Microdroplet microfluidics has gained much interested in the past decade due to its ability to conduct a wide variety of biological and microfluidic experiments with extremely high repeatability on a mass scale. In particular the ability to culture multiple batches of cells by creating microdroplets with a single encapsulated cell and observe their growth overtime allows for specific conditioning of cells. In addition, when conducting co-culture experiment the induction of a certain stimulus may provide observational rare differences in growth that may be characterized by harnessing a single batch of cells out of thousands of samples. This thesis first presents a variety of microdroplet microfluidic devices that use specific techniques to sufficiently produce, synchronize, merge, and analyze microdroplets. Although many of the devices are capable of producing stable droplets and somewhat efficient synchronization, the overall merging efficiency for most passive or active merging methods alone is lacking. Improvements on such methods and the incorporation of multiple merging methods can lead to a higher overall merging efficiency and greater droplet stability. Also, multiple droplet detection methods can be employed to analyze cellular growth under different conditions, while passive or active sorting methods can be used to acquire particular microdroplet samples downstream. The work presented in this thesis entails the characterization and detailed analysis of all aspects of microdroplet microfluidics necessary to adequately produce a microdroplet co-culture device for microbial studies. This includes the incorporation of multiple microdroplet generators for the production of water droplets immersed in oil serving as bio-reactors for cell culture experiments. In addition, multiple microdroplet synchronization devices were tested to sufficiently align multiple trains of droplets for downstream merging using a variety of passive, active, or combination merging methods. In particular, the use of an electric field can cause destabilization of the surfactant surrounding a microdroplet and allow for the formation of a liquid bridge. The formation of this liquid bridge in conjunction with passive merging methods can lead to droplet electrocoalescence. The incorporation of a more uniform electric field that reduces the angle between the droplet dipole moment and E-field can lead to better droplet merging while reducing voltage and frequency requirements observed in previously publications. The testing, observation, and optimization of such aspects of microdroplet microfluidics are crucial for the advancement and production of sound microdroplet culture devices for a variety of applications including the analysis of dangerous pathogenic substances, drug testing or delivery, and genetic studies.

Microfluidic based miniature lab-on-a-chip devices integrate different laboratory functionality in microscale. Microarray technology is evolving as a powerful tool for biomedical and pharmaceutical applications to identify gene sequences or to determine gene expression levels. Preparation of samples and spotting the arrays are the two major steps required for making microarrays. The microfluidic components developed in this research would facilitate performing the above-mentioned steps by a single lab-on-a-chip. Three microfluidic modules were developed: a non-contact microdispenser, an interface connecting the microdispenser with planar Electrowetting on Dielectric (EWOD) sample manipulator and a microvalve that controls the flow at the interface. An electrostatically actuated non-contact type drop-on-demand based microdispenser was developed. The dispenser was designed using finite element modeling technique that solved electrostatically actuated dispensing process. Prototypes were fabricated and tested verifying stable droplet dispensing with error in subsequent droplet generation was less than 15% between each droplet. The frequency of stable generation was 20 Hz and the average volume of dispensed droplet was 1 nL. A closed-channel EWOD actuated interface was developed that allowed a series of droplets to merge inside at the interface converting droplet flow to a continuous flow. An innovative design modification allowed series of droplet merging inside closed-channel. The interface allows integration of pressure driven devices such as: pumps, dispensers, and valves with droplet based planar EWOD devices. A novel EWOD based microvalve was developed that utilizes a thermo-responsive polymer to block and unblock a pressurized continuous flow. EWOD actuation was used to control the positioning of the valving polymer at location of interest. The valve also isolated a pressurized flow from an integrated planar EWOD device. Valves with zero leak rates were demonstrated. Such a valve will be useful in controlling microflows in EWOD to pressure driven flows such as dispensers.

