Satuluri V. K. R. Rajeswari1
                     Ponnusamy Vijayakumar1*
               
                  - 
                           
                        (Department of Electronics and Communication Engineering, SRM Institute of Science
                        and Technology/Kattankulathur-Chennai, Tamilnadu-603203, India   {rs3740, vijayakp}@srmist.edu.in)
                        
 
            
            
            Copyright © The Institute of Electronics and Information Engineers(IEIE)
            
            
            
            
            
               
                  
Keywords
               
                Artificial intelligence,  Diabetes mellitus,  Deep learning,  NIR
             
            
          
         
            
                  1. Introduction
               Diabetes Mellitus(DM) is a chronic disease with a growing incidence worldwide.DM is
                  collectively used for all types of diabetes. According to the diabetes atlas, one
                  in 11 adults (20-79 years of age) have diabetes (463 million people); one in two adults
                  with diabetes is undiagnosed(232 million people); one in five people with diabetes
                  is above 65 years of age (136 million people). More than 1.1 million children and
                  adolescents below 20 years have type 1 diabetes. Three out of four (79%) people are
                  living in low middle-income countries, one in six live births (20 Million) are affected
                  by hyperglycemia in pregnancy, while 84% of them have gestational diabetes, and 10%
                  of global health is spent on diabetes [1].
               
               Individuals with diabetes are always at a higher risk of surgical and nosocomial infections.
                  The possibility of developing cancer is often found in Type-2 DM. Infections due to
                  yeast, fungi, and bacteria are found more in people with DM [2]. It has become crucial to have real-time insight into the glucose levels in an individual.
                  There is a strong need for a self-care management device for the daily monitoring
                  of glucose.
               
               This study evaluated different types of glucose monitoring and how non-invasive glucose
                  monitoring is effective in daily life. Artificial intelligence-powered algorithms
                  that contribute to the novel cause of non-invasive glucose monitoring are also discussed.
               
               
                     1.1 Classification of Diabetes Mellitus
                  Diabetes Mellitus is a collective term used for different types of diabetic conditions.
                     Diabetes is classified into Type-I, Type-II, and Gestational diabetes.
                  
                  1. Type-I Diabetes Mellitus:
                  Type-I diabetes is commonly found in people under the age of 35 years . Due to genetic
                     influences and other environmental factors. It is also known as juvenile diabetes
                     because it occurs during adolescence [3].
                  
                  2. Type-II Diabetes Mellitus:
                  Type-II DM occurs as a result of the insufficient production and secretion of insulin.
                     It is the major type of diabetes worldwide, and the incidence increases with age [3].
                  
                  3. Gestational Diabetes Mellitus:
                  Gestational DM occurs during pregnancy. There are considerable fluctuations in glucose
                     levels during this period, which sometimes leads to other complications, even after
                     the end of the gestational period [3]. Although there are different types of DM, a medium to monitor glucose should be
                     considered of the utmost importance. The following section examines different ways
                     of glucose monitoring.
                  
                
             
            
                  2. Glucose Monitoring 
               There are diagnostic criteria for monitoring glucose in DM. An individual is categorized
                  as diabetic when the upper limit exceeds the described limits [3]:
               
               Random plasma glucose ${\geq}$200mg/dL(11.1mmol/L);
               Fasting plasma glucose${\geq}$126 mg/dL(7mmol/L);
               Oral glucose tolerance test>200mg/dL(11.1 mmol/L).
               The tests to analyze the blood glucose concentrations are generally taken from a pathology
                  lab. Blood is drawn from the arm, and the glucose concentrations are determined from
                  the blood. Furthermore, different types of monitoring glucose will be elaborated on
                  and discussed.
               
