Polyurethane Membranes in Biosensor Preparation

1. Introduction Polyurethanes (PU) are an important polymer type with extensive applications in today's industry across diverse fields such as automotive, construction, furniture, and footwear (Yanılmaz et al., 2013). This broad application range is no coincidence but results from the superior properties polyurethanes possess. Among these are properties such as flexibility, biocompatibility, stability, ease of preparation and application. In recent years, polyurethanes have become a sought-after material in industry due to their excellent mechanical properties, particularly in membrane production. Today, there are numerous successful sensor studies using polyurethane membranes, particularly for eliminating invasive species and sensing applications (Paşahan et al., 2013). The high selective permeability of polyurethanes and their porous structure particularly enhance sensor capability. Through this, a polyurethane coating applied to the surface of the sensor system allows passage of only a specific chemical species while eliminating all other types. For example, Figure 1 shows polyurethane membranes with different pore sizes. The use of polyurethanes as selective membranes on electrode surfaces has attracted particular attention in the medical field due to their unique physicochemical properties, biocompatibility, and low cost (Long et al., 2014). For this reason, in recent years, biocompatible polyurethanes have been actively used in the development of biosensors employed in determining blood glucose levels and detecting medically important hormones such as dopamine and epinephrine (Savan et al., 2016).

2. Biosensors and Polymeric Membranes

Sensors are systems that detect various molecules or groups of molecules or physical environmental conditions. Although their structure, properties, and operating systems differ, they fundamentally consist of a detector, a transducer, and a user interface. Types include mechanical sensors, electrical sensors, thermal sensors, magnetic sensors, radiation sensors, chemical sensors, biosensors, and optical sensors. However, in recent years, biosensor research developed for detecting biologically important molecules has gained significant momentum. Polymeric membranes play an important role in the preparation of these biosensors. Polymer membrane-based sensors of this type are shown in their general structure in Figure 2. Polymers are used successfully for sensor applications due to their unique properties such as ease of production, structural flexibility, and low cost, to sensitively and selectively determine a biochemical molecule in body fluid by eliminating interfering molecules (Wang et al., 1999, Caia et al., 2014, Köytepe et al., 2005, Paşahan et al., 2004, Wang et al., 2006). The use of polymers in sensor and biosensor applications improves the detection response and eliminates the effect of interfering species (Zhang et al., 2013, Liu et al., 2012).

Polymers used in sensor devices both participate in the detection mechanism and immobilize the component responsible for analyte detection (Liu et al., 2012). Although polymer applications in the separation field are extensive, very few polymers can be obtained as biocompatible for biomedical applications (Paşahan et al., 2011, Gavalas et al., 2006, Adhikari et al., 2004).

The choice of polymer in a polymeric sensor depends on the degree of contact between the interfering molecules in the sample solution and other species with the analyte. Particularly in real-time biosensors, more careful consideration must be given to biocompatibility when polymers contact body fluid. Polymers with lower toxicity, better biostability, and inert structure have become preferred in sensor applications. For this reason, in recent years, there have been many studies on the use of bioinert polymers as selective membranes (Pawlak et al., 2014, Badr et al., 2014, Sutter et al., 2006). In addition to being a selective membrane, polymers should also have properties such as high adhesion, appropriate pore structure, and easy application. For this reason, polyurethanes have become a good alternative as a selective membrane material for sensor preparation due to their porous structure, good adhesion, biocompatibility, and easy preparation characteristics. Figure 2. General structure of polymer-based sensors. (a; selective permeable polymeric membrane electrode, b; composite electrode containing additives such as graphene carbon nanotubes and fullerenes, c; enzyme electrode containing enzymes such as glucose oxidase and cholesterol oxidase, and d; immunosensor structures containing antibodies) Many polymers have been studied in the literature for sensor applications. These include polymers such as Nafion, polyimide, poly(methyl methacrylate), polycaprolactone, poly(ethylene oxide), polysulfone, and poly(vinyl chloride). Particularly for polyurethanes, there are numerous chemical studies. Additionally, studies exist on polyurethane composites in polyurethane sensor applications.