Microfluidic based miniature lab-on-a-chip devices integrate different laboratory functionality in microscale. Microarray technology is evolving as a powerful tool for biomedical and pharmaceutical applications to identify gene sequences or to determine gene expression levels. Preparation of samples and spotting the arrays are the two major steps required for making microarrays. The microfluidic components developed in this research would facilitate performing the above-mentioned steps by a single lab-on-a-chip. Three microfluidic modules were developed: a non-contact microdispenser, an interface connecting the microdispenser with planar Electrowetting on Dielectric (EWOD) sample manipulator and a microvalve that controls the flow at the interface. An electrostatically actuated non-contact type drop-on-demand based microdispenser was developed. The dispenser was designed using finite element modeling technique that solved electrostatically actuated dispensing process. Prototypes were fabricated and tested verifying stable droplet dispensing with error in subsequent droplet generation was less than 15% between each droplet. The frequency of stable generation was 20 Hz and the average volume of dispensed droplet was 1 nL. A closed-channel EWOD actuated interface was developed that allowed a series of droplets to merge inside at the interface converting droplet flow to a continuous flow. An innovative design modification allowed series of droplet merging inside closed-channel. The interface allows integration of pressure driven devices such as: pumps, dispensers, and valves with droplet based planar EWOD devices. A novel EWOD based microvalve was developed that utilizes a thermo-responsive polymer to block and unblock a pressurized continuous flow. EWOD actuation was used to control the positioning of the valving polymer at location of interest. The valve also isolated a pressurized flow from an integrated planar EWOD device. Valves with zero leak rates were demonstrated. Such a valve will be useful in controlling microflows in EWOD to pressure driven flows such as dispensers.

This article reports the recent progress of re-search made in the Droplet-Based Manufacturing Laboratory at MIT. The study has been focused on obtaining a fundamental understanding of microdroplet deposition and applying the technology to various practical applications. Specific scientific contributions include the development of an analytical model for droplet splashing/recoiling, an in situ droplet size control methodology, and a study of microstructure design for spray forming. The research per-formed in the lab provides both fundamental knowledge base and practical process developments for a range of manufacturing applications, including electronics packaging, spray forming and freeform fabrication. / Singapore-MIT Alliance (SMA)

Experimental and numerical investigations of water microdroplet evaporation on heated, laser patterned polymer substrates are reported. The study is focused on both (1) validating numerical models with experimental data, (2) identifying how changes in the contact line infuences evaporative heat transfer and (3) determining methods of controlling contact line dynamics during evaporation. Droplets are formed using a bottom-up methodology, where a computer-controlled syringe pump supplies water to a ~200[micro]m in diameter fluid channel within the heated substrate. This methodology facilitates precise control of the droplets growth rate, size, and inlet temperature. In addition to this microchannel supply line, the substrate surfaces are laser patterned with a moat-like trench around the fluid-channel outlet, adding additional control of the droplets contact line motion, area, and contact angle. In comparison to evaporation on non-patterned substrate surfaces, this method increases the contact line pinning time by ~60% of the droplets lifetime. The evaporation rates are compared to the predictions of a commonly reported model based on a solution of the Laplace equation, providing the local evaporation flux along the droplets liquid-vapor interface. The model consistently overpredicts the evaporation rate, which is presumable due to the models constant saturated vapor concentration along the droplets liquid-vapor interface. In result, a modified version of the model is implemented to account for variations in temperature along the liquid-vapor interface. A vapor concentration distribution is then imposed using this temperature distribution, increasing the accuracy of predicting the evaporation rate by ~7:7% and ~9:9% for heated polymer substrates at T[sub]s = 50[degrees]C‰ and 65‰[degrees]C, respectively.

