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Membrane engineering in biotechnology: quo vamus?
Opinion
TRENDS in Biotechnology
Vol.25 No.6
Membrane engineering in biotechnology: quo vamus? Gilbert M. Rios, Marie-Pierre Belleville and Delphine Paolucci-Jeanjean IEM UMR 5635, CC047, Place E. Bataillon, F 34095 Montpellier cedex 05, France
Membranes are essential to a range of applications, including the production of potable water, energy generation, tissue repair, pharmaceutical production, food packaging, and the separations needed for the manufacture of chemicals, electronics and a range of other products. Therefore, they are considered to be ‘dominant technologies’ by governments and industry in several prominent countries – for example, USA, Japan and China. When combined with catalysts, membranes are at the basis of life, and membrane-based biomimetism is a key tool to obtain better quality products and environmentally friendly developments for our societies. Biology has a main part in this global landscape because it simultaneously provides the ‘model’ (with natural biological membranes) and represents a considerable field of applications for new artificial membranes (biotreatments, bioconversions and artificial organs). In this article, our objective is to open up this enthralling area and to give our views about the future of membranes in biotechnology. Introduction A strong, multidisciplinary, highly integrated, holistic approach is a prerequisite for the full development of ‘membrane engineering’ – a new concept that combines material aspects, modeling and new processes and/or systems, and complements all their applications, including biotechnologies. Therefore, it is a sine qua non condition to obtain all the positive outputs that can be envisioned from the advantageous characteristics of membrane technologies. At the heart of every membrane processes there is an interface, which is clearly materialized by a thin barrier that controls the exchange between two phases, not only by external forces and under the effect of fluid properties but also through the characteristics of the film material. This requires the membrane operation to be conceived as a triptych (Figure 1) that simultaneously accounts for fluid properties (which are specific in the field of biotechnology, with most of the time fragile and sticky liquids having high viscosities), membrane material and process conditions. Time-dependant polarization and fouling phenomena cannot be understood correctly out of this approach. In this Opinion article, we give our view on new trends in membrane materials and their most promising operations for biotechnology, while remaining within this thinking framework. Emphasis is placed on biotreatments and bioconversions, with a more marginal description of Corresponding author: Rios, G.M. ([email protected]). Available online 12 April 2007. www.sciencedirect.com
biomedical applications, which lag behind in terms of membrane concepts. Finally, new proposals corresponding to existing programs to achieve the expected integration are presented – the NanoMemPro concept. Functionalized and tailored membranes – towards artificial organs A major limitation in using pressure-driven membrane processes (particularly in the field of biotechnology, where a lot of liquids are sticky and/or present a high viscosity) is the loss of performance due to membrane fouling. This drawback can be reduced thanks to surface-modification treatments. Whatever the route used, the aim is to change the surface properties to get new appropriate functions while preserving or improving the macroporous structure of the material. This is one of the key factors for new membrane processes, such as affinity separation or enzymatic reaction, in several different applications related to separation, reactions and biosensing (e.g. tissue engineering, biofuel cells). Microengineering is one of the most original and promising ways to obtain an appropriate and regular morphology. Surface modification Different ways to obtain surface modifications are proposed in the literature. One of the new and most promising for the field of biotechnology is the deposition of biopolymers (e.g. protein, lipids or chitosan) at the membrane surface [1]. Such modifications do not only enhance filtration performance; they also lead to new materials for novel applications. Membranes coated with lipids can be used for emulsification processes [2] or as a membrane contactor for gas extraction [3]. In other applications, enzymes have been covalently attached onto ceramic membranes through glutaraldehyde bonds created between the enzyme and a polymer layer (comprising gelatin or gelatin/polyethylene imide), which was previously adsorbed on the membrane surface [4,5]. Membranes coated with chitin or chitosan are good candidates for affinity separation [6]. Such treatments are fast and easy to conduct, and the initial membrane characteristics can be easily recovered at the end of operation by simple cleaning. Surface modifications can also be achieved by low-temperature-plasma-induced graft polymerization. Because the new surface layer deposited on the outside or inside of the pore contains reactive groups, it can be easily functionalized [7,8]. Chemical routes are also used to prepare affinity membranes from organic or inorganic porous support, through the grafting of a ligand [9–11].
0167-7799/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2007.04.003
Opinion
TRENDS in Biotechnology
Vol.25 No.6
243
according to the properties of the adsorptive particle [17,18]. In such membranes, the polymeric matrix needs to be highly porous, to have a high degree of pore interconnectivity and low non-specific sorption tendencies for the desired product.
Figure 1. The membrane function.