3. Biosensor Applications of Polyurethane Membranes

Biosensors are analytical devices with ease of use for qualitatively and quantitatively determining various analytes important to humans, featuring high sensitivity and portability, requiring minimal sample volumes. Biosensors have advantages such as sensitivity, specificity, convenience, low production cost, improved detection limit, rapid response time, ease of use, portability, and continuous real-time signal output. They also eliminate the requirement for sample pretreatment and specialist operation for any sample. Additionally, biologically significant toxicity components that cannot be determined by conventional analytical methods can be measured with biosensors (Kaur et al., 2015). A biosensor that enables detection of a biological species such as enzyme, antibody, bacterium, or tissue performs qualitative and quantitative detection of that species using a similar transducer. Because polyurethanes are compatible with biological systems, they can be readily used to immobilize enzymes or biological tissues on electrode surfaces as detection systems. For example, one of these studies is the preparation of a new butyrylcholinesterase enzymatic sensor for determining organophosphate pesticides. For this purpose, the utility of a polyurethane-hydrophilic polyurethane asymmetric membrane was examined in obtaining a potentiometric biosensor. This sensor was successfully applied in the analysis of an organophosphate pesticide, paroxon, and provided advantages such as ease of preparation and time savings during the preparation stage (Cho et al., 1999). The most common biosensor applications are glucose and hormone sensors, with usage at commercial and academic levels.

3.1. Glucose Sensor Applications of Polyurethane Membranes

Close monitoring and control of blood glucose levels is essential for effective diabetes treatment (Wang, 2008). Classical glucose sensors are glucose oxidase-based enzymatic amperometric sensor systems that measure blood glucose levels (Gross et al., 2000, Mastrototaro et al., 2000, Sachedina et al., 2003). The general operating structure of these sensors is shown in Figure 3. Because these sensors are enzyme-based, they require enzymes to be stored under appropriate conditions. If measurement probes are not stored under appropriate conditions, continuous errors occur in glucose measurement and reproducible results are obtained at low levels. For this reason, new approaches such as non-enzymatic measurement systems, optical and infrared measurement systems are continuously being researched (Heo et al., 2013, Oliver et al., 2009, Wisniewski et al., 2011).

Traditional glucose monitoring (for example, the finger prick method) provides instantaneous blood glucose levels and is used to improve diabetes management.

Additionally, new analytical methods are being researched that enable continuous monitoring of glucose concentration fluctuations through implantable enzyme-based electrochemical sensors (Heller et al., 2008, Klonoff, 2005b, Ward et al., 2002). Although some devices have been approved by the U.S. Food and Drug Administration, most sensors have serious limitations including poor accuracy, unpredictable signal stability, sensor response lag time, frequent calibration requirements, and short lifespans that limit clinical use (Klonoff, 2005a, b). Since the sensor's outer surface directly triggers a foreign body response, recent studies have focused on developing more biocompatible polymeric membranes to reduce the foreign body response (Wilson et al., 2005, Wilson et al., 2000, Gifford et al., 2005). For this reason, polyurethane membranes are an important material that can be used to eliminate interfering species in the blood matrix where glucose measurement will be performed. Due to its adhesive properties and porous structure, it can be easily coated on the surface of measurement strips, thereby eliminating species other than glucose and providing a clear sensor signal. One study in this field prepared nitric oxide-releasing polyurethane sensor membranes using nitric oxide-releasing modified silica scaffolds embedded in polyurethane. The adjustable NO release as a function of nitric oxide-releasing silica scaffold and polyurethane compositions and their concentrations was demonstrated, and the effects and kinetics of nitric oxide release on glucose sensor performance were examined.

As a result, it was observed that as the concentration of the nitric oxide-releasing silica scaffold increased, membrane stability decreased. Additionally, it was noted that higher concentrations prevented the production of homogeneous films with consistent nitric oxide release (Koh et al., 2011).

In another study, the in-cell performance of glucose sensors coated with dexamethasone-loaded porous polyurethane membranes used to enter the tissue sensor interface was examined. Two animal studies were conducted to characterize the tissue-modifying effects of porous dexamethasone-loaded coatings placed on sensor implants and investigate their effects on the in-cell performance of glucose sensors. The tissue response to implants was evaluated by determining the amount of macrophage infiltration around implants, blood vessel formation, and collagen density. Implants with porous dexamethasone-loaded polyurethane coatings were found to reduce inflammation and increase vascularization of tissue surrounding the implants. Additionally, it was reported that functional sensors with dexamethasone-loaded porous polyurethane coatings demonstrated improved sensor sensitivity over a 21-day period when compared to controls (Vallejo-Heligon et al., 2016). For this reason, the importance of polyurethane membranes in the preparation of glucose sensors, which are also of major commercial importance, is steadily increasing.