               
                     2.1 Invasive, Non-invasive Monitoring, Minimally Invasive Glucose Monitoring 
                  The glucose concentration is determined by harvesting blood from the hand(venepuncture
                     method). The glucose concentrations are tested from the plasma or serum. This is a
                     general method followed in hospitals for checking the diabetic condition in patients.
                     Continuous blood withdrawal from a patient under ICU care or surgical procedure has
                     a potential risk of a decrease in blood levels. Tests performed at the laboratories
                     can sometimes show false results due to toxicity and cross-reactions due to various
                     agents [4]. Due to the rise in diabetes for a decade, it has become vital to have a continuously
                     monitored glucose device. Different methods for glucose monitoring will be discussed
                     in the following subsection. The glucose concentration can be detected by invasive,
                     minimally invasive, and non-invasive methods [5], as depicted in (Fig. 1). In the invasive method of detecting glucose, blood is drawn from the body. Apart
                     from laboratory tests, which are not suited for continuous monitoring, there are many
                     products in the market that allow continuous monitoring of glucose invasively. A glucometer
                     is a device consisting of a lancet that helps to prick the finger for capillary blood.
                     The glucose strip present reacts and detects the amount of glucose by oxidizing the
                     amount of blood and producing a current proportional to the glucose level. Through
                     the meter, an electron travels to the voltage to the current converter and produces
                     a voltage proportional to the glucose. The drawback of this approach is that pricking
                     the finger for every reading is quite painful, and the lancets and test strips are
                     expensive [5].
                  
                  There are minimally invasive (MI) techniques that need a minimum intervention of the
                     inner body for the analysis. This could be the upper layers of skin, interstitial
                     fluid, or tears that must be extracted to measure the glucose concentration [5] (Fig. 2) shows the different minimally invasive methods.
                  
                  In the ultrasound approach of the MI method, interstitial fluid is sampled using ultrasound
                     methods, and surface plasma resonance is detected. The refractive index is measured
                     using microsystem technology, which gives the glucose concentration. The drawback
                     of this approach is that ultrasound is susceptible to temperature [5,6]. The reverse iontophoresis approach of MI method depends on the circulation of a
                     small electric current between the anode and cathode that is located on the surface
                     of the skin. This approach allows access to the small interstitial fluid. A current
                     is produced when sodium ions migrate by causing the convective flow of the fluid.
                     This fluid also carries glucose molecules that reach the cathode. The glucose sensor
                     at the cathode measures the glucose concentration by following the oxidation process
                     (enzymatic method). The drawback of this method is that it is susceptible to sweat
                     due to heat generation, and skin irritation can occur due to the passage of current.
                     Quick changes in measuring the glucose current are not possible [5,7].
                  
                  The minimally invasive method of sonophoresis measures glucose by extracting the interstitial
                     fluid using an enzymatic method. This method uses low-frequency pressure waves to
                     trigger glucose waves through the skin [5]. This method is used more commonly for drug delivery than for detecting glucose [8]. There may be erroneous readings due to interference from other compounds and pressure
                     changes. This method could also produce incorrect readings due to temperature variations
                     [5,8].
                  
                  The Non-Invasive method of detecting glucose are divided into thermal, electric, and
                     optical methods, as depicted in Fig. 3. The thermal method of detecting glucose uses the heat generation method. Photoacoustic
                     spectroscopy(PAS), metabolic heat conformation(MHC), and thermal emission spectroscopy
                     (TES) are the three types of thermal methods of detecting glucose. In electric methods
                     of detecting glucose, the dielectric properties of glucose at low frequencies are
                     detected. This is achieved using small amounts of electromagnetic radiation, current,
                     and radiation. Bioimpedance spectroscopy, electro-magnetic sensing, and millimeter
                     and microwave sensing are the three electric non-invasive approaches. The third methodology,
                     optical methods, uses light that penetrates the skin or sample at a specific wavelength
                     and detects glucose. The different types of noninvasive optical methods include terahertz
                     time-domain spectroscopy, Raman spectroscopy, FIR spectroscopy(far-infrared spectroscopy),
                     MIR spectroscopy(mid-infrared spectro-scopy), NIR spectroscopy(near-infrared spectroscopy),
                     optical coherence tomography(OCT), and optical polarimetry [5].
                  
                  Among the available market solutions and extensive research on invasive, minimally
                     invasive, and non-invasive methods, extensive research is occurring in the non-invasive
                     monitoring of glucose. The following drawbacks of invasive and minimally invasive
                     techniques justify the need for a non-invasive methodology for detecting glucose [5,9]:
                  
                  1. Anxiety is always triggered when pricking the finger and the pain associated with
                     it. For continuous monitoring of glucose, there is a need to prick the finger continuously,
                     which will not work.
                  
