Starch Nanoparticle Encapsulated Biocompatible Implantable Antenna Design for Smart and Safe Patient Monitoring at 2-45 GHz

Starch Nanoparticle Encapsulated Biocompatible Implantable Antenna Design for Smart and Safe Patient Monitoring at 2.45 GHz

Sanjana Chatterjee, Pratik Das, Piyali Basak School of Bioscience and Engineering (Biomedical Engineering)

Jadavpur University Kolkata 700032, India

Soham Ghosh, Bhaskar Gupta

Electronics and Telecommunication Engineering Jadavpur University

Kolkata 700032, India

Abstract Smart patient monitoring enhances requirement of implantable antenna for transferring the sensed health parameters wirelessly to external monitoring devices. Use of Starch-nanoparticle (St-NP) as a biocompatible encapsulation of implantable antenna instead of previously used materials (Teflon, MACOR etc.) provides several advantages like better biocompatibility and facility of drug delivery along with provision of wireless communication. In this work, St-NP layer is developed to encapsulate a 22X8 mm2 meander-line implantable antenna operating at 2.45 GHz ISM band. St-NP is produced by mixing Corn starch powder in de-ionized water along with using some techniques like ultrasonication, stirring and centrifugation. The characterization results show that the average size of the St- NP is less than 100 nm. A six-meanderline implantable antenna is designed within a homogeneous human body phantom which has return loss and realized gain of 15.87 dB and -23.7 dBi respectively. It is fabricated and encapsulated within gelatinized St-NP. The encapsulated antenna is measured within homogeneous body phantom developed utilizing DI water, DGME and NaCl. The fabricated antenna has return loss of

14.53 dB and realized gain of -25.69 dBi at 2.43 GHz. The use of St-NP as a biocompatible encapsulation of implantable antenna is the novel approach of this work.

Keywords Starch nanoparticle, biocompatible, implantable antenna, Patient monitoring, TEM

1. INTRODUCTIONAccording to the latest statistical analysis, about 30% of urban people and about 20% of rural people in India are suffering from chronic diseases such as cardiovascular disease, diabetes, hyper tension, etc. [1, 2]. In the modern era of health science, safe and continuous patient monitoring is one of the prime focus of engineers as well as doctors. The use of implantable antenna can increase the quality of smart patient monitoring. Implantable antenna embedded in Implantable Medical Device (IMD) can provide a facility of transferring sensed parameters (blood pressure, sugar content, hormonal imbalance etc.) wirelessly to the external monitoring devices [3-5]. The significant factors required for crafting implantable antenna are miniaturization of antenna, high lossy tissue layers, low input power restriction, Specific Absorption Rate (SAR) etc. [4-6]. Implantable antennas can be implemented in different medicalapplications such as glucose monitoring [4], intracranial pressure monitoring [7-8], cardiac telemetry [9] etc. In previous works, there are many investigations on shape, bandwidth, size, operation frequency, gain etc. as a novelty in the research regarding implantable antenna [10]. However, there are very limited works available related to the importance of biocompatibility for implantable antenna design, which is very important for patients safety.Biocompatibility of implantable antenna is important for preserving patients safety and preventing the rejection of implants [11]. Tissue layers of human body are conductive in nature. As the patch element and ground plane of antenna are also conductors, there may be a possibility of short circuit condition between human tissue and the implantable antenna while direct contact will happen. It can hamper as well as cause death to the patient [12]. Extensively used biocompatible materials for implantable antenna design are alumina (ceramic), Teflon (polymer), MACOR (machinable glass- ceramic), zirconia (ceramic), biomedical-grade base elastomer (polymer) etc. [9, 13-14]. In this work, a novel approach of biocompatible encapsulation of the implantable antenna using starch nanoparticle (St-NP) is proposed. Starch is a biocompatible, biodegradable, eco-friendly biopolymer with large number of glucose units connected with glycosidic bonds. Starch mainly contains amylopectin and amylase. It is highly compatible with human tissues due to similar biochemical components as the extra cellular matrix of human body [15]. Starch is widely used in different industrial applications such as drug delivery, pharmaceuticals etc. Nanoparticle takes leading role in drug delivery.In this manuscript, a starch nanoparticle based encapsulated implantable antenna operated at 2.45 GHz Industrial, Scientific and Medical band has been designed. St-NP is chosen here as a biocompatible encapsulation not only to ensure biocompatibility but also to provide a facility of controlled drug delivery to the patient and sensing continuous health parameters such as blood sugar, pressure level etc. Nanomaterial composes of atoms or molecules, whose size ranges from 1-100 nm [16]. Performing Dynamic light scattering (DLS), the average size of the starch solution is obtained as 62.36 nm which is within the range of being

   nanoparticle. Transmission Electron Microscope (TEM) is also carried out to verify the size as well as the distribution of nanoparticles. Then a meander line implantable antenna is designed and fabricated. It is placed into gelatinized St-NP and is kept at 10ºC for 2 days to solidify. The encapsulated antenna is measured within human body phantom.