3.2. Hormone Sensor Applications of Polyurethane Membranes

Today, many important diseases such as Parkinson's, Hashimoto's, acromegaly, and myxedema are related to hormones being secreted too little or too much. For this reason, in many hormonal diseases, the level of these hormones must be determined regularly and accurately. Blood, serum, or cerebrospinal fluid is used as the measurement fluid, and measuring in all these fluids is quite difficult. This is because they are very small in quantity and contain many types structurally. The most commonly used techniques in this field are HPLC, GC, electrophoresis, and spectroscopic methods. However, these techniques require a long and tedious pretreatment process. For this reason, they are not practical in intensive clinical applications. Voltammetric sensor techniques are promising as a practical technique with high accuracy and sensitivity in this field. Measurement of hormones such as dopamine, epinephrine, serotonin, and melatonin has been accomplished with high sensitivity at low analyte concentrations in this field. The most important role of polyurethane membranes in such sensors is to create a selective permeable surface that transports only the measured analyte species to the electrode surface. Figure 4 shows the general structure of sensors that eliminate species present in body fluids that respond on the electrode surface, providing only dopamine signals. Dopamine is one of naturally occurring catecholamines, which are an important class of neurotransmitters (Li et al., 2014). It plays an important role on the kidneys, hormonal, cardiovascular, and central nervous systems (Raj et al., 2003, Cui et al., 2012). Low levels of dopamine are associated with neurological disorders such as Parkinson's disease. For this reason, rapid and accurate determination of dopamine is important in clinical diagnosis (Gingrich et al., 1993). Various methods have been reported for effective detection of dopamine, including fluorescence, electrochemistry, and chromatography (Wang et al., 2002, He et al., 2012, Seçkin et al., 2005, Díaz et al., 2009). The design of electrochemical sensors for dopamine determination is under intense research effort in terms of low detection limit, very rapid response time, and ease of production (Temoçin et al., 2013, Arrigan et al., 2004). However, electrochemical determination of dopamine is compromised by the simultaneous presence of ascorbic acid and uric acid, whose oxidation potential is close to dopamine's response.

For this reason, membrane development for electrochemical sensors that perform high-selectivity measurement of dopamine rapidly without being affected by uric acid and ascorbic acid is an important field to eliminate this selectivity problem (Liu et al., 2014).

In studies conducted for this purpose, polyurethane membranes are increasing day by day. For example, one of these studies is the synthesis of polyurethanes containing maltose and the preparation of dopamine-selective electrodes. In this study, polyurethanes containing 1%, 3%, 5%, and 10% maltose were synthesized using 1,2-ethanediol and 4,4'-diphenylmethane diisocyanate. These polyurethanes were cast on the electrode surface to form a film. The prepared polyurethane films were then used as a selective membrane for dopamine sensor preparation. Additionally, whether these polyurethane films could be used as a membrane for voltammetric determination of dopamine was also investigated. Voltammetric results showed that polymer electrodes modified with 3% maltose-containing polyurethane films prevented high-level passage of electroactive ascorbic acid and uric acid while allowing dopamine entry. For this reason, it was demonstrated that this polyurethane electrode could be readily used as a selective membrane for dopamine in the presence of electroactive and non-electroactive species (Paşahan et al., 2013). Another study on this subject examined whether a solid composite electrode based on graphite and polyurethane resin modified with [Cu(II) Salen] complex, a symmetric tetradentate Schiff base type, could be used as a dopamine sensor. The voltammetric behavior of this [Cu(II) Salen] complex was investigated on a modified carbon paste electrode. The analytical potentials of the graphite and polyurethane resin-based composite electrode modified with [Cu(II) Salen] complex were evaluated using dopamine as the probe.

It was found that graphite and polyurethane resin modified with [Cu(II) Salen] provided the best results with electrocatalytic effect when compared to graphite and polyurethane resin without modification.

