Diabetess mellitus is a chronic disease, that occurs when the pancreas does non bring forth adequate insulin, or when insulin is non efficaciously used. Insulin is a endocrine that regulates blood sugar degree. This leads to an instability in concentration of glucose within the blood ; doing either hyperglycemia or hypoglycemia. This over a period of clip leads to serious harm to the nervousnesss and blood vass. It has been noted to impact more than 346 million people world-wide ( WHO, 2012 ) and passing on complications caused by diabetes, cost the NHS about & A ; lb ; 10 billion in 2011 ( Diabetes UK, 2012 ) .
These complications can either be acute or chronic, both classs cause terrible disadvantages to the patient. By 2035 it is predicted that the NHS will pass & A ; lb ; 16.8 billion, 17 % of its full budget for diabetes intervention. ( NHS, 2012 )
Figure 2.1 shows significantly lifting incidence of diabetes in England ( Diabetes UK, 2012 )
Two general types of diabetes:
Type 1, Insulin-dependent diabetes mellitus ( IDDM ) by and large afflicted at a younger age ( between ages 10 and 16 ) ( Harvard Health Publications, 2012 ) besides known as juvenile diabetes mellitus and it is caused by the inability of the beta cells ( in the pancreas ) to bring forth or release insulin ( figure 2 ) . Approximately 10 % of diabetics have type 1 ( Newman and Turner, 2005 ) .
Type 2, Non-insulin dependant diabetes mellitus ( NIDDM ) which makes up the bulk of diabetes, 90 % ( Guyton and Hall, 2005 ) affliction normally occurs after the age of 40 old ages and it more prone to happen due to lifestyle. Obesity greatly increases the hazard of diabetes ( Harvard Health Publications, 2012 ) . In this type, insulin has an inability to adhere to cell receptors for the consumption of glucose ( figure 3 ) .
type 2 diabetes.jpg
type 1 diabetes.jpg
Figure 2.2 ( left ) and 2.3 ( right ) shows the physiological difference between type 1 and type 2 diabetes, severally ( Harvard Health Publications, 2012 ) .
A biosensor is a compact analytical device or unit which transduces biological or biologically derived stimulations, be it physical or chemical, into clear signal for monitoring ( figure 4 ) . ( Chambers et al. , 2008 ) There are three chief parts of a biosensor:
The biological acknowledgment elements that distinguish the mark molecules in the presence of assorted chemicals
A transducer that converts the biorecognition event into mensurable signal
A signal processing system that converts signal into a clear signifier
Figure 2.4 shows conventional presentation of a biosensor ( Belluzo et al. , 2008 )
There is a overplus of different biosensors developed, which use different feeling modes like electrochemical, optical, thermometric, piezoelectric and magnetic ( Yoo and Lee, 2010 ) . The most researched being glucose biosensors which preponderantly use an electrochemical detection mode, characteristically implied in holding simple operation, suited sensitiveness, dependable, speedy and low fabrication cost ( Newman and Turner, 2005 ) .
2.2.1 Glucose Biosensors
Glucose is the primary fuel for cells of most living beings and commercially, it is used as a precursor for the production of molecules such as vitamin C, citric acid, gluconic acid, polylactic acid, sorbitol and bio-ethanol. Such big scale use of glucose necessitates the call for efficient feedback control systems, for which the usage of glucose detectors is compulsory.
Glucose biosensors, as mentioned earlier, preponderantly use an electrochemical detection mode. Electrochemical detectors measure the negatrons produced as a byproduct during an electrochemical reaction. This is done by aiming a chief feature of most enzymes, whereby a redox reaction is performed, when catalysing a substrate ( Ratner et al. , 2004 ; Heller and Feldman, 2008 ) . Electrochemical detectors may be subdivided into Amperometric, Potentiometric, Conductometric and Ion-selective field-effect transistor ( ISFET ) based detectors ( Li et al. , 2007 ) .
Figure 2.5 shows a chronological history of glucose biosensors. ( Yoo and Lee, 2010 )
Enzymatic Amperometric glucose biosensors are the most common devices commercially available, and have been widely researched over the last few decennaries ( Yoo and Lee, 2010 ) . Amperometric detectors comprise of two to three cardinal electrodes ; a on the job electrode, a mention electrode and a counter electrode. The enzyme involved in catalysing the mark reaction is immobilized on the working electrode surface and the consequence of the biochemical reaction can so be found at the anode or cathode.