                  2. For continuous monitoring of glucose, it is essential for a system to be small
                     so that it can be carried anywhere.
                  
                  3. Lancets and test strips are only used once. They are quite expensive.
                  4. There is a possibility of incorrect recordings caused by environmental changes.
                  5. Incorrect readings can occur if the testing strip is misplaced.
                  6. Erroneous readings are unavoidable when the finger is not cleaned when the test
                     is taken.
                  
                  7. In a minimally invasive method, removing the interstitial fluid is painful. This
                     method is not recommended for the continuous monitoring of glucose.
                  
                  8. In a minimally invasive method, sensors are inserted inside the human body and
                     send important information to IoT devices. This method is associated with allergies
                     and emergencies.
                  
                  The above drawbacks justify the need for the non-invasive monitoring of glucose. Former
                     sections have shown that non-invasive methods can solve the above-discussed challenges.
                     The next section evaluates non-invasive methods for detecting glucose using different
                     approaches.
                  
                  
                        Fig. 1. Glucose detection approaches.
 
                  
                        Fig. 2. Minimally invasive approach to detecting glucose.
 
                  
                        Fig. 3. Non-invasive approach of detecting glucose.
 
                  
                        Fig. 4. Categorization the ML algorithms.
 
                
             
            
                  3. Non-invasive Glucose Monitoring
               The World Health Organization (W.H.O) reported a higher number of deaths due to strokes,
                  ischemia, and diseases, such as blindness, kidney failure, heart attacks, and lower
                  limb amputation associated with diabetes [10]. There is a need to monitor glucose continuously for emergency conditions, before
                  and after surgery, after release from ICU, at home, and to monitor the chronic condition
                  itself. Few studies have worked on different non-invasive glucose monitoring methods,
                  as shown in Fig. 3. Different approaches towards detecting glucose non-invasively are discussed further.
               
               In the thermal method of detecting glucose, PAS uses short laser pulses that implement
                  wavelengths absorbed by any molecule and produce microscopic localized heating. It
                  is susceptible to noise and temperature changes [11-13]. By implementing NIR wavelengths, glucose is detected at 1540-1840nm [11] and 905 nm-1550nm [12]. By implementing MIR in an in-vivo method of detecting glucose from the skin, glucose
                  molecules are detected at 1070,1105, and 1140cm$^{-1}$ [13]. This approach fails in the in vivo method because of the poor sensitivity of glucose
                  detection.
               
               In the thermal method of MHC, physiological parameters in the form of radiation, convection,
                  and evaporation are measured using the metabolic oxidation of glucose [14,15].Glucose is detected from the amount of heat dissipated. Thermal and optical sensors
                  are generally used to measure the physiological parameters, such as blood flow rate
                  [14,15] of local tissue, blood oxygen [14], hemoglobin, and oxyhemoglobin concentrations [15]. The susceptibility to temperature and sweat are the main drawbacks of this approach.
                  In the TES thermal method of detecting glucose, the heat emitted by the human body
                  is in the form of energy, which can be detected at the FIR range of 8 ${\mathrm{\mu}}$m
                  to 14${\mathrm{\mu}}$m.Glucose molecules in the body absorb this radiation and provide
                  a detection mechanism for glucose. A handheld approach to detecting glucose was presented
                  in [16], where the device was made from a filter spectrometer and the rmobile detector. While
                  sensitive to temperature and heat, tissue density varies for each person, which may
                  produce fault readings. Continuous glucose monitoring is not possible, which is another
                  drawback of this approach.
               
               In the electrical method of bioimpedance sensing, the permittivity and conductivity
                  of the membrane in red blood cells produce dielectric impedance when current is applied.
                  Bioimpedance strip electrodes are placed on the wrist to collect glucose measurements
                  [17]. As this procedure may produce fluctuated data due to temperature sensitivity and
                  physiological conditions that can affect the cell membrane, another approach for obtaining
                  better accuracy was proposed [18], where MIR was also implemented. The glucose levels are detected at (5-10)${\mathrm{\mu}}$m
                  at 100 Hz-30MHz. Sensitivity to water is another drawback for both approaches. In
                  the electrical method of the microwave/ millimeter detection of glucose, radiation
                  is present in a low energy per photon and less scattering in the tissue, which is
                  advantageous for detecting glucose concentration much more accurately. A radar-driven
                  microwave sensor is proposed in [19] to detect glycemia levels in patients with diabetes. The transmission of mm waves
                  to detect glucose from two microstrip patch antennas is proposed in [20]. This method is used widely in communications that need detection through the exploitation
                  of transmission and absorbance characteristics owing to the advantages of deeper penetration.
                  This approach faces challenges when there is a variation in blood molecules, and physiological
                  parameters, including breathing and temperature changes in the body.
               