<p>      Chemical reactions in micron-sized droplets under ambient conditions are often orders of magnitude faster than the equivalent bulk reactions due to the large interfacial effects. The investigation of the underlying mechanisms driving the unique surface chemistry of droplets, as well as their applications and implications in synthesis, has garnered considerable interest. This dissertation delves into three key subtopics: (1) Exploring partial solvation as a mechanism for accelerating reactions in microdroplets, (2) Investigating the spontaneous oxidation and reduction of heteroatom double bonds induced by water radical cations and anions generated from water, and (3) Examining the role of oxazolone intermediates in prebiotic peptide synthesis and the emergence of homochirality in living systems.</p> <p>      Chemical reactions can be accelerated in microdroplets but with previously unclear mechanisms. Here we report a systematic study of organic reactions of common types in microvolumes and compare their rates with those in bulk solution. The observed interfacial area effect, molecularity effect and solvent effect provided experimental evidence for partial solvation at gas/liquid interface as one of the major contributors to the observed more than 10⁴-fold acceleration in microdroplets.</p> <p>      Recent spectroscopic results as well as computations demonstrate the existence of a strong electric field at aqueous droplet surfaces, which can result in microdroplet-specific reactions, especially their intrinsic redox properties. Spontaneous oxidation or reduction without external oxidants or reductants has been reported. One explanation for the existence of active species is dissociation of the radical cation/anion pair (H₂O^+∙/ H₂O^-∙), recently argued to occur in pure bulk water, to provide the free radical cation and radical anion. In this work, we reported spontaneous oxidation of heteroatom double bonds (e.g. sulfone to sulfonic acid, ketone to carboxylic acid) in non-aqueous microdroplets containing traces of water (<1%). Meanwhile, the simultaneous oxidation and reduction of several phosphonates was discovered, supporting the radical pair as the source of reactive species in water microdroplets.</p> <p>      One implication of microdroplet chemistry lies in its connection to prebiotic synthesis. Peptide formation from amino acids is thermodynamically unfavorable but a recent study provided evidence that the reaction occurs at the air/solution interfaces of aqueous microdroplets. Here we show that (i) the suggested amino acid complex in microdroplets undergoes dehydration to form oxazolone; (ii) addition of water to the oxazolone forms the dipeptide; and (iii) reaction of oxazolone with other amino acids forms tripeptides. Furthermore, the chirality of the reacting amino acids is preserved in the oxazolone, and strong chiral selectivity is observed when converting the oxazolone to tripeptide. This last fact ensures that optically impure amino acids will undergo chain extension to generate homochiral peptides. Peptide formation in bulk by wet-dry cycling shares a common pathway with the microdroplet reaction, both involving the oxazolone intermediate.</p>

