire permet d’étudier la génération de différentes morphologies métalliques, ainsi que la micro-structuration sur des objets conducteurs grâce au développement de nouveaux setups expérimentaux. Le concept s’est également montré très utile dans le domaine de la mise en mouvement de particules. D’une part, les objets asymétriques qui ont été préparés par électrodéposition bipolaire peuvent agir comme des micro-nageurs capables de mouvement de translation ou de rotation. D’autre part, l’application d’un champ électrique peut directement induire le déplacement d’objets isotropes par génération localisée de bulles. Un mouvement de lévitation combinée à l’émission de lumière est également possible. Finalement, l’électrochimie bipolaire a été utilisée pour étudier la conductivité de biomolécules (ADN), ce qui est d’une grande importance dans le domaine de la nanotechnologie. / Bipolar electrochemistry deals with the exposure of an isolated conducting substrate that has no direct connection with a power supply except via an electric field. Therefore it can be considered as a “wireless technique”. The polarization of the substrate with respect to the surrounding medium generates a potential difference between its opposite ends which can support localized electrochemical oxidation reduction reactions and break the surface symmetry of the substrate. The method was applied in the present thesis to materials science and biological systems. In the frame of designing asymmetric particles, also called “Janus” particles, bipolar electrochemistry was adapted for the bulk preparation of these objects. Conductive substrates with different nature, sizes and shapes have been modified with various materials such as metals, ionic and inorganic compounds using this approach. Moreover, a control over the deposit topology could be achieved for substrates at different length scales. Bipolar electrodeposition is also a good tool for investigating the generation of different metal morphologies. Further developments in the bipolar setup allowed us to use the technology for microstructuration of conductive objects. Furthermore the concept has shown to be very useful in the field of the induced motion of particles. The asymmetric objects that have been prepared by bipolar electrodeposition were employed as microswimmers which could show both translational and rotational motion. The application of electric fields in the bipolar setup can be used for the direct generation of motion of isotropic objects through bubble generation. A levitation motion of objects combined with light emission was possible using this concept. Finally, bipolar electrochemistry was also used for studying the intrinsic conductivity of biological molecules (DNA), which is of great importance in the nanotechnology.

Les micronageurs et parmi eux les microangeurs artificiels sont en général, limités à exister dans des écoulements dominés par des forces visqueses. Ces écoulements sont caractérisés par un bas nombre de Reynolds (Re). Cela impacte la stratégie de nage et plus particulièrement les séquences de forme possibles, qui doivent nécessairement être non-réciproques dans l'espace de déformation pour espérer induire un déplacement net non-nul. De plus, due aux forts effets de traînée, les vitesses de nage sont limités à des valeurs faibles.Dans cette thèse, on examine la possibilité d'utiliser un mécanisme de nage basé sur l'instabilité de flambage d'une sphère creuse. Cette instabilité est provoquée en soumettant la sphère à une onde de pression. La particularité de ce mécanisme est qu'il satisfait par construction la condition nécessaire de nage à bas Reynolds exposée précédemment. De plus, la rapidité de la déformation lors de l'instabilité pousse à prévoir l'apparition d'effets inertiels, et ce même à l'échelle microscopique.Une étude expérimentale a été conduite à l'échelle macroscopique dans le but de comprendre la dynamique de l'instabilité et son impact sur le fluide qui entoure la coque creuse. Ces expériences nous permettent de montrer qu'un déplacement net non-nul est produit pour tous les régimes d'écoulements.On met en évidence le rôle de paramètres géométriques, des propriétés du matériau composant la coque creuse et de la rhéologie du fluide sur l'efficacité de la nage.On montre l'existence d'un optimum de déplacement net pour des valeurs intermédiaires du nombre de Reynolds. Pour expliquer cela, on se sert de mesures de PIV résolues temporellement pour mettre en évidence la présence d'effets d'histoire non-triviaux qui augmentent le déplacement net.On dérive un simple modèle en se basant sur les observations expérimentales pour montrer que ce régime optimal de nage est atteignable pour des sphères microscopiques, ceci est possible grâce l'activation rapide de l'instabilité. Cette propriété permet aussi une excitation à haute fréquence en utilisant des ultrasons. Une étude d'échelle nous permet de prédire une vitesse de nage de 1 cm/s pour un micro-robot contrôlé à distance. Cet ordre de grandeur de vitesse est idéal pour des applications biologiques comme la distribution ciblée de médicaments. / Microswimmers, and among them aspirant microrobots, are generally bound to cope with flows where viscous forces are dominant, characterized by a low Reynolds number (Re). This implies constraints on the possible sequences of body motion, which have to be nonreciprocal. Furthermore, the presence of a strong drag limits the range of resulting velocities.Here, we propose a swimming mechanism which uses the buckling instability triggered by pressure waves to propel a spherical hollow shell. The particularity of this mechanism is that it fulfills naturally the necessary condition of swimming at low Re. In addition, the swiftness of the instability might produce inertial effects even at the microscopic scale.With a macroscopic experimental model we show that a net displacement is produced at all Re regimes. We put in evidence the role of geometrical parameters, shell material properties and rheology of the surrounding fluid on the swimming efficiency.An optimal displacement is reached at intermediate Re. Using time-resolved PIV measurements, we explain that non-trivial history effects take place during the instability and enhance net displacement.Using a simple model, derived from the study of shell dynamics, we show that due to the fast activation induced by the instability, this regime is reachable by microscopic shells. The rapid dynamics would also allow high frequency excitation with standard traveling ultrasonic waves. Scale considerations predict a swimming velocity of order 1 cm/s for a remote controlled microrobot, a suitable value for biological applications such as drug delivery.