               There are many approaches to the optical method of measuring glucose molecules. Terahertz
                  or FIR spectroscopy is based on the principle of absorption due to the vibration and
                  rotation of weak and heavy bonds of atoms. This is achieved at wavelengths between
                  0.3THz (1000${\mathrm{\mu}}$m)and 30THz(10${\mathrm{\mu}}$m). The silicon Dowe prism
                  for internal reflection was used in [21]. Glucose molecules were detected at 0.1-0.5THz, and similarly at 0.1-1.0THz [22]. When the blood glucose levels rise above the normal value, the amplitude and phase
                  of the reflection coefficient change on human skin. If there is a strong absorption
                  of water, identification of glucose molecules becomes difficult, which is a drawback
                  of this approach. Raman spectroscopy is the principle of determining the scattered
                  light from a monochromatic light. Miniaturized Raman spectroscopy was designed from
                  the benchtop [23]. Glucose molecules were detected at 1125cm$^{-1}$.The collection time for glucose
                  in this method was large, which is a drawback. In [24], ellipsoidal reflector-based spectrometry was proposed to detect glucose. Glucose
                  was also detected at 980nm while not using reflector spectrometry to increase the
                  accuracy. The interferences in blood with other molecules, such as hemoglobin, challenge
                  this approach. Another disadvantage of this approach is the predictability of the
                  wavelength. In the MIR optical method, glucose can be detected at 12-30THz. Owing
                  to the longer wavelength, there is light scattering, but the water molecules cannot
                  be absorbed in MIR ,resulting in less penetration. Hence lasers, such as quantum cascade
                  lasers, were proposed by [25] with this approach. An external cavity QC laser between 8-10${\mathrm{\mu}}$m was
                  focused on a hollow core fiber to deliver light. Glucose molecules are detected at
                  1080 cm$^{-1}$. Similarly, an ATR prism was used in [26]. Two FTIR spectrometers were used to detect the glucose. Three different wavenumbers,
                  i.e, at 1050 cm$^{-1}$, 1070 cm$^{-1}$, and 1100 cm$^{-1}$, were chosen to detect
                  glucose, but glucose peaks were obtained at 1036 cm$^{-1}$,1080 cm$^{-1}$, and 1100
                  cm$^{-1}$.The prototypes designed in [25,26] are expensive, which is a challenge in continuous glucose monitoring. The NIR approach
                  to detecting glucose is a wider area of research for designing wearables [27,28]. Glucose can be detected between 780 nm to 2500nm.Using a PPG circuit and NIT amplifier,
                  glucose is detected at on a single wavelength, 940nm in [27]. This method has the drawback of accuracy, which is much enhanced in [28], where glucose is detected in the range of 940nm by reflection spectroscopy and at
                  1300 nm by detection spectroscopy. Owing to the low wavelength, there is high light
                  scattering, which is a challenge in NIR spectroscopy.
               
               Optical coherence tomography is an imaging method of detecting optical characteristics
                  in bio-tissues at micrometer resolutions. The method works on the principle of low-coherence
                  interferometry and coherent radiation. The blood glucose level is obtained at the
                  dermis layer of the skin using this approach. A Michelson interferometer, fiberoptic
                  coupler, and broadband light source for illumination are gathered to detect glucose
                  [29]. Glucose is detected by measuring the exponential slope of the light attenuation
                  in the tissue. This approach is sensitive to skin thickness, temperature changes,
                  and tissue homogeneity. Optical polarimetry works on the principle of chiral molecules
                  that can rotate on the plane of polarized light. Glucose is a chiral molecule when
                  it rotates in the polarization plane of a light beam by an angle ' ${\alpha}$' in
                  a clockwise direction. The rotation is proportional to the concentration of glucose,
                  optical path length, temperature, and wavelength of the laser beam. Glucose is detected
                  between 635nm and 830 nm. This method is quite sensitive to interference, temperature,
                  and motion changes because the optical rotation is minimal, and there is high scattering
                  of light [30].
               