<p dir="ltr">Scientists across many disciplines have observed a striking phenomenon: chemical reactions that do not appreciably occur in large volumes often proceed readily in microdroplets. At the core of suggested mechanisms is the influence of interfacial chemistry on the overall reaction; when the interfacial area dominates the reactor volume, the measured reaction rate is often accelerated. For instance, microdroplets with a high surface area-to-volume ratio (generally with radii smaller than 10 µm) provide a unique reaction environment and have been observed to accelerate a wide variety of chemical reactions. This is likely surprising to most readers, as much of our chemical intuition comes from experiments performed on benchtops in beakers (large, single-phase systems). However, microdroplets are regularly exploited by nature, from multiphase atmospheric aerosols to biomolecular condensates in cells. Thus, it is vital to have measurement tools capable of studying multiphase, nanoscale reactors. An electrochemical perspective on measuring multiphase chemistry under nanoconfinement is given in Chapters 2-4. To my knowledge, there were no reports of accelerated reactivity in microdroplets from electrochemical studies until the 2021 observation that enzyme turnover rates are inversely-related to the size of the containing nanodroplet (given in Chapter 6). In this dissertation work, we developed new electroanalytical tools to probe chemical transformations/reactions at micro- and nano-interfaces and made use of new reaction schemes that capitalize on multiphase microenvironments.</p><p dir="ltr">Much of the method development was built on the foundation of stochastic nanoelectrochemistry, a technique that is reviewed thoroughly in Chapters 2, 4, and 5. Briefly, stochastic nanoelectrochemistry is the measurement of single nano-entities, one-at-a-time, as the collide with a micron-sized electrode. The nano-entities studied in this dissertation were aqueous droplets, either suspended in an immiscible oil continuous phase or propelled through air. We dove deeply into these studies, from using correlated microscopy to watch how these micro- and nanodroplets collide with other interfaces to building simulations to quantify changes to the chemistry inside. We showed how the surface environment directs water nanodroplet collisions (Chapter 10) and measured the sub-diffraction-limited nanometer contact area that forms between a microdroplet and a metal surface (Chapter 11). Using the nanodroplets as tiny reactors, we measured accelerated rate constants and promoted unfavorable nucleation events in attoliter-femtoliter aqueous droplets (see Chapter 6-7 and Chapter 12, respectively) and in microliter aqueous droplets (see Chapter 8 and Chapter 9, respectively).</p><p dir="ltr">As mentioned above, microdroplets are ubiquitous in air (<i>e.g.,</i> aerosols). However, electrochemistry is not an obvious choice for the measurement of intact aerosols because electrochemistry is traditionally performed in a conductive solution, and electrochemistry in air is difficult. In this dissertation we laid the groundwork for a path forward that allows electrochemical access the air|microdroplet interface. We designed and characterized a novel electrochemical cell, where the working electrode is a microwire traversing a suspended liquid film (Chapters 13-15). The early results were born from pure curiosity: Can we do electrochemistry in a soap bubble wall? Chapter 13 shows that the answer is “Yes!”, and that electrochemistry can report on aerosol contents that are collected from the air into this thin film. However, the soap bubble wall was severely limited by the lifetime of the bubble wall (bubbles pop), so in Chapters 14 and 15, we introduce a suspended ionic liquid film that does not pop from evaporation. With the more robust system, we realized the ability to probe intact single microdroplets, one-at-a-time (Chapter 14), giving electrochemical access to the air|water interface.</p><p dir="ltr">As detection of illicit substances from aerosols has the potential for immediate impact on first responder, user, and bystander safety, we employed the new technology to electroanalyze aerosolized methamphetamine (Chapter 13) and fentanyl (Chapter 15). Electrochemistry is small, simple, and affordable, making it a realistic candidate for an in-field sensor. We overcame selectivity challenges by using our understanding of interfacial microenvironments to leverage local pH changes, as demonstrated by the reliable detection of low purity cocaine in mixed powders (Chapter 16). This patented method provides a highly selective technique for cocaine identification in the presence of adulterants without the need to bring any chemicals to the scene (water is our only reagent!).</p><p dir="ltr">In sum, this body of work contributes to the electrochemical studies in nano- and microdroplets, extending the reach to account for droplet size on measured rates and to include microdroplets with a water|air boundary. Applications of the work were focused on in-field detection of illicit substances.</p>