Finally, an innovative approach to obtain new properties is based on ‘adaptive supramolecular nanosystems’. This overcomes the multidisciplinary research required to develop new ‘smart’ nanomaterials with applications to new membrane nanotechnologies [12]. Piletsky et al. [13] reported a new synthetic method for preparing imprinted polymer membranes, based on the formation of a self-assembled template–monomer complex. New polymeric or mixed-matrix structures Owing to their low cost, polymer membranes remain the primary materials used in biomedicine, although it is worth noting that single-use devices are sometimes preferable. The preparation of such membranes requires the use of organic solvents, which can be advantageously replaced by supercritical carbon dioxide (SC CO2). SC CO2 has many advantages compared with organic solvents: it can dry the membrane rapidly without the collapse of the structure, owing to the absence of liquid–vapor interface; the process does not require additional post treatments; and it enables the recovery of the solvent. Furthermore, it is possible to continuously modulate membrane characteristics, such as pore size, by changing the pressure and/or temperature conditions [14]. When prepared from biocompatible polymers, such as PMMA (Poly(methyl methacrylate)) [14] or PPA (Polylactide) [15], such membranes are good candidates for biomedical and pharmaceutical applications, such as drug delivery devices or films for cell culture. Because the poor mechanical resistance of polymer membranes limits their uses, novel membrane materials that are produced by blending organic and inorganic materials have gained interest. These have excellent separation performances and can adapt to rigorous environmental conditions. The addition of inorganic fillers affects the morphology and performance of the resulting membrane, whereby permeability and retention are generally enhanced [16]. In analogy to these membranes, Wessling and co-workers have developed a new absorber membrane, called the mix-matrix membrane (MMM). To produce this, they incorporated functional particles into a porous polymeric support, and the resulting membrane can be applied to isolate biomolecules through ion-exchange functionalities, molecular recognition or other specific interactions www.sciencedirect.com
Micro-engineering Membrane science and nano-micro engineering techniques merge into new exciting technologies for tailoring new membrane support in a regular and reproducible way. [19]. Conventional micro perforation methods, for example, laser drilling or precision etching, can obtain well-defined pore sizes >10 mm. Using micro engineering techniques that originated in the semiconductor industry (for example, mask or lithography), and using silicon as the primary substrate material, it is relatively easy to downscale pore sizes to between 10 and 0.1 mm. These membranes are durable filters, and are particularly suitable for applications where long life or easy cleaning are required, for example, crossflow filtration of industrial biological fluids or tissue engineering. When disposable filters are preferred (for medical or microbiological analysis), it is better to elaborate materials of lower cost. A new and efficient way to prepare them is by the deposition of polymers on patterned substrate structures, such as with phase separation micro molding. These new developments will enable materials with suitable properties for tissue engineering and bioartificial organs, such as being able to stimulate specific cell responses and maintain the differentiated functions of the cell, to be obtained in the near future. Thus, separation and physicochemical and morphological surface properties must be taken into account [20]. Recently published studies prove the suitability of this kind of membrane for tissue engineering [21] and bioartificial organs [22] (Figure 2). New contacting and reacting systems – towards improved processes In parallel with the classical pressure-, electric- or concentration-driven membrane processes that prevail in the literature, new membrane technologies with great potential have been developed during past decade. With these so-called membrane-contacting devices, or membrane contactors (MC), membrane materials (mostly porous and hydrophobic) are used as tools for inter-phase mass transfer. Here the membrane does not act as a selective barrier but rather as a new high-technology contacting device, which improves separation based on phase equilibrium. MC is a more efficient alternative to conventional contacting systems (such as plate columns or packed beds) because of its large and well-controlled specific interfacial areas, which are independent of fluid velocities, its capacity for high-throughput without flooding, its independence of phase density difference, the modularity of its design, its compactness and easy scale-up. Several classical unit operations for separation, phase distribution or even catalytic reactions, which are particularly important for biotechnology, can be carried out according to this configuration. Some examples are given below.
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through nanoprecipitation or interfacial polymerization. Based on this principle, Drioli and co-workers have developed a new membrane crystallization process, which might be useful for protein crystallization [25]. Separation Osmotic evaporation (OE) – also called osmotic distillation (OD) – is a concentration technique that is regarded with great interest for the processing of liquid foods, such as fruit juice, or aqueous solutions of thermally labile pharmaceutical or biological products [26–29]. Because it is performed at or below ambient temperature and under atmospheric pressure, a high concentration level can be reached without any damage or loss of the solute. The principle of osmotic evaporation is in Figure 3. Absorption and extraction can be also carried out according to these principles. Affinity-membranes also have potential for separation, particularly those designed to replace membrane chromatographic separations using a classical chromatographic column. Membrane chromatography has several advantages, mainly owing to the short, wide bed of the membranes systems, which induces a drop in pressure and limited resistance to diffusion [11]. This process is particularly suitable for the purification of proteins [10].
