Microfluidic devices as gas – Ionic liquid membrane contactors for CO2 removal from anaesthesia gases
Introduction
The most commonly used anaesthetic gas for surgical operations is nitrous oxide (N2O). The mixture of nitrous oxide and volatile anaesthetic compounds, mainly isoflurane, desflurane and sevoflurane, is introduced to the closed breathing system [1]. The main problem with such mixture is the risk of hypoxia. Hypoxia caused by the excessive amount of N2O, also called a ‘third gas effect’, influences the partial pressure of O2 within the alveolar channels. When high quantity of nitrous oxide is present in the alveoli, O2 and CO2 are diluted by this gas which leads to the decrease of their respective partial pressures resulting in insufficient blood oxygenation [2].
An alternative approach in order to replace nitrous oxide is to use Xe as an anaesthetic gas. The anaesthetic properties of Xe were discovered, analysed and described in the early 1950s [3]. Xe possesses a number of characteristics that make it a perfect anaesthetic compound. It is hemodynamically stable, which results in lack of cardiac depression, it produces high regional blood flow and it possesses low solubility in liquids, greatly reducing the risk of hypoxia [2], [4]. Xe is particularly attractive for the neonates as it attenuates isoflurane neurotoxicity [5]. Moreover, Xe is non – flammable, non – toxic, and does not contribute to the depletion of the ozone layer [2], [6] compared to N2O; however, it is far more expensive (1 l of produced liquid Xe consumes 792 kJ which costs approximately 1000 $, that is 100 times more than the price of N2O) [4], [6]. Hence, recycling of Xe from the anaesthesia exhaled gas rather than wasting it to the atmosphere is the only way to ensure economic and efficient Xe use.
The gas mixture exhaled from the patient during the surgical operation, where Xe is used as an anaesthetic gas, consists mainly of Xe (≈ 65%), O2 (≈ 27%), N2 (≈ 3.3%) and CO2 (5%) [1]. The conventional method of removing CO2 from the exhaled gas mixture in a closed – circuit technology relies on the use of soda lime that is composed of: calcium hydroxide, water, sodium hydroxide and potassium hydroxide. In this system the removal of CO2 is limited by the size of the CO2 absorber canister [5]. Apart from this, the process is relatively efficient and commonly used, but it suffers from a number of problems. The soda lime, if allowed to dry up, can produce hydrogen and heat resulting in an explosion hazard; or can react with volatile anaesthetic compounds producing toxic side products (e.g. fluoromethyl-1-2,2-difluoro-a-(trifluoromethyl) vinyl ether [7] or carbon monoxide [1]) in the gas circuit. Therefore, several attempts, with alternative methods, are being investigated for the on – line removal of CO2 from anaesthetic closed circuits down to 0.5%. Mendes et al. proposed the use of a membrane contactor technology with commercial carbon molecular sieve membranes (CMSM) in the form of hollow fibres [8] or flat sheets [1], and diamine as CO2 absorber. High permeation and ideal selectivities for CO2/Xe and N2/Xe were obtained in the case of single gas experiments. On the other hand, multicomponent performance was adversely affected by Xe, that possesses a kinetic diameter of 4.04 Å, which is able to block the pores close to the CMSM surface (with diameters in the range of 3–5 Å) resulting in the reduction of the free pore space and limitation of the diffusion of other species. Hollow-fibre based non-volatile liquid membranes have been also successfully applied for CO2 removal from N2O anaesthesia mixtures containing halogenated hydrocarbons [9].
Recently, Yong et al. have described the use of a hollow fibre membrane-based gas-liquid contactor for CO2 capture using potassium carbonate as a solvent [10]. The authors propose the coating of the membrane surface, non-porous PDMS-PS or porous PP, with carbonic anhydrase (CA) enzyme by LbL technique to: i) increase mass transfer rates due to the reduction in pore wetting by the adsorbed polyelectrolytes; ii) to promote the CO2 absorption kinetics into K2CO3. However, a slight loss in enzyme activity due to immobilization is observed even during the short term kinetic studies.
Additionally to all the trials focused on the membrane type, material and morphology, it was discovered that some ionic liquids possess high affinity towards CO2 and the addition of carbonic anhydrase increases this property remarkably [11], [12]. CA is a naturally occurring thermoresistant metalloenzyme that works as a catalyst for the reversible conversion of CO2 into bicarbonate (HCO3-) at extremely high turnover rates. It regulates important biological processes within humans and other living organisms. This catalytic activity has been exploited for promoting the absorption of CO2 from N2 containing gas streams [12], [13], [14]. Neves et al. [12] described the use of supported liquid membranes for the integrated CO2 capture and enzymatic bioconversion. A commercial hydrophobic porous membrane made of polyvinylidene fluoride (PVDF) was used as a support for the immobilization of CO2 absorbing solvents, i.e. polyethylene glycol (PEG) 300 and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ionic liquid. The CO2 solubility increased between 20% and 30%, even at low enzyme concentration (0.01% w/w) due to the chemical reaction enhancement factor [12].