Les suspensions d'objets microscopiques ayant la faculté de se déplacer par eux-mêmes dans le fluide qui les entoure sont des systèmes qui présentent un intérêt croissant dans la communauté scientifique. Du fait de leur dynamique intrinsèquement hors-équilibre au sens de la physique statistique, ils génèrent des effets particulièrement complexes. Parmi les micro-objets autopropulsés existants, les micro-algues vertes représentent une part importante de la biomasse de la Terre et participent activement au retraitement du CO2 par leur activité photosynthétique. Elles présentent de plus un remarquable potentiel dans les domaines de la production de bio-carburants, du retraitement des déchets, de la fabrication de cosmétiques et de compléments alimentaires. La compréhension de la dynamique de nage de ce type de microorganisme est d'un intérêt primordial d'un point de vue industriel. Cet ouvrage présente l'étude de la dynamique de la micro-algue Chlamydomonas Reinhardtii. En utilisant un système de suivi de particules en imagerie optique que nous avons développé, nous analysons ici le mécanisme fondamental de nage utilisé par cette algue jusqu'à ses implications en terme d'effets collectifs sur la dynamique de nage d'une suspension semi-diluée. / The suspensions of microscopic objects with the ability to propel themselves into the surrounding fluid are systems of growing interest in the scientific community. Due to their intrinsic out-of-equilibrium dynamics in the sense of statistical physics, they generate complex effects. Among the existing self-propelled micro-objects, green micro-algae are an important part of the biomass of Earth and they actively participate to the recycling of CO2 by their photosynthetic activity. Moreover they have remarkable potential for the production of bio-fuels, waste reprocessing, cosmetics and dietary supplements production. From an industrial point of view, understanding the dynamics of this type of swimming microorganism is of primary interest. This work presents the study of the dynamics of microalgae Chlamydomonas Reinhardtii. Using a system of particle tracking with optical imaging that we have developed, we analyze the mechanism of stroke used by the algae up to its implications in terms of collective effects on the dynamics of swimming in a semi-dilute suspension.

What is flagellar swimming? Cilia and flagella are whip-like cell appendages that can exhibit regular bending waves. This active process emerges from the non-equilibrium dynamics of molecular motors distributed along the length of cilia and flagella. Eukaryotic cells can possess many cilia and flagella that beat in a coordinated fashion, thus transporting fluids, as in mammalian airways or the ventricular system inside the brain. Many unicellular organisms posses just one or two flagella, rendering them microswimmers that are propelled through fluids by the flagellar beat including sperm cells and the biflagellate green alga Chlamydomonas. Objectives. In this thesis in theoretical biological physics, we seek to understand the nonlinear dynamics of flagellar swimming and synchronization. We investigate the flow fields induced by beating flagella and how in turn external hydrodynamic flows change speed and shape of the flagellar beat. This flagellar load-response is a prerequisite for flagellar synchronization. We want to find the physical principals underlying stable synchronization of the two flagella of Chlamydomonas cells. Results. First, we employed realistic hydrodynamic simulations of flagellar swimming based on experimentally measured beat patterns. For this, we developed analysis tools to extract flagellar shapes from high-speed videoscopy data. Flow-signatures of flagellated swimmers are analysed and their effect on a neighboring swimmer is compared to the effect of active noise of the flagellar beat. We were able to estimate a chemomechanical energy efficiency of the flagellar beat and determine its waveform compliance by comparing findings from experiments, in which a clamped Chlamydomonas is exposed to external flow, to predictions from an effective theory that we designed. These mechanical properties have interesting consequences for the synchronization dynamics of Chlamydomonas, which are revealed by computer simulations. We propose that direct elastic coupling between the two flagella of Chlamydomonas, as suggested by recent experiments, in combination with waveform compliance is crucial to facilitate in-phase synchronization of the two flagella of Chlamydomonas.