Figure 2. Nanoporous alumina membrane to improve bone cell response [21]. The debris seen on the membrane is dust.
Generation of finely divided phases Nanoparticles, which can potentially be used as drug carriers, can be prepared in a MC [23]. The contactor can control the addition of one reactant to another reactant under the principle of membrane emulsification [24] (Figure 3). Nanoparticle formation is then performed
Catalytic reaction The enzymatic membrane reactor (EMR) is a new configuration designed to ensure contact between the substrates to be transformed and the enzyme, which is immobilized at the membrane surface. The main advantages of these reactors are presented in a recent review [30]. EMRs have numerous applications, mainly in pharmaceuticals and biomedical treatments [31]. Among these applications is lipase-catalyzed ester synthesis using conventional [32] or non-conventional solvents [33,34].
Figure 3. Two examples of membrane contacting systems, for (a) emulsification and (b) osmotic evaporation. www.sciencedirect.com
Opinion
TRENDS in Biotechnology
Integration of membrane technologies – Towards system intensification Hybrid operations The idea to associate a membrane to another unit operation to get synergies and to develop new functionalities can generate also favorable repercussions. In the field of biotechnologies, the idea to use a membrane to circumscribe the liquid space where reaction takes place, to avoid the loss of catalyst and sometimes substrates, is not new. The first membrane reactors were studied in the 60s and such reactors still present interest. The reaction is conducted in continuous operation mode, the biocatalyst is reused and the recovery of product is facilitated [35,36]. This concept can be applied in many different types of reactions and for a wide range of catalysts and substrates, even in the fields of fine chemistry. In the past decade, as the enzymatic synthesis reactions in organic media or ionic liquid gained in interest, a new hybrid process emerged, the coupling of an enzymatic reactor with a pervaporation membrane. This coupling enables an effective in situ removal of water (a by- product of the reaction) through the pervaporation membrane. By this way, the reaction equilibrium is displaced in favour of synthesis and the conversion rate is increased. [37–39]. Other nice hybrid processes have been also presented in the literature, which involve at least two separation principles: membrane plus electrophoresis or cascades of membrane operations such as MF/UF/NF for fractionating complex solutions of proteins or amino-acids. . . Modeling and simulation for membrane integration in industrial processes Membrane separation is, today, recognized as a suitable downstream separation process; however, in practice its use is somewhat limited. The reasons proposed for this limitation are usually membrane fouling and high investment costs. From our viewpoint, the real bottlenecks can be summarized as follows: (i) How do we take into account the complexity of real solutions without a complete description, which is usually unrealistic? (ii) How do we practice scaling-up and -down between laboratory and industry in a controlled way? (iii) How do we predict the performance and acceptability of membrane technology in practice? In other words how, with the limited available information, do we reliably link the macroscale process performance to local phenomena at the meso-scale (the membrane pore) or the nano-scale (solute–solute or solute– barrier interactions)? By making generalizations, the same questioning could be extended to the other growing applications of membranes for biocatalysis, health and sensors. To answer these questions, there is a strong need for new methodologies based on chemical engineering principles, with a holistic approach to problems and with wellbalanced experimental and simulation and/or modeling components. Unfortunately, published studies on these points are few and far-between. One exception is the work of Darnon et al. [40–41], which concerns the purification by ultrafiltration (UF) of a small neutral molecule from a fermentation broth that is mainly www.sciencedirect.com
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composed of large peptides and proteins. In this study, a new approach based on process engineering principles and typical of what should be termed ‘membrane engineering’, was developed. Such an approach will provide stakeholders with the information they need to make decisions regarding costs, safety, product quality and environmental affects, and will cement membranes into their welldeserved place within industrial processes. Conclusions and proposals for a better future – the NanoMemPro project Membrane technologies are essential tools for process or system intensification – a general trend in technology to create cheaper processes or systems, with better performance, using much smaller equipment. Similar to biological membranes, which control the main functions of life, these technologies should develop quickly and strongly during the next few years in most of the activities on which the life of our societies is based. This is particularly true for biotechnology and health, where biomimetism will provide many opportunities to inspire new materials (particularly those based on the use of biological molecules or the supramolecular structures mimicking them) and original operation and/or system designs. This is what we have tried to emphasize in this Opinion article. Based on past experiences, our feeling is that without an integrated and holistic approach comprising all the necessary competences and scientific expertise (e.g. physics, chemistry, biology, modeling, and systems approaches) at both the university and industrial levels, to a great ‘membrane’ transverse project related to all the application fields (e.g. environment, energy, food, health and biotechnology), the development of all these technologies could be much slower, particularly, for example, for artificial organs. This idea was the start of the NanoMemPro project, a European network of excellence supported by the European Commission The main objectives of this project are to create a virtual institute, the European Membrane House (EMH), which will enable the sharing of means and tools (e.g. equipment, databases and research expertise), facilitate communication, induce strong changes in knowledge management, favour cross-fertilization and act as a interface between research, industry and society at large. Within the framework of this program, several of the activities are related to food, health and biotechnology as a whole, for example, one group is working on bioartificial organs, and preparing common proposals within the FP7. Important outcomes should follow from this innovative approach. In our opinion, there is no doubt that this will lead to the recognition that membranes deserve in the field of biotechnology. Acknowledgements NanoMemPro, the European network of excellence on nanoscale-based membrane technologies is supported by the European Commission (EC) in the 6th Framework Programme for Research and Development (Contract No. NMP3-CT-2004–500623; http://www.nanomempro.com). 13 universities or research centers, representing 13 different countries, are directly involved, in addition to related industrial partners. The financial support of the EC is equal to s 6 380 000 for a 4-year period. Prof. Gilbert M. Rios is the coordinator.