               This section described the different approaches for monitoring glucose non-invasively.
                  These approaches have been adapted to achieve sensitivity, coefficient of determination,
                  and accuracy. Methods that provide the best accuracy and reliability for a system
                  to work with the least error are vital in healthcare. The following section discusses
                  the ML algorithms implemented for the different approaches to detecting glucose non-invasively.
                  
               
             
            
                  4. Integration of IoT and Machine Learning for Monitoring Blood Glucose Non-invasively
               AI is the discipline of making computers learn human intelligence. ML is a part of
                  AI that helps train computers to learn.AI applications in healthcare ranging from
                  detection, prediction, diagnosis, self-management, and personalization of disease
                  therapy [31].
               
               The three categories of ML algorithms are supervised learning, unsupervised learning,
                  and reinforcement learning. In supervised learning, machines are trained using 'labeled'
                  data. Classification and reinforcement learning are two categories of supervised learning.
                  In unsupervised learning, machines are trained using 'unlabeled' data.
               
               The two learning classifications of unsupervised learning are association learning
                  and clustering learning. The third classification in ML is reinforcement learning
                  which is implemented to make decisions. For detection, ML techniques from different
                  studies are gathered and discussed further .For a non-invasive diabetic management
                  system to thrive, understanding proper models in ML and choosing suitable algorithms
                  for the nature of data is very important. The glucose levels in blood (non-invasively)
                  were predicted, and many approaches of ML were explored.
               
               Every ML algorithm follows the stages from collecting the data/datasets, data pre-processing,
                  feature selection/ extraction, detection, and diagnosis of glucose monitoring [31].
               
               
                     4.1 Discussion on ML Techniques in Diagnosing Diabetes/Glucose Monitoring by Integrating
                     IoT 
                  
                  Data collection/Datasets: In healthcare scenarios and applications, ML algorithms
                     are first evaluated on publicly available datasets, such asthePIMA dataset and Kaggle.
                     The public dataset on non-invasive monitoring of glucose is unavailable. This paper
                     described the number of volunteers whose glucose levels were tested while implementing
                     different ML approaches. The accuracy of the model depends on the quality and quantity
                     of data [31].
                  
                  1. Data Pre-Processing: Once the data is collected, the next step is to clean and
                     process it. The data are processed using different techniques depending on the data
                     gathered and the nature of the system [31]. The nature of data collected in [32, 33, 37, 38] is the PPG signal that comprises noise, artifacts, and baseline drifts. These are
                     reduced using Digital wavelet transform(DWT), Wavelet decomposition, and Moving average(MA)
                     [32]. The activity detection module in [33] extracted cleaner data without noise, whereas filters were implemented in [37,38] to remove unwanted observations. The pulse waveforms(PSW) data in [34,35] are processed polynomial fitting for removing baseline drift [34] and customized modeling to remove noise from raw optical signals. The spectroscopy
                     data in [36, 41,42] were filtered using filters, such as row normalization, butter worth filter [36], and Kalman filter [42].
                  
                  2. Feature Selection: The efficiency of an algorithm depends on the selection and
                     extraction of features from a dataset. It helps in decreasing the complexity and time
                     taken to process, decreasing overfitting while increasing accuracy [31]. The nature of data, e.g., image, text, texture, color, and shape, depends on the
                     modality used in the application. The features selected are glucose molecules in optical
                     NIR spectroscopy [27, 36, 37, 39-42]. The features extracted in NIR spectroscopy are the amplitude, difference of optical
                     density, variance, skewness, and standard deviation [32]. The pulse waveform from arteries was collected; the distinct feature extracted was
                     the amplitude [34]. By implementing optical granular computing, the low and high pressure, rising phase,
                     and temperature are extracted from the pressure signal [35]. The blood flow rate is extracted from the PPG signal and blood oxygen saturation
                     [38]. The above data, it can be inferred that for glucose molecules, the body signals
                     are common features that may provide extraordinary results for research in the detection
                     and diagnosis of th DM.
                  