Figure 2.6 shows the constituents of a coil-type electrochemical, Amperometric glucose biosensor ( Yu et al. , 2005 ) .
The enzyme used for the above glucose biosensor assembly is Glucose Oxidase ( GOx ) . This is one of two enzyme households that are used as the acknowledgment component for the electro-oxidation of glucose, the other household being PQQ-glucose dehydrogenase ( PQQ-GDH ) .
Although both enzymes can be used for electrochemical glucose biosensor, GOx has been more widely used for in vivo glucose biosensor research. This is due to GOx being more specific to glucose compared to PQQ-GDH. In the electrochemically relevant half-reaction, glucose is oxidized by GOx at the rate of 5×103 glucose molecules per 2nd whereas PQQ-GDH oxidizes glucose at rate of 11,800 glucose molecules per second ( Heller and Feldman, 2008 ) .
2.3 The importance of blood glucose Monitoring
Blood glucose measuring has been highlighted for a diabetes intervention ; it has been shown that effectual monitoring of blood glucose can take to 30-70 % lessening in patterned advance of the disease. ( Diabetes control and complications test research group, 1993 ) With careful disposal, these complications can be delayed and even prevented. ( Newman and Turner, 2005 ) Thus the American Diabetes Association recommends Self-Monitoring of Blood Glucose ( SMBG ) for Type 1 diabetics at least four times a twenty-four hours ( American Diabetes Association, 1994 cited in Newman and Turner, 2005 ) while Type 2 diabetics should supervise their glucose degrees twice a twenty-four hours. Blood glucose monitoring are undertaken utilizing commercial ex-vivo/in-vitro glucose biosensors.
The commercial glucose biosensors available for place and clinical usage are by and large classified into two classs, self-monitoring of blood glucose ( SMBG ) and uninterrupted glucose monitoring ( CGM ) devices. The former requires finger to be pricked to pull blood which is used on trial strips to observe blood glucose concentration while the ulterior involves non-invasive, minimally invasive or invasive methods for CGM. However, commercially merely invasive devices like needle type electrochemical glucose biosensor are available which is inserted in the dermal or hypodermic skin infinite and map for a maximal period of 7 yearss.
Figure 2.7 shows the advantage of CGM over SMBG ( Medtronic, 2012 )
The figures even with conformity of proving often are a snapshot for the fluctuating blood glucose measurings present, therefore losing episodes of sodium thiosulphate or hyper-glycemia. Besides assorted other facets need to be taken into consideration such as what and when the patient last Ate ; patients exercise government and the sum of medicine that has been taken, cost of glucose trial strips. Hence due to the assorted complexnesss, it appears that the frequence of SMBG proving is non at the needed degree in many parts of the universe. ( Newman and Turner, 2005 ) There is demand for improved designs of glucose proctors for better diabetes intervention which all glucose biosensor company ‘s research and design. Focus are extremely involved in developing glucose biosensors for a long term Continuous Glucose Monitoring ( CGM ) system which will supply better apprehension and monitoring of the fluctuating glucose degrees for better single intervention. It is besides the constriction for the design of an unreal pancreas.
2.4 Continuous Glucose Monitoring
Continuous Glucose Monitors ( CGM ) measures interstitial glucose degrees continuously and updates the glucose degree every 1 to 5 proceedingss. The CGM system consists of the followers:
A proctor to expose the information
Detector that is normally inserted in the hypodermic tissue utilizing detector inserter
Figure 2.8 shows Continuous Glucose Monitoring System where A -Sensor, B- Sensor inserter, C- Monitor, D- Monitor connected to a docking station to download informations into a computing machine ( Gross et al. , 2000 ) .
A sender that transmits the detector informations to the proctor.
CGM can supply both retrospective every bit good as real-time information for the sensing of hypoglycaemic and hyperglycemic jaunts, anticipation of impending hypoglycaemia and broad fluctuations in glucose degrees besides known as glycemic variableness ( Yee and Klonoff, 2010 ) .