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References 1 Paolucci-Jeanjean, D. et al. (2005) Biomolecules applications for membrane-based phase-contacting systems: distribution, separation and reaction – a first state of the art. Chem. Eng. Res. Des. 83, 302–308 2 Vladisavljevic´, G.T. and Williams, R.A. (2005) Recent developments in manufacturing emulsion and particulate products using membranes. Adv. Colloid Interface Sci. 112, 1–20 3 Romero, J. et al. (2006) New hydrophobic membranes for contactor processes – applications to isothermal concentration of solutions. Desalination 193, 280–285 4 Belleville, M.P. et al. (2001) Preparation of hybrid membranes for enzymatic reaction. Sep. Purif. Technol. 25, 229–233 5 Lozano, P. et al. (2002) Active membranes coated with Candida antarctica lipase B: preparation and application for continuous butyl butyrate synthesis in organic media. J. Membr. Sci. 201, 55–64 6 Yang, L. et al. (2002) Chitosan–cellulose composite membrane for affinity purification of biopolymers and immunoadsorption. J. Membr. Sci. 197, 185–197 7 Chen, H. and Belfort, G. (1999) Surface modification of poly(ether sulfone) ultrafiltration membranes by low-temperature plasmainduced graft polymerization. J. Appl. Polym. Sci. 72, 1699–1711 8 Liu, Z-M. et al. (2003) Surface modification of microporous polypropylene membranes by the grafting of poly(c-stearyl-Lglutamate). Eur. Polym. J. 39, 2291–2299 9 Charcosset, C. (1998) Purification of proteins by membrane chromatography. J. Chem. Technol. Biotechnol. 71, 95–110 10 Zou, H. et al. (2001) Affinity membrane chromatography for the analysis and purification of proteins. J. Biochem. Biophys. Methods 49, 199–240 11 Klein, E. (2000) Affinity membranes: a 10-year review. J. Membr. Sci. 179, 1–27 12 Barboı¨u, M. et al. (2004) Ion-driven ATP pump by self-organized hybrid membrane materials. J. Am. Chem. Soc. 126, 3545–3550 13 Piletsky, S.A. et al. (1999) Receptor and transport properties of imprinted polymer membranes – a review. J. Membr. Sci. 157, 263–278 14 Reverchon, E. et al. (2006) Flexible supercritical CO2-assisted process for poly(methyl methacrylate) structure formation. Polym. Eng. Sci. 46, 188–197 15 Xu, Q.L. et al. (2005) Effect of different experimental conditions on biodegradable polylactide membranes prepared with supercritical CO2 as nonsolvent. J. Appl. Polym. Sci. 98, 831–837 16 Yang, Y. et al. (2006) Preparation and properties of polysulfone/tiO2 composite ultrafiltration membranes. J Polym. Sci. Part B: Polym. Phys. 44, 879–887 17 Avramescu, M-E. et al. (2003) Mixed-matrix membrane adsorbers for protein separation. J. Chromatogr. A. 1006, 171–183 18 Borneman, Z. and Wessling, M. (2006) Enzyme capturing and concentration with mixed matrix membrane adsorbers. J. Membr. Sci. 280, 406–417 19 Van Rijn, C.J.M. (2004) In Nano- and micro-engineered membrane technology. Membrane Science and Technologies Series (vol. 10), Elsevier
20 Drioli, E. and De Bartolo, L. (2006) Membrane biorector for cell tissues and organoids. Artif. Organs 30, 793–802 21 Popat, K.C. et al. (2005) Influence of nanoporous alumina membranes on long-term osteoblast response. Biomaterials 26, 4516–4522 22 Gerlach, J.C. (2006) Bioreactors for extracorporeal liver support. Cell Transplant. 15, S91–S103 23 Charcosset, C. et al. (2005) Preparation of nanoparticles with a membrane contactor. J. Membr. Sci. 266, 115–120 24 Joscelyne, S.M. et al. (2000) Membrane emulsification – a literature review. J. Membr. Sci. 169, 107–117 25 Di Profio, G. et al. (2003) Membrane crystallization of lysozyme: kinetics aspects. J. Cryst. Growth 257, 359–369 26 Cassano, A. et al. (2003) Clarification and concentration of citrus and carrot juices by integrated membrane processes. J. Food Eng. 57, 153– 163 27 Rodrigues, R.B. et al. (2004) Evaluation of reverse osmosis and osmotic evaporation to concentrate camu-camu juice (Myrciaria dubia). J. Food Eng. 63, 97–102 28 Vaillant, F. et al. (2005) Clarification and concentration of melon juice using membrane processes. Innov. Food Sci. Emerg. Technol. 