In the last decade, biodegradable, biocompatible and environmental-friendly ILs that are synthesized by naturally-derived materials such as sugars and aminoacids are gaining increasing importance. This novel class of ILs may provide an optimum media to stabilize proteins (i.e. enzyme) [15] which is of our interest for the increase in CO2 capturing effect. Cholinium cations are quaternary ammonium cations ([N,N,N - trimethylethanolammonium]+) fully derived from natural products [11], [16]. It was shown that the cholinium cations combined with a range of alkanoate anions or amino acids provide a media where living cells can actively grow [17], [18]. Moreover, they show high CO2 capturing effect by absorption plus chemical reaction that take place due to their ammonium cations [19]. When compared to the commonly used ionic liquids for CO2 capture [20], the CO2 solubility values are lower, i.e 0.209 mol/mol for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide compared to 0.152 mol/mol for cholinium propionate at 10 bar and 303 K. Nevertheless, cholinium based ILs show high biocompatibility and provide an environment where CA is active.
Martins et al. [11] evaluated the CO2 solubility and diffusivity values in cholinium based ionic liquids as a function of water activity content and in the presence of CA (0.01% w/w). The highest CO2 solubility was obtained at the lowest water activity due to the lower solubility of CO2 in an ionic liquid with higher water content. However, in case of the ionic liquid combined with the enzyme, the highest enhancement in CO2 solubility was obtained with the largest aw because, under these conditions, the required water is available to assure the enzyme activity. As a result, the optimal water activity value was identified to be 0.753. Among the tested ILs, cholinium propionate (CP) exhibited the highest potential for CO2 capture with a maximum transport enhancement of 63% in the presence of CA [11].
The present work pursues the proof of concept validation of a PDMS based microfluidic device as a gas – liquid contactor, with cholinium propionate and CA as a liquid phase, for the removal of CO2 from Xe anaesthetic gas. A priori, the high gas permeability values of dense free standing PDMS membranes could allow to fully exploit the advantageous effect of the CO2 enzymatic reaction in the liquid phase. In addition, due to miniaturization, the volume of ionic liquid required to fill the micro – chamber and the amount of CA are notably reduced while providing high throughput for the preliminary screening of different ionic liquid-enzyme formulations at several working conditions. Furthermore, this approach could be of interest for Xe recovery systems that require low capacity as is the case of recirculating machines for neonates. Moreover, the general advantages of PDMS microfluidic devices in terms of fabrication (simplicity and cost), weight, size, easy integration and scaling up by replication and stacking by O2 plasma are also worthy to mention. The microdevice consists of two independent chambers (one dedicated to the liquid phase and the second one devoted to the gas phase) and a non-porous flat PDMS membrane in between. The gas, CO2 or Xe, is introduced in one of the chambers, permeates through the membrane and dissolves in the ionic liquid on the other compartment. An alveolar design has been mainly adopted for the liquid chamber to ensure that mass transfer of CO2 through the liquid phase, in order to reach the CA active reaction sites, does not become rate controlling. As in previous works, enhanced CO2 transport is expected due to the high solubility of CO2 in cholinium propionate assisted by the enzymatic CO2 conversion, both leading to greater driving force. In order to confirm such hypothesis the following tasks were performed: 1 – design and fabrication of a polymeric membrane and a microfluidic device with different geometries depending on the chamber purpose (liquid or gas); 2 – characterization of the fabricated chip in terms of leaks, membrane detachment and maximum tolerable pressure for its validation as G-L contactor; 3 – determination of the single gas permeability values of free standing membrane; and, 4 – evaluation of the G-L microcontactor performance for CO2 capture due to the presence of CP solvent flowing in the liquid chamber and the use of CA to accelerate CO2 transport.
Section snippets
Ionic liquid tested
Cholinium propionate (CP) was used in the CO2/Xe separation procedure. The ionic liquid was prepared by neutralization of propionic acid (purchased from Sigma-Aldrich, USA) in choline hydroxide. The resulting ionic liquid (IL) was equilibrated into a specific water activity, i.e. 0.753, with a sodium chloride salt (Applichem, Panreac, Germany). The water content corresponding to a given water activity was measured with a Karl-Fisher coulometer (Metrohm, model 831 KF coulometer).
Carbonic anhydrase enzyme
Carbonic
Permeability of a free – standing membrane
Fig. 4 shows the normalized ΔP/ΔP0 decay (defined using t=0 as reference) as a function of time divided by the membrane thickness, for two different free-standing membranes (60 µm thick and 126 µm thick). The measurements were performed for O2, Xe and CO2. The raw data (see Fig. S4, SI) shows that the pressure decrease of the feed is equal to the increase of pressure at the permeate indicating that the volumes of the feed and permeate chambers are identical. As it was expected, the slowest ΔP/ΔP0
Conclusions
In summary, we have investigated the effect of an ionic liquid and its combination with carbonic anhydrase to remove CO2 from Xe used in anaesthesia. The microfluidic experimental system was designed as a membrane contactor working in a semi-continuous operation mode. Even though the permeability of PDMS for Xe and CO2 was similar, the cholinium propionate and cholinium propionate in combination with carbonic anhydrase showed the enhancement in the CO2 capturing effect, while there was no
Acknowledgments
The authors would like to acknowledge the financial support from the Government of Aragon and the Education, Audiovisual and Culture Executive Agency (EU-EACEA) within the EUDIME – “Erasmus Mundus Doctorate in Membrane Engineering” program (FPA 2011-0014, SGA 2012-1719, http://eudime.unical.it). CIBER-BBN is an initiative funded by the VI National R&D&i Plan 2008–2011 financed by the Instituto de Salud Carlos III European Regional Development Fund. Authors acknowledge the LMA-INA for offering
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