Therefore they are good to both the doctor and the patient for tendency analysis, better insulin intervention and for long term decrease to metabolic complications. However the major end of CGM devices still focuses upon its impact within the diabetic society and for the commercial production of an unreal pancreas, which proctors and delivers effectual therapy to fluctuations in blood glucose to keep the healthy scope within a diabetic, forestalling ague and chronic complications associated with fickle blood glucose degrees within patients.
The CGM systems available commercially include Guardian REAL-Time, Minimed Paradigm REAL-Time, Minimed CGMS system Gold, SEVEN by Dexcom ( San Diego, CA, USA ) and Freestyle Navigator by Abbott ( Abbott Park, IL, USA ) . All these systems are based on enzymatic, amperometric glucose biosensors which measure glucose in the interstitial fluid ( ISF ) . While the Glucoday system is based on the rule of microdialysis, where ISF is drawn utilizing minimally invasive dialysis tubing to an amperometric device located outside the organic structure. This device is moderately bulky and is frequently limited to infirmary. The dependable usage of these devices for CGM is limited to 2 to 7 yearss, due to concern related with biocompatibility.
Therefore there is a demand for better CGM engineering, with aspiration for bring forthing devices with a longer life span. The demand for bettering length of service is ab initio for improved glucose monitoring, but besides for implicating the potency of planing an effectual unreal pancreas, with a closed cringle system. ( Hovorka, 2008 )
Glucose biosensors have been shown to neglect for assorted grounds, but one big country of public presentation failure in vivo, is the job of detector biocompatibility. Previous research workers have focused on changing the outer membrane constituent of glucose biosensors for bettering biocompatibility, as the outer membrane Acts of the Apostless as the interface between the functional electrochemical constituents of the glucose biosensor and the external biological environment. Therefore developing an outer membrane with better surface belongingss will command and forestall negative biological responses. ( Wisniewski et al, 2000 )
2.5 Biocompatibility – the constriction for implantable glucose biosensors
Wound mending response
Figure 2.9 shows host response to the interpolation of a biomaterial ( Chan et al. , 2008 )
Secretion of ECM by fibroblasts and capillary formation
Macrophages and FBGCs
Monocytes differentiate to macrophages
Host tissue in-growth, Implant debasement and reabsorption
Macrophages merger to organize FBGCs, Frustrated phagocytosis
Proliferation of fibroblasts and endothelial cells
Hempen capsule formation
Resolution of hurt
Implant site remodelling
Host tissue replacement/regeneration
Protein surface assimilation, Water surface assimilation, Implant debasement
The outer membrane constituent of glucose biosensors is by and large composed of polymer stuff, most often a man-made polymer, known to hold better lastingness, mechanical strength, viscoelastic belongingss and easiness for industry and processing. However they lack biocompatibility when compared to natural polymers. ( Wang et al, 2004 ; Liang et Al, 2007 )
Therefore through the usage of electrospinning, a procedure capable of manufacturing nanofiber mesh-like membranes advantageous for improved outer membrane features every bit good as bio-mimicry of biological constituents, such as the extracellular matrix ( ECM ) . ( Sill and Von Recum, 2008 )
Electrospinning is a non-woven fiber whirling engineering that allows spinning of fibers holding diameter runing from 2 nanometers to 10s of µm ( Bhardwaj and Kundu, 2010 ) . Making fibre meshes, in the nanometer and micrometre scope, is of import for biological application, as little pore sizes created in nanofibres composings, prevent the attachment of certain cells. ( Sill and Von Recum, 2008 ) Therefore the production of nanofibres meshes would be advantageous for the outer membrane of in vivo glucose biosensors for length of service of the detector in vivo.