6, 213– 220 29 Babu, R.B. et al. (2006) Mass transfer in osmotic distillation of phycocyanin colorant and sweet-lime juice. J. Membr. Sci. 272, 58–69 30 Rios, G.M. et al. (2004) Progress in enzymatic membrane reactors – a review. J. Membr. Sci. 242, 189–196 31 Giorno, L. et al. (2000) Biocatalytic membrane reactors: applications and perspectives. Trends Biotechnol. 18, 339–349 32 Magnan, E. et al. (2004) Immobilization of lipase on a ceramic membrane: activity and stability. J. Membr. Sci. 241, 161–166 33 Lozano, P. et al. (2004) Membrane reactor with immobilized Candida antarctica lipase B for ester synthesis in supercritical carbon dioxide. J. Supercrit. Fluids 28, 121–128 34 Pomier, E. et al. (2005) A new reactor design combining enzyme, membrane and SC CO2: application to castor oil modification. J. Membr. Sci. 249, 127–132 35 Prazeres, D.M.F. et al. (1994) Review: enzymatic membrane bioreactors and their applications. Enzyme Microb. Technol. 16, 738–750 36 Paolucci-Jeanjean, D. et al. (2000) Kinetics of continuous starch hydrolysis in a membrane reactor. Biochem. Eng. J. 6, 233–238 37 Bartling, K. et al. (2001) Lipase-catalysed synthesis of geranyl acetate in n-hexane with membrane-mediated water removal. Biotechnol. Bioeng. 75, 676–681 38 Be´lafi-Bako´, K. et al. (2002) Application of pervaporation for removal of water produced during enzymatic esterification in ionic liquids. Desalination 149, 267–268 39 Won, K. et al. (2006) Lipase-catalysed enantoselective esterification of racemic ibuprofen coupled with pervaporation. Process Biochem. 41, 267–269 40 Darnon, E. et al. (2002) Modelisation of ultrafiltration of complex biological solutions. A.I. Ch.E.J. 48, 1727–1737 41 Darnon, E. et al. (2003) Ultrafiltration within downstream processing: some process design considerations. Chem. Eng. Proc. 42, 299–309
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TRENDS in Biotechnology
Vol.25 No.6
Membrane engineering in biotechnology: quo vamus? Gilbert M. Rios, Marie-Pierre Belleville and Delphine Paolucci-Jeanjean IEM UMR 5635, CC047, Place E. Bataillon, F 34095 Montpellier cedex 05, France
Membranes are essential to a range of applications, including the production of potable water, energy generation, tissue repair, pharmaceutical production, food packaging, and the separations needed for the manufacture of chemicals, electronics and a range of other products. Therefore, they are considered to be ‘dominant technologies’ by governments and industry in several prominent countries – for example, USA, Japan and China. When combined with catalysts, membranes are at the basis of life, and membrane-based biomimetism is a key tool to obtain better quality products and environmentally friendly developments for our societies. Biology has a main part in this global landscape because it simultaneously provides the ‘model’ (with natural biological membranes) and represents a considerable field of applications for new artificial membranes (biotreatments, bioconversions and artificial organs). In this article, our objective is to open up this enthralling area and to give our views about the future of membranes in biotechnology. Introduction A strong, multidisciplinary, highly integrated, holistic approach is a prerequisite for the full development of ‘membrane engineering’ – a new concept that combines material aspects, modeling and new processes and/or systems, and complements all their applications, including biotechnologies. Therefore, it is a sine qua non condition to obtain all the positive outputs that can be envisioned from the advantageous characteristics of membrane technologies. At the heart of every membrane processes there is an interface, which