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The objective of this study was to investigate the role of GnRH on the preimplantation development of mouse embryos in vitro. GnRH-I, GnRH-II, and GnRH agonists: Des-Gly, Des-Trp and histrelin did not improve embryo development. However, treatment with the specific GnRH antagonist SB-75 blocked embryo development at morula stage. The inhibition of embryo development by SB-75 could be rescued by the addition of histrelin. To determine which intracellular signaling cascade is involved following binding of GnRH to the GnRHR, embryos were cultured in the presence of specific PKC (GFX) or PKA (SQ22536) inhibitors. The PKC inhibitor blocked embryo development at a similar stage as SB-75, whereas SQ22536 had an inhibitory effect, diminishing blastocyst formation and hatched rates. There are evidences that GnRH has an essential autocrine effect on mouse embryonic development via GnRHR, probably by activating PKC signaling cascade while the inhibition of the GnRH signaling does not activate apoptotic mechanisms involving caspase-3. In another experiment, development in vitro of embryos from Chinese Meishan (M) and occidental white crossbred (WC) females were investigated after improving the vitrification protocol for pig embryos. Efficient cryopreservation of zona pellucida-intact porcine embryos and studies of the difference among breeds could greatly impact the swine industry. The percentage of embryos surviving 24 h after cryopreservation without lysis or degeneration was higher for M (72%) than WC (44%). However, in vitro development of embryos that survived cryopreservation was not different between M and WC at the expanded (64%) or hatched (22%) blastocyst stages. Developmental rates were significantly higher for control embryos than frozen embryos from both breeds at expanded blastocyst stage, but not at hatched blastocyst stage. Rates of expanded blastocyst formation did not differ between M and WC control embryos (98 and 95%, respectively). With a new procedure to warm vitrified pig embryos, the survival rates may be improved. The optimal stages to vitrify pig embryos using the microdroplet method ranges from late compact morula to early expanded blastocyst. The results suggest that M embryos have a higher capacity to survive the vitrification process than WC embryos. / O objetivo do presente estudo foi investigar a importância do GnRH no desenvolvimento embrionário precoce em camundongos. GnRH-I, GnRH-II e os GnRH agonistas: Des-Gly, Des-Trp e histrelina não incrementaram o desenvolvimento embrionário. Entretanto, o tratamento com SB-75, um antagonista específico do GnRH, bloqueou o desenvolvimento embrionário no estádio de mórula. A inibição do desenvolvimento embrionário pelo SB-75 pôde ser revertida com a adição de histrelina. Para determinar a cascata do sinal intracelular desencadeada pela ligação do GnRH com o seu receptor, embriões foram cultivados na presença de inibidores específicos da PKC (GFX) e da PKA (SQ22536). O inibidor da PKC bloqueou o desenvolvimento embrionário em estádio similar ao bloqueio mediado pelo SB-75, enquanto o SQ22536 teve efeito inibitório diminuindo a formação de blastocisto e taxas de eclosão. Os resultados sugerem que o GnRH tem um efeito autócrino essencial no desenvolvimento embrionário através do GnRHR, provavelmente, ativando a cascata da PKC. Por outro lado, a inibição do sinal do GnRH não ativa mecanismos apoptóticos que involvam caspase-3. Em outro experimento, foi investigado o desenvolvimento in vitro de embriões da raça Meishan (M) e branco cruzado (WC) após vitrificação pelo método microgota. O desenvolvimento de protocolos eficientes para criopreservação de embriões suínos com a zona pelúcida intacta e a avaliação das diferenças entre raças pode ter um significativo impacto na suinocultura. A percentagem de embriões que sobreviveram à criopreservação depois de 24 h foi maior na M (72%) do que na WC (44%). No entanto, o desenvolvimento in vitro dos embriões que sobreviveram à criopreservação não foi diferente entre M e WC nos estádios de blastocisto expandido (64%) ou eclodido (22%). Os índices de desenvolvimento foram significativamente mais altos para os embriões controle do que para os embriões vitrificados nas duas raças no estádio de blastocisto expandido, porém não foram diferentes para o estádio de blastocisto eclodido. A formação de blastocisto expandido não diferiu entre os embriões controle M e WC (98 e 95%, respectivamente). Com o novo procedimento ( hot warm ) para descongelar embriões vitrificados pelo método de microgota, pode-se aumentar dos índices de sobrevivência. Os melhores estádios embrionários para a vitrificação de embriões suínos variam de mórula compacta tardia até blastocisto expandido inicial. Os resultados sugerem que embriões M têm mais capacidade de sobreviver ao processo de vitrificação do que embriões WC.

<p>Microdroplets have large and varied areas of application ranging from document printing to complex lab-on-chip devices. Lab-on-chip systems often require precise volume control as well as high throughput operations. Microdroplets fulfill these requirements and have become a staple in these devices. The work presented in this thesis involves the design and characterization of two individual devices capable of droplet generation utilizing flow focusing and electrogeneration methods.</p> <p>The first design involved the generation of gel microdroplets utilizing the flow focusing technique. This device proved to be robust and reliable producing large volumes of uniformly mixed droplets. Long term operation of this device was analyzed and determined to be a feasible route for the manufacture of large quantities of droplets. The device was operated for over 30 hours creating gel droplets ranging from 40-200 μm in diameter with acceptable polydispersities for use in drug release studies.</p> <p>The second device involved the design and characterization of a system for the electrogeneration of microdroplets. This novel device involved the injection of droplets via high voltage and high frequency signals into a cross-flow of oil. The droplet generation was characterized and different droplet generation modes were observed. With the careful selection of parameters ideal conditions were obtained to generate monodisperse droplets of sizes ranging from under 5 to over 100 μm in a highly repeatable manner.</p> <p>To conclude, two separate microfluidic droplet generation devices operating in distinct modes were designed and analyzed. These devices are robust, reliable, and flexible with some applications being tested.</p> / Master of Applied Science (MASc)

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