Aqueous two-phase systems (ATPSs), traditionally utilized in industrial bioseparations, are showing increasing potential as an approach for concentrating components in paper-based point-of-care bioassays. Specifically, our lab was the first to demonstrate that ATPSs can increase the sensitivity of the lateral-flow immunoassay (LFA) by concentrating biomarkers into one of the two phases, after which, the phase containing the concentrated biomarkers was extracted prior to their detection. While this method demonstrated consistent improvements to the LFA, its applicability in point-of-care settings was restricted by two main factors. First, the time to separate into two distinct phases varied among different classes of ATPSs, but usually required hours to achieve effective biomolecule concentration. Second, the method required several user steps in the form of sample mixing with the ATPS components, and the subsequent extraction and application of the phase containing the concentrated biomolecule to the LFA after phase separation occurred. More recently, our lab demonstrated that when the mixed, homogenous ATPS was applied to a paper membrane, phase separation was observed within the paper membrane itself as the solution wicked across the paper. This largely unexplored phenomenon reduced the phase separation time of a polyethylene glycol (PEG)-salt ATPS from hours in a tube to minutes on paper.
This thesis focuses on advancing the phenomenon of paper-based phase separation as a means of enhancing the phase separation rate of ATPSs and making them more suitable for point-of-care applications. First, we extended the paper-based phase separation phenomenon to a naturally slow phase separating system, the Triton X-114 ATPS. Next, we investigated the dehehydration of the phase forming components directly into the paper matrix, and the subsequent phase separation upon resolubilization of the components by a liquid sample. Within these two investigations, the ATPS was then integrated with the LFA to improve the detection of biomarkers of infectious diseases such as malaria and chlamydia. Furthermore, we investigated the use of the Washburn equation as a mathematical framework to describe the flow behavior of ATPS phases within porous media in order to better predict phase separation behavior within paper.
The Triton X-114 ATPS is a micellar ATPS that is comprised of the Triton X-114 nonionic surfactant. This particular ATPS, previously used by our lab to concentrate biomarkers for the LFA, is one of the slowest separating systems, partially due to the small interfacial tension and density difference between its two phases (the micelle-poor and micelle-rich phases). We applied the Triton X-114 ATPS to paper membranes and demonstrated, using a distinctively different design in which the solution flows vertically up a multilayered paper wick, that paper-based phase separation can also be achieved with the Triton X-114 system. In this case, we found that gravitational effects had no influence on the flow of the dense gold nanoparticles as the less dense micelle-poor phase containing the gold wicked ahead of the micelle-rich phase. This was the first time that a micellar ATPS was applied directly to a fiberglass paper membrane to significantly speed up its macroscopic separation from at least 8 hrs in a test tube to approximately 3 min on paper. The paper-based Triton X-114 ATPS was then integrated with the LFA to simultaneously concentrate a malaria protein biomarker into the leading micelle-poor phase, and then detect it without the need of a user-dependent phase extraction step. The single-step integration improved the LFA detection limit for the protein by 10-fold in buffered saline and complex serum media. This was also the first time within our lab that we concentrated an infectious disease biomarker in complex biological fluids.
The design used for the abovementioned Triton X-114 study is applicable when dealing with oral and vaginal swab samples as the swab would need to be mixed with a buffer solution to solubilize the target. However, in urine, saliva, or blood applications, one would prefer to just add the biological fluid to the device. The second focus of the thesis was therefore to further improve the user-friendliness of the ATPS-LFA integration by removing initial sample preparation steps. To achieve this, we investigated the novel concept of sequential rehydration of the two-phase components that were initially dehydrated into the paper matrix as a way to achieve paper-based ATPS phase separation. We used two different polymer-salt ATPSs: the PEG-salt system and the UCON-50-HB-5100 (UCON)-salt system, optimizing the component concentrations and rehydration order to yield the appropriate phase separation conditions for each system. Upon rehydration of the components, phase separation successfully occurred within the paper, leading to the formation of a leading polymer-poor phase and a lagging polymer-rich phase. As mentioned above, the benefit of this method is that a biological sample no longer needs to be manually mixed with the components and instead can be directly added to the device. These dehydrated systems were then integrated with the LFA to produce paper-based assays in which all the necessary components for target concentration and detection were stored within the paper matrix. The dehydrated PEG-salt ATPS and UCON-salt ATPS were integrated with the LFA to simultaneously concentrate and detect large Chlamydia trachomatis whole bacteria and smaller human IgM antibodies, respectively. Ultimately, our designs demonstrated 10-fold improvements to the detection limits for both Chlamydia trachomatis and IgM, and could therefore improve LFA sensitivity without adding steps to the user. These exciting developments solve the initial limitations of the ATPS-LFA integration, making their use as point-of-care diagnostic devices for infectious diseases one step closer to reality.
Although paper-based phase separation of several different aqueous two-phase systems have been experimentally demonstrated, there is a need for a mathematical model that can accurately predict two-phase system wicking behavior in paper-based devices, as it would benefit the process of device design. We decided to evaluate the Washburn model as a framework for fluid flow of the PEG-salt ATPS and Triton X-114 ATPS in porous media. Using a combination of imbibition studies and characterization of the Washburn fluid parameters for each individual phase, we determined that the viscosity difference between the two phases is a dominant factor in the ability of a given ATPS to phase separate in paper. More specifically, the Washburn model correctly predicts that the less viscous phase will constitute the leading phase, independent of flow direction in a horizontal or vertical orientation. In this validation of the model, we then applied it to predict the phase separation capabilities of two polymer-polymer systems, the PEG-Dextran system and the PEG-polyacrylic acid (PEG-PAA) system. We predicted and successfully demonstrated that the PEG-PAA system could phase separate due to its large enough difference in phase viscosities, while the PEG-Dextran system could not phase separate due to its small viscosity difference. Furthermore, this theoretical framework was extended to predict the phase separation of ATPSs in polyethylene glycol dimethacrylate-based microporous hydrogels. We were able to predict and show for the first time that phase separation of various ATPSs could be achieved within hydrogels, demonstrating that the phase separation enhancement phenomenon can occur in various types of porous media.