                  3. Detection and diagnosis of glucose monitoring: In this study, many researchers
                     have explored and implemented various approaches of ML when surveying for the best
                     papers while integrating the IoT and ML.
                  
                  In applications of NIR spectroscopy of detecting glucose molecules, a deep neural
                     network(DNN) with R=0.97,Mean absolute relative difference(mARD)= 4.86,Average error(AvgE)=4.88,Mean
                     absolute deviation (MAD)=9.42,Relative mean square error(RMSE)=13.57 is achieved [27]. In another study implementing NIR spectroscopy, partial least square (PLS) was used
                     ,where the average correlation coefficient Rp is 0.86 [32]. An artificial neural network (ANN) [36] is implemented. ANNs perform better with a Clarke-error grid=86% compared to bioimpedance
                     spectroscopy and Alternating least square(ALS). The blood oxygen and blood flow rate
                     is extracted from the PPG signal to analyze the glucose concentration non-invasively.
                     The coefficient of determination (R$^{2}$=0.91), Clarke error grid =83% in class A
                     and 17% in class B is obtained [38]. ANN is applied to build a model on glucose molecules extracted by breathing acetone.
                     A low mean square error, Regression=1, is achieved. Neural network (NN) performs better
                     than time and frequency, and single pulse analysis showed regression=1 and an error
                     limit of ${\pm}$7.5% [39]. The DNN, ANN, and NN are deep neural networks with advantages in dealing with intra
                     class differences and noisy information. NN consists of enormous numbers of neurons
                     and is advantageous in DM, where the data are massive. A supervised ML Support vector
                     machine (SVM) is implemented on the extracted glucose levels and amplitude. It helps
                     classify the nonlinear data to linear data [34,41]. This alters the DM training data in a higher dimension. The root mean square error(RMSE),
                     which depicts the difference between actual outcome and predictions, achieved RMSE=19.90mg/dL
                     [41] when compared with the autoregressive integrated moving average (ARIMA), random forest
                     (RF). The RF is the next commonly implemented ML algorithm for regression and classification
                     problems. Compared to SVM, ridge linear regression (RLR), multilayer neuron perceptron
                     network (MNPN) RF has a better coefficient of determination (R\-$^{2}$ =0.90) and
                     a Clarke error grid of 87.7% [33]. The sweat sensor is designed to extract glucose levels from sweat. Linear regression(LR)
                     and polynomial regression (PR)analysis is applied to building the model [37,40]. Correlation factors P=0.672,0.574, and 0.343 were achieved, whereas R$^{2}$=0.851
                     was achieved [37]. On a nonlinear dataset, PR was implemented, which produced a correlation coefficient
                     of R$^{2}$=0.99 [40]. A customized physique-based fuzzy granular modeling(PbFG) achieved R$^{2}$=0.9 and
                     Clarke(A+B)>90% when compared with PbFG SVR and PbFG ANN [35]. In a similar modality of detecting blood glucose non-invasively, SVM with the linear
                     kernel is applied and showed a classification of 80.7% [34].
                  
                  Different approaches toward the prediction and diagnosis of non-invasive methods of
                     glucose monitoring were elaborated. The following section discusses the justification
                     of the suitable methodology for detecting glucose from blood non-invasively.
                  
                  
                        Table 1. Literature review on modality and ML algorithms implemented for a non-invasive method of monitoring glucose.
                     