The electrospinning procedure consists of a polymer solution held by its surface tenseness at the terminal of a capillary tubing which is subjected to an electric field. Here the
charge is induced onto the liquid surface by an electric field. As a consequence, common charge repulsive force causes a force which is straight opposite to the surface tenseness. And as the strength of the electric field is increased, the hemispherical surface of the solution at the tip of the capillary tubing elongates to organize a conelike form known as the Taylor cone. When the electric field reaches a critical value at which the repulsive electric force overcomes the surface tenseness force, a charged jet of the solution is ejected from the tip of the Taylor cone. The flight of the jet can be controlled by an electric field as it is charged. When the jet travels in air, the dissolver evaporates, go forthing behind a charged polymer fiber which is laid indiscriminately on a roll uping surface. Therefore uninterrupted fibers are laid to organize a non-woven cloth. ( Doshi and Reneker, 1995 )
Figure 2.9 Conventional diagram shows the general setup used for electrospinning. ( Ziabari et al, 2009 )
Figure 2.10 Photographs exemplifying the instability part of a liquid jet electrospun from an aqueous solution of poly ( ethylene oxide ) PEO A ) conventional and B ) high velocity camera with exposure times of 1/250 s and 18 N severally. ( Li and Xia, 2004 )
In real-time, the flexing instability of the jet appears to naked oculus or utilizing conventional picture taking as the individual watercourse of polymer solution dividing into big figure of jets in a cone form. However, utilizing high velocity camera, Shin et Al. demonstrated that it is fundamentally a individual fiber that bends and whips quickly to give the semblance of splitting and multiple jets ( Shin et al. , 2001 )
2.5.1 Electrospinning setup
The setup required for electrospinning is simple in assembly and consists of a high electromotive force power supply, a syringe pump with tubing to transport solution from the syringe to the spinneret, and a conducting aggregator. The spinneret and aggregator are aligned opposite to each other and together they are normally oriented either horizontally or vertically. Further fluctuations in the equipment can be achieved with assorted spinneret and aggregator combinations.
Spinneret buildings are used to accomplish greater versatility in the nano-fibre constructions. They can be categorised into individual or multiple and needle-type or needleless constellations ( Teo and Ramakrishna, 2006 ) . The usual electrospinning apparatus consists of a individual blunt terminal acerate leaf for whirling individual polymer fibers ( figure 2.11a ) . To obtain composite fiber construction, either coaxal ( Figure 2.11b ) or dual-capillary ( side-by-side ) ( Mistake: Reference beginning non found 2.11e ) spinnerets are used. The former produces a core-shell while the latter produces side-by-side fused fibre constructions severally. The advantage of utilizing coaxal spinnerets is to obtain fibers from incompatible or non-electrospinnable polymer solutions ( Teo and Ramakrishna, 2006 ) . For case, a non-electrospinnable polymer solution can be extruded in the interior capillary, while the spinnable solution extruded in the outer capillary. Thus the interior solution is enclosed by the outer solution ensuing in a nucleus fiber. Next the outer solution can so be dissolved to obtain the fibers of the non-electrospinnable solution. This method can besides be used to bring forth hollow fibers by fade outing the inner nucleus fiber. Overall the coaxal fiber consequences in a strengthened nucleus polymer fiber encapsulated by a outer covering of a different polymer, whereas in the side-by-side fused composite fibers there are two different polymers placed aboard. Using assorted substitutions and combinations of two polymers different membranes can be obtained for varied applications, peculiarly in the promising field of tissue technology. In add-on, the coaxal spinnerets can besides be farther modified to integrate, for illustration, three homocentric polymer fibers ( Figure 2.11c ) , or a big fiber reinforced with three different polymer fibers within its composite construction ( Figure 2.11d ) .
Figure 2.11 shows needle-based spinnerets and their cross-sections, a ) individual acerate leaf, B ) two homocentric acerate leafs, degree Celsius ) three homocentric acerate leaf, vitamin D ) three side-by-side needle inside a big acerate leaf and vitamin E ) side-by-side acerate leafs. The Numberss 1 to 4 indicate different polymer solutions ( Teo and Ramakrishna, 2006 ) .