is clearly materialized by a thin barrier that controls the exchange between two phases, not only by external forces and under the effect of fluid properties but also through the characteristics of the film material. This requires the membrane operation to be conceived as a triptych (Figure 1) that simultaneously accounts for fluid properties (which are specific in the field of biotechnology, with most of the time fragile and sticky liquids having high viscosities), membrane material and process conditions. Time-dependant polarization and fouling phenomena cannot be understood correctly out of this approach. In this Opinion article, we give our view on new trends in membrane materials and their most promising operations for biotechnology, while remaining within this thinking framework. Emphasis is placed on biotreatments and bioconversions, with a more marginal description of Corresponding author: Rios, G.M. ([email protected]). Available online 12 April 2007. www.sciencedirect.com
biomedical applications, which lag behind in terms of membrane concepts. Finally, new proposals corresponding to existing programs to achieve the expected integration are presented – the NanoMemPro concept. Functionalized and tailored membranes – towards artificial organs A major limitation in using pressure-driven membrane processes (particularly in the field of biotechnology, where a lot of liquids are sticky and/or present a high viscosity) is the loss of performance due to membrane fouling. This drawback can be reduced thanks to surface-modification treatments. Whatever the route used, the aim is to change the surface properties to get new appropriate functions while preserving or improving the macroporous structure of the material. This is one of the key factors for new membrane processes, such as affinity separation or enzymatic reaction, in several different applications related to separation, reactions and biosensing (e.g. tissue engineering, biofuel cells). Microengineering is one of the most original and promising ways to obtain an appropriate and regular morphology. Surface modification Different ways to obtain surface modifications are proposed in the literature. One of the new and most promising for the field of biotechnology is the deposition of biopolymers (e.g. protein, lipids or chitosan) at the membrane surface [1]. Such modifications do not only enhance filtration performance; they also lead to new materials for novel applications. Membranes coated with lipids can be used for emulsification processes [2] or as a membrane contactor for gas extraction [3]. In other applications, enzymes have been covalently attached onto ceramic membranes through glutaraldehyde bonds created between the enzyme and a polymer layer (comprising gelatin or gelatin/polyethylene imide), which was previously adsorbed on the membrane surface [4,5]. Membranes coated with chitin or chitosan are good candidates for affinity separation [6]. Such treatments are fast and easy to conduct, and the initial membrane characteristics can be easily recovered at the end of operation by simple cleaning. Surface modifications can also be achieved by low-temperature-plasma-induced graft polymerization. Because the new surface layer deposited on the outside or inside of the pore contains reactive groups, it can be easily functionalized [7,8]. Chemical routes are also used to prepare affinity membranes from organic or inorganic porous support, through the grafting of a ligand [9–11].
0167-7799/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2007.04.003
Opinion
TRENDS in Biotechnology
Vol.25 No.6
243
according to the properties of the adsorptive particle [17,18]. In such membranes, the polymeric matrix needs to be highly porous, to have a high degree of pore interconnectivity and low non-specific sorption tendencies for the desired product.
Figure 1. The membrane function.
