Bacterial populations housed in microfluidic environments can serve as transceivers for molecular communication, but the data-rates are extremely low (e.g., 10 −5 bits per second.). In this work, genetically engineered Escherichia coli bacteria were maintained in a microfluidic device where their response to a chemical stimulus was examined over time. The bacteria serve as a communication receiver where a simple modulation such as on-off keying (OOK) is achievable, although it suffers from very poor data-rates. We explore an alternative communication strategy called time-elapse communication (TEC) that uses the time period between signals to encode information. We identify the limitations of TEC under practical non-zero error conditions and propose an advanced communication strategy called smart time-elapse communication (TEC-SMART) that achieves over a 10x improvement in data-rate over OOK. We derive the capacity of TEC and provide a theoretical maximum data-rate that can be achieved.

Echogenic liposomes (ELIP) are an excellent candidate for concurrent imaging and drug delivery applications. They combine the advantages of liposomes—biocompatibility and ability to encapsulate both hydrophobic and hydrophilic drugs—with strong reflections of ultrasound. The objective of this study is to perform a detailed in vitro acoustic characterization—including nonlinear scattering that has not been studied before—along with an investigation of the primary mechanism of echogenicity. Both components are critical for developing viable clinical applications of ELIP. Mannitol, a cryoprotectant, added during the preparation of ELIP is commonly believed to be critical in making them echogenic. Accordingly, here ELIP prepared with varying amount of mannitol concentration are investigated for their pressure dependent linear and non-linear scattered responses. The average diameter of these liposomes is measured to be 125–185 nm. But they have a broad size distribution including liposomes with diameters over a micro-meter as observed by TEM and AEM. These larger liposomes are critical for the overall echogenicity. Attenuation through liposomal solution is measured with four different transducers (central frequencies 2.25, 3.5, 5, 10 MHz). Measured attenuation increases linearly with liposome concentration indicating absence of acoustic interactions between liposomes. Due to the broad size distribution, the attenuation shows a flat response without a distinct peak in the range of frequencies (1–12 MHz) investigated. A 15–20 dB enhancement is observed both for the scattered fundamental and the second harmonic responses at 3.5 MHz excitation frequency and 50–800 kPa amplitude. It demonstrates the efficacy of ELIP for fundamental as well as harmonic ultrasound imaging. The scattered response however does not show any distinct subharmonic peak for the acoustic excitation parameters studied. Small amount of mannitol proves critical for echogenicity. However, mannitol variation above 100 mM shows no effect.

The stabilizing encapsulation of a microbubble based ultrasound contrast agent (UCA) critically affects its acoustic properties. Polymers, which behave differently from commonly used materials—e.g. lipids or proteins—for the monolayer encapsulation, hold potential for better stability and control over encapsulation properties. Air-filled microbubbles coated with Poly (D, L-lactide) (PLA) are characterized here using in vitro acoustic experiments and several models of encapsulation. The interfacial rheological properties of the encapsulation are determined according to each of these models using attenuation of ultrasound through a suspension of these microbubbles. Then the model predictions are compared with scattered nonlinear—sub- and second harmonic—responses. For this microbubble population (average diameter 1.9 μm), the peak in attenuation measurement indicates a weighted average resonance frequency of 2.5–3 MHz, which, in contrast to other encapsulated microbubbles, is lower than the resonance frequency of a free bubble of similar size (diameter 1.9 μm). This apparently contradictory result stems from the extremely low surface dilatational elasticity (around 0.01–0.07 N/m) and the reduced surface tension of the PLA encapsulation as well as the polydispersity of the bubble population. All models considered here are shown to behave similarly even in the nonlinear regime because of the low value of the surface dilatational elasticity. Pressure dependent scattering measurements at two different excitation frequencies (2.25 and 3 MHz) show strongly non-linear behavior with 25–30 dB and 5–20 dB enhancements in fundamental and second-harmonic responses respectively for a concentration of 1.33 μg/mL of suspension. Subharmonic responses are registered above a relatively low generation threshold of 100–150 kPa with up to 20 dB enhancement beyond that pressure. Numerical predictions from all models show good agreement with the experimentally measured fundamental response, but not with the second harmonic response. The characteristic features of subharmonic response and the steady response beyond the threshold are matched well by model predictions. However, prediction of the threshold value depends on property values and the size distribution. The variation in size distribution from sample to sample leads to variation in estimated encapsulation property values—the lowest estimated value of surface dilatational viscosity better predicts the subharmonic threshold.

In this work we consider nano-scale communication using bacterial populations as transceivers. We demonstrate using a microfluidic test-bed and a population of genetically engineered Escherichia coli bacteria serving as the communication receiver that a simple modulation like on-off keying (OOK) is indeed achievable, but suffers from very poor data-rates. We explore an alternative communication strategy called time elapse communication (T EC) that uses the time period between signals to encode information. We identify the severe limitations of T EC under practical non-zero error conditions in the target environment, and propose an advanced communication strategy called smart time elapse communication (T EC-SM ART ) that achieves over a 10x improvement in data-rate over OOK. The thesis is organized as follows.Chapter 2 presents a detailed description of the bacterial strain used and the microfluidic system that houses the bacteria. Chapter 3 presents the results from microfluidic experiments with E. coli bacteria and establishes the motivation for timeelapse communication in super-slow networks. Chapter 4 presents the key design principles of time-elapse communication. Chapter 5 presents the theoretical maximum achievable data-rate using time-elapse communication and Chaper 6 shows the simulation results of time-elapse communication along with the optimization proposed.

Liposomes are submicron sized vesicles with a lipid bilayer encapsulating an aqueous phase inside. Due to their favorable properties like longer circulation time, lesser toxicity, and greater uptake, they are a prime candidate for drug delivery. Recently, they are being specially prepared so as to encapsulate air, making them good scatterers of ultrasound wave. These echogenic liposomes, therefore, can be used both for ultrasound contrast imaging and drug delivery. We will report in vitro attenuation and scattering measurement from echogenic liposomes loaded with carboxyfluorescein (used as a surrogate for small molecular weight drugs). The results will be compared with non-dye-loaded ones. Effects of dye loading and presence of bovine serum albumin (BSA) on size distribution, echogenicity, and release characteristics will also be discussed.