In cardinal constellation, the spinneret acerate leaf is aligned perpendicular to a inactive grounded level home base aggregator. During electrospinning procedure, the seamless fiber gets collected as a sheet, normally on an aluminum foil placed on top of the level home base aggregator. The attendant sheet has random fibre orientation. However, the orientation, 3D architecture and belongingss of the electrospun constructions can be varied by the application of an electric field between the spinneret and aggregator or by revolving the aggregator. In add-on, the usage of alternating-current ( AC ) high electromotive force supply alternatively of the traditional direct-current ( DC ) high electromotive force supply for bear downing the electrospinning solution is reported to bring on better alliance of the fibers. ( Kessick et al. , 2004 )
Assorted aggregator constellations used in research are illustrated below in figure 2.12. ( Teo and Ramakrishna, 2006 )
Large country of aligned fibers
Highly aligned hempen assemblies hard to manufacture
If revolving velocity is excessively high fibre breakage may happen
Easy to obtain extremely aligned fibers
Easy transportation of aligned fiber to another subtrate
Thick bed of aligned fibers are non possible
Limit in length og aligned fibers
Revolving wire membranophone aggregator
Highly aligned fibers are accomplishable
Can non accomplish thicker bed of aligned fiber
Fibers may non be aligned right through the whole assembly
Drum aggregator with wire lesion on it
Highly aligned fibers possible
Area of aligned fibers on the wire can be adjusted by changing wire thickness
Aligned fibers are concentrated on the wire alternatively of the whole membranophone
Revolving tubing aggregator with knife-edge electrodes below
Highly aligned fibers
Whole tubing covered by aligned fibers
Thick bed of aligned fibre deposition possible
Set-up requires negative electrode to be effectual
Merely little diameter tubing possible
Highly aligned fibers accomplishable
Direction of fibre alliance on tubing controlled
Thicker bed of aligned fibre deposition
Set-up requires negative electrode to be effectual
Merely little diameter tubing possible
Highly aligned fibers accomplishable
Fabrication of panoplied fibers by attaching rotatable tabular array on border of phonograph record possible
Incapable to retain high fiber alliance at same revolving velocity when deposited fibers are thicker
Small country of fibre alliance
Array of counter-electrodes
Inconsistent fiber form throughout assembly
Limited Assembly country
Thicker fiber assembly non possible
Revolving membranophone with crisp pin inside
Fabrication of big country of panoplied fibers
Thicker country of panoplied fiber assembly non possible
Blade placed in line
Highly aligned collected narration
Limited fabricated yarn length
Deposited fibers need to be dipped in H2O before narration formation
Ringing aggregator placed in analogue
Fabrication of distorted narration
Limited fabricated yarn length
One ring needs to be rotated to writhe fibers that sedimentation into narration
Controlled deposition utilizing pealing electrode
Area of fibre deposition can be minimized
Ringss have to be given positive charge
Figure 2.12 Table demoing a assortment of aggregator constellations that have been tested for act uponing fibre deposition on the aggregator. ( Teo and Ramakrishna, 2006 )
Fibre deposition can be controlled over an country
Ringss have to be given positive charge
Area of fibre deposition is big although confined within the ring
2.5.2 Electrospinning Parameters
The many parametric quantities which affect and/or command the procedure of electrospinning of fibers and their resulting morphology and diameter are as follows:
Polymer molecular weight
Solution surface tenseness
Applied electromotive force
Distance of electrode beginning from the mark substrate
Solution flow rate
All the variables mentioned above non all are neither cardinal control parametric quantities nor they independent of each other. For illustration, solution viscousness is a map of both concentration and molecular weight. ( Shenoy et al. , 2005 )
The electrospinnability of a polymer depends on its solution viscousness and electrical electric resistance. If the viscousness is excessively low, the coherence between the polymer ironss ( surface tenseness ) is low to keep the fluid jet emerging from the Taylor cone together. Similarly, if the solution is excessively conductive, the abhorrent forces between the polymer molecules cause the fluid jet to breakdown, organizing droplets. With increasing viscousness and electrical electric resistance, the dissolution of the jet into droplets passages into the formation of beads-on-string fiber construction before proper fibers form ( Shenoy et al. , 2005 ) . Further addition in viscousness consequences in increasing fibre diameter until a maximal viscousness beyond which the Taylor cone becomes excessively large, doing the jet to go unstable ( Megelski et al. , 2002, Jarusuwannapoom et al. , 2005, Demir et al. , 2002, Deitzel et al. , 2001a ) . Therefore, for a given polymer, there exists a scope of solution viscousness, wherein proper fibres signifier. This scope varies for any specific polymer depending on the dissolver it is dissolved in and besides varies for different polymers ( Huang et al. , 2003 ) . The solution parametric quantities non merely find the electrospinnability of a polymer, but besides affect the fiber morphology and diameter.