Finally, an innovative approach to obtain new properties is based on ‘adaptive supramolecular nanosystems’. This overcomes the multidisciplinary research required to develop new ‘smart’ nanomaterials with applications to new membrane nanotechnologies [12]. Piletsky et al. [13] reported a new synthetic method for preparing imprinted polymer membranes, based on the formation of a self-assembled template–monomer complex. New polymeric or mixed-matrix structures Owing to their low cost, polymer membranes remain the primary materials used in biomedicine, although it is worth noting that single-use devices are sometimes preferable. The preparation of such membranes requires the use of organic solvents, which can be advantageously replaced by supercritical carbon dioxide (SC CO2). SC CO2 has many advantages compared with organic solvents: it can dry the membrane rapidly without the collapse of the structure, owing to the absence of liquid–vapor interface; the process does not require additional post treatments; and it enables the recovery of the solvent. Furthermore, it is possible to continuously modulate membrane characteristics, such as pore size, by changing the pressure and/or temperature conditions [14]. When prepared from biocompatible polymers, such as PMMA (Poly(methyl methacrylate)) [14] or PPA (Polylactide) [15], such membranes are good candidates for biomedical and pharmaceutical applications, such as drug delivery devices or films for cell culture. Because the poor mechanical resistance of polymer membranes limits their uses, novel membrane materials that are produced by blending organic and inorganic materials have gained interest. These have excellent separation performances and can adapt to rigorous environmental conditions. The addition of inorganic fillers affects the morphology and performance of the resulting membrane, whereby permeability and retention are generally enhanced [16]. In analogy to these membranes, Wessling and co-workers have developed a new absorber membrane, called the mix-matrix membrane (MMM). To produce this, they incorporated functional particles into a porous polymeric support, and the resulting membrane can be applied to isolate biomolecules through ion-exchange functionalities, molecular recognition or other specific interactions www.sciencedirect.com
Micro-engineering Membrane science and nano-micro engineering techniques merge into new exciting technologies for tailoring new membrane support in a regular and reproducible way. [19]. Conventional micro perforation methods, for example, laser drilling or precision etching, can obtain well-defined pore sizes >10 mm. Using micro engineering techniques that originated in the semiconductor industry (for example, mask or lithography), and using silicon as the primary substrate material, it is relatively easy to downscale pore sizes to between 10 and 0.1 mm. These membranes are durable filters, and are particularly suitable for applications where long life or easy cleaning are required, for example, crossflow filtration of industrial biological fluids or tissue engineering. When disposable filters are preferred (for medical or microbiological analysis), it is better to elaborate materials of lower cost. A new and efficient way to prepare them is by the deposition of polymers on patterned substrate structures, such as with phase separation micro molding. These new developments will enable materials with suitable properties for tissue engineering and bioartificial organs, such as being able to stimulate specific cell responses and maintain the differentiated functions of the cell, to be obtained in the near future. Thus, separation and physicochemical and morphological surface properties must be taken into account [20]. Recently published studies prove the suitability of this kind of membrane for tissue engineering [21] and bioartificial organs [22] (Figure 2). New contacting and reacting systems – towards improved processes In parallel with the classical pressure-, electric- or concentration-driven membrane processes that prevail in the literature, new membrane technologies with great potential have been developed during past decade. With these so-called membrane-contacting devices, or membrane contactors (MC), membrane materials (mostly porous and hydrophobic) are used as tools for inter-phase mass transfer. Here the membrane does not act as a selective barrier but rather as a new high-technology contacting device, which improves separation based on phase equilibrium. MC is a more efficient alternative to conventional contacting systems (such as plate columns or packed beds) because of its large and well-controlled specific interfacial areas, which are independent of fluid velocities, its capacity for high-throughput without flooding, its independence of phase density difference, the modularity of its design, its compactness and easy scale-up. Several classical unit operations for separation, phase distribution or even catalytic reactions, which are particularly important for biotechnology, can be carried out according to this configuration. Some examples are given below.
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through nanoprecipitation or interfacial polymerization. Based on this principle, Drioli and co-workers have developed a new membrane crystallization process, which might be useful for protein crystallization [25]. Separation Osmotic evaporation (OE) – also called osmotic distillation (OD) – is a concentration technique that is regarded with great interest for the processing of liquid foods, such as fruit juice, or aqueous solutions of thermally labile pharmaceutical or biological products [26–29]. Because it is performed at or below ambient temperature and under atmospheric pressure, a high concentration level can be reached without any damage or loss of the solute. The principle of osmotic evaporation is in Figure 3. Absorption and extraction can be also carried out according to these principles. Affinity-membranes also have potential for separation, particularly those designed to replace membrane chromatographic separations using a classical chromatographic column. Membrane chromatography has several advantages, mainly owing to the short, wide bed of the membranes systems, which induces a drop in pressure and limited resistance to diffusion [11]. This process is particularly suitable for the purification of proteins [10].
Figure 2. Nanoporous alumina membrane to improve bone cell response [21]. The debris seen on the membrane is dust.
Generation of finely divided phases Nanoparticles, which can potentially be used as drug carriers, can be prepared in a MC [23]. The contactor can control the addition of one reactant to another reactant under the principle of membrane emulsification [24] (Figure 3). Nanoparticle formation is then performed
Catalytic reaction The enzymatic membrane reactor (EMR) is a new configuration designed to ensure contact between the substrates to be transformed and the enzyme, which is immobilized at the membrane surface. The main advantages of these reactors are presented in a recent review [30]. EMRs have numerous applications, mainly in pharmaceuticals and biomedical treatments [31]. Among these applications is lipase-catalyzed ester synthesis using conventional [32] or non-conventional solvents [33,34].
