N2 - Two types of polydimethylsiloxane PDMS membranes with two thicknesses, 0.125 mm and 0.25 mm, were studied experimentally for a series of environmental conditions in a closed chamber. The conditions were changing oxygen concentration, ambient pressure and normoxic conditions. The results for piloted ignition are linearly dependent on oxygen concentration and ambient pressure via chemistry. A non-monotonic dependency was found in normoxic conditions, but the dominant mechanisms are not clear. The flame spread has a linear dependence on oxygen concentration via chemistry and thermal transfer. Two dependency regimes (linear and asymptotic) emerge for flame spread as a function of ambient pressure. The mechanisms are due to thermal transfer (linear regime) and a combination of thermal transfer and kinetics (asymptotic regime). Flame lengths are exponentially dependent on oxygen concentration via oxygen/fuel supply and soot oxidation. On the contrary, ambient pressure affects flame length non-monotonically via fuel supply (increasing regime) and an excess of oxidiser (decreasing regime). Under normoxic conditions, increasing oxygen concentration had a stronger influence than decreasing pressure with respect to both flame spread and flame length. The extinction conditions at low pressure are due to a combination of increased radiative losses (similar to those of microgravity conditions) and kinetic effects. Furthermore, extinction at low pressures occurred well below normoxic or hypoxic conditions. Thus, the two PDMS materials tested are more flammable under normoxic conditions, which needs to be considered when assessing the fire risk associated with spacecraft design and operation. It remains unclear how these results directly translate to microgravity, where other phenomena might dominate, so future experiments in microgravity are recommended.
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AB - Two types of polydimethylsiloxane PDMS membranes with two thicknesses, 0.125 mm and 0.25 mm, were studied experimentally for a series of environmental conditions in a closed chamber. The conditions were changing oxygen concentration, ambient pressure and normoxic conditions. The results for piloted ignition are linearly dependent on oxygen concentration and ambient pressure via chemistry. A non-monotonic dependency was found in normoxic conditions, but the dominant mechanisms are not clear. The flame spread has a linear dependence on oxygen concentration via chemistry and thermal transfer. Two dependency regimes (linear and asymptotic) emerge for flame spread as a function of ambient pressure. The mechanisms are due to thermal transfer (linear regime) and a combination of thermal transfer and kinetics (asymptotic regime). Flame lengths are exponentially dependent on oxygen concentration via oxygen/fuel supply and soot oxidation. On the contrary, ambient pressure affects flame length non-monotonically via fuel supply (increasing regime) and an excess of oxidiser (decreasing regime). Under normoxic conditions, increasing oxygen concentration had a stronger influence than decreasing pressure with respect to both flame spread and flame length. The extinction conditions at low pressure are due to a combination of increased radiative losses (similar to those of microgravity conditions) and kinetic effects. Furthermore, extinction at low pressures occurred well below normoxic or hypoxic conditions. Thus, the two PDMS materials tested are more flammable under normoxic conditions, which needs to be considered when assessing the fire risk associated with spacecraft design and operation. It remains unclear how these results directly translate to microgravity, where other phenomena might dominate, so future experiments in microgravity are recommended.
Three simple approaches for the selective immobilization of biomolecules on the surface of poly(dimethylsiloxane) (PDMS) microfluidic systems that do not require any specific instrumentation, are described and compared. They are based in the introduction of hydroxyl groups on the PDMS surface by direct adsorption of either polyethylene glycol (PEG) or polyvinyl alcohol (PVA) as well as by a liquid-based oxidation step. The hydroxyl groups are then silanized using a silane containing an aldehyde end-group that allows the surface to interact with a primary amine moiety of the biomolecule structure to be immobilized. The entire process takes 4.5h. The required steps can be characterized in less than 15 hours by contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The performance of the biofunctionalization process can be assessed by using peroxidase enzyme as a model biomolecule. Its correct immobilization and stability is easily tested by developing an analytical approach for hydrogen peroxide (H2O2) detection in the biofunctionalized microfluidic system and carrying out analytical measurements for a period of up to two months.