Materials and Methods
Electrodes: Platinum-iridium ( 10IR5t ) ( Pt: Ir, 9:1 weight ratio ) and silver wires ( AG5T ) , each holding a diameter of 0.125 millimeters and covered with an insulating Teflon-coating, were obtained from ABC
Sensor Coatings: Bovine serum albumen ( BSA ) , glutaraldehyde ( GTA ) class I ( 50 % ) , glucose oxidase ( GOD ) ( EC 220.127.116.11, Type X-S, Aspergillus Niger, 157,500U/g, Sigma ) , ATACS 5104/4013 epoxy adhesive, non-ionic wetting agent Brij 30, polyurethane ( PU ) Z1A1, , Tetrahydrofunan ( THF ) ( THF, ACS reagent, & A ; gt ; 99.0 % )
Sensor Function Testing: D- ( + ) -Glucose and phosphate buffered saline ( PBS ) tablets ( 0.01 M incorporating 0.0027 M K chloride and 0.137 M Na chloride, pH 7.4, at 25 & A ; deg ; C ) were purchased from Sigma-Aldrich-Fluka, UK.
A illumination coil-type implantable glucose biosensor, developed in Moussy ‘s group ( Yu et al. , 2005 ) , is used as theoretical account detector in this survey. The amperometric detector is a two electrode system based on Pt/Ir working and silver/silver chloride ( Ag/AgCl ) mention electrodes.
Figure shows the constituents of implantable glucose biosensor
Working electrode: It was prepared by foremost taking 1 cm length of Teflon covering at either terminals of the 8 centimeter long Pt-Ir wire holding 0.125 mm diameter. On one terminal, the bare Pt-Ir wire was wound around 18 gage acerate leaf ( 1/2 inch, BD ) to do the working electrode spiral. The nucleus of the spiral was so filled with cotton to heighten the immobilisation of enzyme and besides to forestall formation of an air bubble. The cotton reinforced spirals were so cleaned utilizing absolute ethyl alcohol, every bit good as by being placed in deionised H2O and sonicated utilizing an supersonic bath ( 1510 Branson ) for 30 proceedingss. The detectors were so heated at 60 & A ; deg ; C for 20 proceedingss before immobilisation of glucose oxidase ( GOD ) took topographic point.
Enzyme Load: For immobilising GOD on the working electrode spiral, 1.0-1.5 µl of enzyme solution ( 39.3 mg/ml BSA, 8.2 mg/ml GOD and 1.6 % ( v/v ) GTA ( 50 % , v/v ) dissolved in DI H2O ) was loaded onto the cotton reinforced Pt-Ir spiral and allowed to dry at room temperature for 30 proceedingss. This enzyme loading process was repeated 3 times for each working electrode. After the last coating, the enzyme bed was left nightlong to let GTA cross-linking to finish.
Mass-transport restricting membrane: To surface epoxy-PU ( EPU ) mass-transport restricting membrane, 1.5 µl of the EPU lading solution ( 26.7mg of PU, 8.9mg each of Part A and Part B of epoxy adhesive and 1 µl of Brij 30 dissolved in 4ml THF ) was applied on the enzyme bed ( Yu et al. , 2007 ) . The epoxy-PU coating has been shown to heighten map and length of service of the detectors ( Yu et al, 2006 ) . After air-drying at room temperature for 30 proceedingss, the dissolver dramatis personae EPU bed was cured in an oven at 80oC for 20 proceedingss.
Mention electrode: The mention electrode was prepared by foremost depriving 1 centimeter of Teflon surfacing from both terminals of a 7 centimeter long Teflon covered silver wire holding 0.125 mm diameter. One terminal was carefully wound around a 30 gage 1/2 inch subcutaneous acerate leaf to acquire the spiral terminal. The Ag spiral was treated with ammonia solution for 30 seconds followed by 10 seconds in 6 M azotic acid. The spiral was so washed in DI H2O and electroplated in 0.01 M HCl at a changeless current of 0.1 mas utilizing a galvanostat ( 263A, Princeton Applied Research, TN, US ) for 5-6 hours in a 0.01M HCl solution of deionised H2O ( 100 milliliter ) and Hydrochloric acid ( 100 ?L ) . Electroplating consisted of the connexion of the mention electrodes and a counter Pt mesh electrode within the beaker. The ensuing Ag/AgCl mention electrode spirals were rinsed with DI H2O.
Detector Assembly: The detectors were assembled by infixing the Pt-Ir wire of working electrode through the spiral of mention electrode until the two electrodes were separated by 5 millimeters and the Teflon covered portion of the two wires were entangled along their full length.
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