Figure 3. Two examples of membrane contacting systems, for (a) emulsification and (b) osmotic evaporation. www.sciencedirect.com
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TRENDS in Biotechnology
Integration of membrane technologies – Towards system intensification Hybrid operations The idea to associate a membrane to another unit operation to get synergies and to develop new functionalities can generate also favorable repercussions. In the field of biotechnologies, the idea to use a membrane to circumscribe the liquid space where reaction takes place, to avoid the loss of catalyst and sometimes substrates, is not new. The first membrane reactors were studied in the 60s and such reactors still present interest. The reaction is conducted in continuous operation mode, the biocatalyst is reused and the recovery of product is facilitated [35,36]. This concept can be applied in many different types of reactions and for a wide range of catalysts and substrates, even in the fields of fine chemistry. In the past decade, as the enzymatic synthesis reactions in organic media or ionic liquid gained in interest, a new hybrid process emerged, the coupling of an enzymatic reactor with a pervaporation membrane. This coupling enables an effective in situ removal of water (a by- product of the reaction) through the pervaporation membrane. By this way, the reaction equilibrium is displaced in favour of synthesis and the conversion rate is increased. [37–39]. Other nice hybrid processes have been also presented in the literature, which involve at least two separation principles: membrane plus electrophoresis or cascades of membrane operations such as MF/UF/NF for fractionating complex solutions of proteins or amino-acids. . . Modeling and simulation for membrane integration in industrial processes Membrane separation is, today, recognized as a suitable downstream separation process; however, in practice its use is somewhat limited. The reasons proposed for this limitation are usually membrane fouling and high investment costs. From our viewpoint, the real bottlenecks can be summarized as follows: (i) How do we take into account the complexity of real solutions without a complete description, which is usually unrealistic? (ii) How do we practice scaling-up and -down between laboratory and industry in a controlled way? (iii) How do we predict the performance and acceptability of membrane technology in practice? In other words how, with the limited available information, do we reliably link the macroscale process performance to local phenomena at the meso-scale (the membrane pore) or the nano-scale (solute–solute or solute– barrier interactions)? By making generalizations, the same questioning could be extended to the other growing applications of membranes for biocatalysis, health and sensors. To answer these questions, there is a strong need for new methodologies based on chemical engineering principles, with a holistic approach to problems and with wellbalanced experimental and simulation and/or modeling components. Unfortunately, published studies on these points are few and far-between. One exception is the work of Darnon et al. [40–41], which concerns the purification by ultrafiltration (UF) of a small neutral molecule from a fermentation broth that is mainly www.sciencedirect.com
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composed of large peptides and proteins. In this study, a new approach based on process engineering principles and typical of what should be termed ‘membrane engineering’, was developed. Such an approach will provide stakeholders with the information they need to make decisions regarding costs, safety, product quality and environmental affects, and will cement membranes into their welldeserved place within industrial processes. Conclusions and proposals for a better future – the NanoMemPro project Membrane technologies are essential tools for process or system intensification – a general trend in technology to create cheaper processes or systems, with better performance, using much smaller equipment. Similar to biological membranes, which control the main functions of life, these technologies should develop quickly and strongly during the next few years in most of the activities on which the life of our societies is based. This is particularly true for biotechnology and health, where biomimetism will provide many opportunities to inspire new materials (particularly those based on the use of biological molecules or the supramolecular structures mimicking them) and original operation and/or system designs. This is what we have tried to emphasize in this Opinion article. Based on past experiences, our feeling is that without an integrated and holistic approach comprising all the necessary competences and scientific expertise (e.g. physics, chemistry, biology, modeling, and systems approaches) at both the university and industrial levels, to a great ‘membrane’ transverse project related to all the application fields (e.g. environment, energy, food, health and biotechnology), the development of all these technologies could be much slower, particularly, for example, for artificial organs. This idea was the start of the NanoMemPro project, a European network of excellence supported by the European Commission The main objectives of this project are to create a virtual institute, the European Membrane House (EMH), which will enable the sharing of means and tools (e.g. equipment, databases and research expertise), facilitate communication, induce strong changes in knowledge management, favour cross-fertilization and act as a interface between research, industry and society at large. Within the framework of this program, several of the activities are related to food, health and biotechnology as a whole, for example, one group is working on bioartificial organs, and preparing common proposals within the FP7. Important outcomes should follow from this innovative approach. In our opinion, there is no doubt that this will lead to the recognition that membranes deserve in the field of biotechnology. Acknowledgements NanoMemPro, the European network of excellence on nanoscale-based membrane technologies is supported by the European Commission (EC) in the 6th Framework Programme for Research and Development (Contract No. NMP3-CT-2004–500623; http://www.nanomempro.com). 13 universities or research centers, representing 13 different countries, are directly involved, in addition to related industrial partners. The financial support of the EC is equal to s 6 380 000 for a 4-year period. Prof. Gilbert M. Rios is the coordinator.
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