                           
                              
                                 | Ref | Modality | ML Algorithm Applied | Performance Metrics | 
                        
                        
                              
                                 | [27] | NIR spectroscopy | DNN | R=0.97 mARD=4.86 AvgE)=4.88 MAD)=9.42 (RMSE)=13.57 | 
                           
                                 | [32] | NIR spectroscopy | PLS | Rp=0.86 | 
                           
                                 | [33] | PPG waveform | RF | R=0.90 Clarke(A+B)=87.7% | 
                           
                                 | [34] | Pulse waveforms | SVM-linear kernel | Classification of diabetic and pulse waveform=80.7% | 
                           
                                 | [35] | Pulse waveforms | Fuzzy granular modeling | R$^{2}$=0.851 Clarke(A+B)>97.9% | 
                           
                                 | [36] | Bioimpedance spectroscopy | ANN | Clarke-error grid=86% | 
                           
                                 | [37] | Electrochemical method | LR | P=0.672,0.574,0.343 R$^{2}$=0.95 | 
                           
                                 | [38] | Single pulse analysis | ALS | R$^{2}$=0.91, Clarke error grid =83% in class A and 17% in class B | 
                           
                                 | [39] | Biosensor | ANN | Regression=1 Error limit=±7.5 | 
                           
                                 | [40] | Electrochemical impedance spectroscopy | PR | R$^{2}$=0.99 | 
                           
                                 | [41] | Time series forecasting | RF | RMSE=19.90mg/dL | 
                        
                     
                   
                
             
            
                  5. Challenges and Future Works
               Researchers have focused on developing desktop models and larger equipment by integrating
                  IoT and ML to design a non-invasive model for detecting glucose. There is research
                  potential in designing a portable model. A non-invasive glucose-monitoring device
                  should provide real-time measurements irrespective of interference and noise. The
                  data collected from the device must channel toward a cloud or any database to store
                  the measurements, transfer them to healthcare providers, and generate alerts during
                  an emergency. The predictions from the data can also make future doctor visits much
                  easier to access and understandable and provide a better quality of treatment. The
                  predictions should be modeled to help the doctors go through the patient's history,
                  analyze the disease status with predictions, and provide better quality treatment.
                  Wearables proposed in [27, 32, 37, 41, 42] detect glucose levels and make predictions but do not generate alerts or enhancements.
                  Researchers can consider this as future work. Future work can be done on the ML algorithms
                  applied in [32-35, 37, 40-42] that can be replicated with the different DL algorithms and improve the accuracy
                  of the proposed models. Future work can also be done by considering more features,
                  i.e., heartbeat, blood oxygen level, and blood pressure, in the developed model. Correlation
                  analysis of the blood sugar with additional features can be taken as future work.
                  A wearable that can detect blood glucose levels by providing accurate real-time predictions
                  with built-in features of the heartbeat pattern, blood oxygen levels, and pulse rate
                  can improve healthcare. 
               
             
            
                  6. Conclusion
               This paper presented the integration of IoT and ML in diagnosing non-invasiveglucose
                  monitoring. This paper elaborated on the various approaches todetect glucose molecules
                  from blood non-invasively. Furthermore, this paper presents comprehensive research
                  activities on different ML algorithms that address the detection anddiagnosis of glucose
                  issues non-invasively. On the other hand, much potential for designing portable, detection,
                  predictions, alerts, and other enhancements with correlation analysis with additional
                  features can be addressed in future work.
               
             
          
         
            
            
                  
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            Author
            
            
               			Vijayakumar Ponnusamy received his Ph.D. from SRM IST in 2018. His area of research
               was Applied Machine Learning in Wireless Communication (cognitive radio). He completed
               his Masters in Applied Electronics from the College of Engineering, Guindy in 2006.
               In 2000, he received his B.E in Electronics and Communication Engineering from Madras
               University. He is currently working as a Professor in the ECE Department, SRM IST,
               Chennai, Tamil Nadu, India. He is a Certified “IoT specialist” and “Data scientist.“
               He is also a recipient of the NI India Academic award for excellence in research (2015).
               His current research interests are Machine and Deep learning, IoT-based intelligent
               system design, Blockchain technology, and cognitive radio networks. He has authored
               or co-authored more than 100 International journals and more than 65 International,
               National conferences. He is a senior member of IEEE.
               		
            
            
            
               			S. V. K. R. Rajeswari is a Research Scholar in SRMIST, Kattankulathur, Chennai-Tamil
               Nadu, India. She is pursuing her Ph.D. at the faculty of Electronics and Communication
               Engi-neering department. She received her M.Tech with Embedded Systems Technology
               as her specialization. and Bachelor of Technology from J.N.T.U.H, India. Her research
               interests include data science, machine learning algorithms, deep learning, artificial
               intelligence, IoT, and embedded systems.