Here, the details of the protocol used in a previous work21 for different liquid-based surface chemical biofunctionalization methods areprovided. The developed methods can easily be performed in standard chemical and biological laboratories avoiding the need of special instrumentation.Both physical adsorption and covalent modification methods are analyzed. On one hand, physical adsorption of two different polymers containing hydroxylgroups, such as polyethylene glycol (PEG) or polyvinyl alcohol (PVA) (figure 1b) enable the further silanization of the surface for the introduction ofchemical functional groups and the eventual covalent immobilization of the bioreceptor. In some applications, as Yu et al did (22), theaim of the PVA immobilization was to avoid the non-specific binding of proteins. Other groups used PEG instead of PVA, since it offers the sameadvantage (23,24). In the present application, the objective is totally different. These polymers are used as anchoring points for furthersilanization and final protein receptor immobilization. On the other hand, a covalent modification approach was tested based on the chemical oxidationof the PDMS surface that generates silanol groups (figure 1c) onto which a silanization process and further immobilization of the protein receptor arecarried out, as above. This chemical oxidation protocol was already described by Sui et al. for creating hydroxyl groups that could be used asanchoring points for the immobilization of other molecules (13). A deep structural characterization of the resulting modified surfaces iscarried out. The analytical performance and the stability of the modified surfaces following the different methods are also tested using a PDMS-basedphotonic LoC (PhLoC) microsystem consisting of a hollow Abbe prism transducer configuration (25).
The described PDMS biofunctionalization approaches were applied to the modification of a photonic Abbe prism based LoC system. Horseradish peroxidasewas selected as a model biomolecule. The analytical performance of the resulting modified systems was then tested by carrying out the analysis of H2O2. HRP was chosen because it is a widely used enzyme that exhibits a high turnover number and can be applied with a high numberof different mediators. Also, as H2O2 is the product of many other enzymatic reactions, HRP catalysis can be coupled in morecomplex enzymatic systems in order to get a cascade reaction to be applied for signal amplification.
As it can be seen in Figure 12, the absorbance at 420 nm increased together with the H2O2 concentration for all the testedsystems. This increase was lineal in the range 0-24.3 µM H2O2 and then saturation occurred. A linear fitting was carried outin this range and the analytical parameters were calculated (Table 3). There were no significant differences among the different approaches, but theestimated error was higher for the adsorption approach. The lowest LOD was 0.10 µM H2O2. This result was between 10 and 100 times lower than the previously reported values in similar analytical systems based on the use of HRP as a receptor (25,36). In addition, thesensitivity of the modified PhLoC was 150 times better than in other applications using the same system (30).
A larger scale PMA consisting of 25 two- and four-way valves was fabricated using the optimized parameters detailed above and tested for functionality after long term storage and up subject to high numbers of actuations. Briefly, SU-8 molds for both pneumatic and fluidic layers were fabricated using a standard photolithography protocol4,12. The fabrication then proceeded as previously described for the 5-valve chip. A stamp was created and applied using the optimized parameters determined before the PDMS was bonded to a glass wafer. A pumping sequence was used to demonstrate the actuation of all microvalves for transporting liquid from an inlet to an outlet. Flow rate measurements were taken using a Sensirion (Switzerland) SLI-1000 flow rate monitor and compared either after long term dry storage or over an extended period of repeated actuation. Furthermore, the PMA was challenged on a rocket launch to be exposed high g-force condition and rocket launch-induced vibration to confirm the stability of PFTCS coating. The flight parameters are included in Table SI1 and a link to view the flight video is also included. A video of the launch is also included with the supplemental info. A VB300 (Extech; Nashua, NH) g-force data logger was used to measure acceleration and vibration during this launch.
As an additional demonstration for this selective bonding technique, the stretchable microdevice was fabricated and tested by observing the assembly and quality of the designed 3D structures. To fabricate the stretchable device shown in Fig. SI8, thin PDMS was unidirectionally stretched 15%, masked, PFTCS deposited, and then bonded to a PDMS ribbon after alignment with the PFTCS pattern before relaxation. Multiple arched shaped 3D structures were formed successfully without any heavy microfabrication procedures when stretching of thin PDMS substrate was relaxed. 2ff7e9595c
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