In addition, it was stated that the used electrode was long-lived and robust. Only one of the electrodes was used once throughout the entire study and it was observed that the analyte or its oxidation products adhered to the surface without requiring surface renewal between measurements (Santos et al., 2014). Another study involved the preparation of dopamine-selective electrodes based on polyurethane containing 3,4-divanillytetrahydrofuran groups. In this study, after polyurethane was synthesized with 3,4-divanillytetrahydrofuran and different diisocyanates, polyurethane films were prepared by casting on the electrode surface. These polyurethane films obtained exhibited good adhesion, chemical resistance, and flexibility. Additionally, whether these films could be used as a membrane for voltammetric determination of dopamine was also investigated. A simple, rapid, and sensitive electrochemical sensor was developed for dopamine determination in the presence of electroactive (uric acid and large amounts of ascorbic acid) and non-electroactive species (lactose, sucrose, and urea). The selective permeability behavior of the polyurethane electrode containing 3,4-divanillytetrahydrofuran toward dopamine was examined in the presence of electroactive (uric acid and large amounts of ascorbic acid) and non-electroactive species (lactose, sucrose, and urea). As a result, interference from ascorbic acid and uric acid was effectively eliminated. Particularly with the polyurethane film synthesized with 4,4'-diphenylmethane diisocyanate, an excellent dopamine-selective electrode was obtained with excellent sensitivity, selectivity, stability, reusability, and low response time (18 s) and wide linear ranges (Savan et al., 2016). Similar approaches have been developed for serotonin, melatonin, and adrenaline electrodes. In fact, with conductive structures such as carbon nanotubes, fullerenes, and graphene added to the polyurethane structure, electrocatalytic effects are provided and hormones with similar chemical structures are analyzed simultaneously. In this approach, the reduction potentials of species such as dopamine, melatonin, and ascorbic acid with overlapping reduction potentials were changed to enable simultaneous reading.

4. Conclusion

In recent years, one of the important applications of polyurethanes is the preparation of polymeric membrane materials used in biosensor development. The biocompatibility properties of polyurethanes are increasingly being preferred specifically for this type of membrane application. Studies particularly indicate that polyurethane-containing structures provide significant advantages in the preparation of membranes for use in measuring glucose and hormones, which have commercial value. These studies are important evidence that applications of polyurethanes as polymeric membranes in the preparation of various biosensors will further increase. Büşra Aksoy / Doctoral Student - Department of Chemistry - Faculty of Arts and Sciences - İnönü University Assoc. Prof. Dr. Süleyman Köytepe / Department of Chemistry - Faculty of Arts and Sciences - İnönü University Prof. Dr. Burhan Ateş / Department of Chemistry - Faculty of Arts and Sciences - İnönü University Prof. Dr. Turgay Seçkin / Department of Chemistry - Faculty of Arts and Sciences - İnönü University

References

• Adhikari, B., Majumdar, S. (2004). Prog Polym Sci. 29; 699–766.

• Arrigan, D.W.M., Ghita, M., Beni, V. (2004). Chem Commun. 40; 732–733.

• Badr, I.H., Gouda, M., Abdel-Sattar, R., Sayour, H.E. (2014). Carbohydr Polym. 997; 83-790.

• Caia, W., Laia, T., Dub, H., Yea, J. (2014). Sens Actuators B. 193; 492–500.

• Cho, Y.A., Lee, H.S., Cha, G.S., Lee, Y.T. (1999). Biosens Bioelectron. 14; 435-438.

• Cui, R., Wang, X., Zhang, G., Wang, C. (2012). Sens Actuators B. 161; 1139.

• Díaz, A.N., Sánchez, F.G., Aguilar, A., Bracho, V., Algarra, M.J. (2009). Liq Chromatogr Relat Technol. 32; 849–859.

• Gavalas, V.G., Berrocal, M.J., Bachas, L.G. (2006). Anal Bioanal Chem. 384; 65–72.

• Gingrich, J.A., Caron, M.G. (1993). Ann Rev Neurosci. 16; 299–321.

• Gross, T.M., Bode, B.W., Einhorn, D., Kayne, D.M., Reed, J.H., White, N.H., Mastrototaro, J.J. (2000). Diabetes Technol Ther. 2; 49–56.

• He, M.F., Zheng, X.H.J. (2012). Mol Liq. 173; 29–34.

• Heller, A., Feldman, B., (2008). Chem Rev. 108; 2482–2505.

• Heo, Y.J., Takeuchi, S. (2013). Adv Healthc Mater. 2; 43–56.

• Kaur, H., Kumar, R., Babu, J.N., Mittal S. (2015). Biosens. Bioelectron. 63; 533–545. • Klonoff, D.C. (2005a). Diab Care. 28; 1231–1239.

• Klonoff, D.C. (2005b). Diab Technol Ther. 7; 770–775.

• Koh, A., Riccio, D.A., Sun, B., Carpenter, A.W., Nichols, S.P., Schoenfisch, M.H. (2011). Biosens. Bioelectron. 28; 17–24.

• Köytepe, S., Paşahan, A., Ekinci, E., Seçkin, T. (2005). Eur Polym J. 41; 121–127.

• Li, M., Li, H., Zhong, W., Zhao, Q., Wang, D. (2014). ACS Appl Mater Interfaces. 6; 1313−1319.

• Liu, B., Lian, H.T., Yin, J.F., Sun, X.Y. (2012). Electrochim Acta. 75; 108–114.

• Liu, X., Zhua, H., Yang, X. (2014). RSC Adv. 4; 3706–3712.

• Long, C., Jian, K., Sachiko, S. (2014). Biomed Mater Eng. 24; 1979–1989.

• Mastrototaro, J.J. (2000). Diabetes Technol Ther. 2; 13–18.

• Oliver, N., Toumazou, C., Cass, A., Johnston, D. (2009). Diabet Med. 26; 197–210.

• Paşahan, A., Köytepe, S., Cengiz, M.A., Seçkin, T. (2013). Int J Polym Mater Polym Bio. 62; 642–647.

• Paşahan, A., Köytepe, S., Cengiz, M.A., Seçkin, T. (2013). Polym Int. 62; 246–250.

• Paşahan, A., Köytepe, S., Ekinci, E. (2011). Polym Plast Technol Eng. 50; 1239–1246.

• Paşahan, A., Köytepe, S., Ekinci, E., Seçkin, T. (2004). Polym Bull. 51; 351–358.

• Pawlak, M., Bakker, E. (2014). Electroanalysis. 26; 1121–1131.

• Raj, C.R., Okajima, T., Ohsaka, T.J. (2003). Electroanal Chem. 543; 127–133.

• Sachedina, N., Pickup, J. (2003). Diabet Med. 20; 1012–1015.

• Santos, S.X., Cavalheiro, E.T.G. (2014). J Braz Chem Soc. 25; 1071-1077.

• Savan, E.K., Paşahan, A., Aksoy, B., Güngör, Ö., Köytepe, S., Seçkin, T. (2016). Int J Polym Mater Polym Bio. 65; 402-408.

• Seçkin, Z.E., Volkan, M. (2005). Anal Chim Acta. 547; 104–108.

• Sutter, J., Pretsch, E. (2006). Electroanalysis. 1; 19–25.

• Temoçin, Z. (2013). Sens Actuators B. 176; 796–802.

• Vallejo-Heligon, S.G., Brown, N.L., Reichert, W.M., Klitzman, B. (2016). Acta Biomaterialia. 30; 106–115.

• Wang, C.Y., Wang, Z.X., Zhu, A.P., Hu, X.Y. (2006). Sensors. 6; 1523–1536.

• Wang, H., Mu, S. (1999). Sens Actuators B. 56; 22–30.

• Wang, H.Y., Sun, Y., Tang, B. (2002). Talanta. 57; 899–907.

• Wang, J. (2008). Chem Rev. 108; 814–825.

• Ward, W.K., Jansen, L.B., Anderson, E., Reach, G., Klein, J.C., Wilson, G.S. (2002). Biosens Bioelectron. 17; 181–189.

• Wisniewski, N.A., Helton, K., McMillan, W.A. (2011). Tissue-integrating sensors, in, Google Patents.

• Yanılmaz, M., Erbay, B.T., Serhatli, E. Sarac, A.S.J. (2013). App Polym Sci. 127; 4957–4966.

• Zhang, H., Zhou, Y., Zhang, J., Gou, L., Zheng, J.J. (2013). Mol Liq. 184; 43–50.