2. ST-NP GENERATION AND CHARACTERIZATION
   1. MaterialsCornstarch powder is obtained from Foodfrillz, De-ionized (DI) water is from IC centre, Jadavpur University, and 90% Ethanol obtained from Merck India Pvt Ltd.
   2. Synthesis of Corn Starch NanoparticleCorn Starch powder (5% w/v) is mixed with DI water. It is gelatinized at 85 ºC, 800 rpm for 30 minutes with Remi 2MLH magnetic stirrer. Gelatinized starch paste is ultrasonicated using Labman Probe sonicator for 5 to 25 minutes and lastly starch nanoparticle (St-NP) is produced under constant stirring within 90% ethanol at 1:1 ratio. An aliquot of colloidal suspension is diluted with distilled water after thorough mixing for characterization.
   3. Characterization
      1. Dynamic LightScattering (DLS): Dynamic Light Scattering (DLS) is performed using Malvern Zetasizer Nano-ZS, Malvern Instruments, Life Science and Biotechnology Department, Jadavpur University to estimate average hydrodynamic diameters and polydispersity index (PDI) of the 1 mL obtained 5% w/v from St-NPs stock solution at 25º C. The intensity (%) vs size (nm) of this sample is illustrated in Fig. 1. From this plot, it is observed that the average particle size is 62.36 nm which is justifying this sample as a Nanomaterial. The PDI value obtained from this measurement is 0.281 which is sufficiently low. As the PDI is nearly 0, it is concluded that the sample is monodispersed.
         

         10

         Intensity (%)

          

         8

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         0 100 200 300 400

         Size (nm)

         Fig. 1: DLS particle size for St-NP

         Fig. 2: TEM images indicating size and distribution of St-NP

      2. Transmission Electron Microscope: The Transmission Electron Microscope (TEM) is performed to understand the shape, morphology and size of St-NP. It is performed in Central Facility of Jadavpur University, India. Diluted St-NP solution is casted on copper-grid. Next, the grd is dried for48 h before recording TEM image. The TEM snapshots (Fig. 2) suggest that the St-NP sizes are between 13.4159.69 nm. As DLS indicates the hydrodynamic size of St-NP, therefore the size of St-NP obtained in DLS is larger than TEM. The distribution of the NPs is also observed from Fig. 2 (a) and (b).Fig. 3: (a) Equivalent human body model design and (b) its dimensions
3. IMPLANTABLE ANTENNA DESIGN
   1. 1. 1. Human Body ModelHuman body has multiple numbers of layers. It is very difficult to replicate the multilayer human body model practically for in- vitro measurement of implantable antenna due to possible formation of air gaps between two layers, chemical reactions between two layers etc. while placing one layer on other. Therefore, it is recommended to use homogeneous human body model for testing [17].Therefore, it is very important to choose the dielectric properties of homogeneous equivalent human body model because the impedance and radiation characteristics of the antenna are highly dependent on the dielectric properties in terms of relative permittivity and conductivity of its environment. According to some standard guidelines [18-22], the relative permittivity, i.e., rt, and conductivity, i.e. t, are taken as 52.7 and 1.95 S/m at 2.45 GHz ISM band. The schematic view of human body model utilized to simulate a biocompatible St-NP encapsulated meander-line implantable antenna in CST Microwave Studio 2021 is shown in Fig. 3.Measurement Setup 

            Dielectric probe

            Solution of Starch Nanomaterial

            Fig. 4: Dielectric Measurement of the solution containing St-NP

         2. Biocompatible St-NP layer ModelingTo design an encapsulation in St-NP of the implantable antenna, it is important to test its dielectric properties practically. Therefore, the dielectric properties of the solution containing St-NPs are measured using 85070E dielectric measurement kit (based on open ended coaxial probe technique) as shown in Fig. 4. The measured relative permittivity and conductivity of starch solution are 2.74 and0.051 S/m. Then, the starch solution is magnetically rotated at 30oC for 1.5 hr to make a gel-like structure.
         3. Implantable Antenna ModelingA meander-line implantable antenna is designed here to operate at 2.45 GHz as shown in Fig. 5. Here a low loss dielectric substrate, Arlon AD 430 (relative permittivity of 4.3, loss tangent of 0.003 and thickness of 30 mil), is used for antenna designing. Fig. 5 (a) and (b) are showing the top and bottom views of the antenna respectively and corresponding design parameters are tabulated in Table I. The antenna isTABLE I. Antenna Design Parameters
            

            Parameters	Values (mm)	Parameters	Values (mm)
            Ground Length (Lground)	22	Ground Width (Wground)	8
            Substrate Length (Lsubstrate)	22	Substrate Width (Wsubstrate)	8
            Patch Length (Lpatch)	8.5	Patch Width (Wpatch)	3.5
            Location of the feed point	(7.2, 2.2)	Thickness of Meander- Line stub (x)	0.5

         4. Design Evolution of Antenna Part

   Use of meander-line concept to design miniaturized antenna is a popular practice. As per previous discussion, the size of the implantable antenna should be very small for proper placement within patients body. Therefore, a meander-line implantable antenna is designed here as shown in Fig. 5. The design evolution is discussed here. From rigorous literature survey [23-25], it is observed that the typical foot-print area of implantable antennas is less than 200 mm2. Here 22 mm X 8 mm antenna is designed with area of 176 mm2. A simple patch with 8.5 mm X 3.5 mm (Fig. 6(a)) is considered first including St-NP layer within homogeneous human body model where no cuts are present. From the return loss plot of Step 1 in Fig. 6(b), it is clear that this antenna is not radiating at 2.45 GHz.

   encapsulated by 1 mm thick biocompatible St-NP layer as shown in Fig. 5 (c).

   (a)

   (1)

   (3)

   (5)

   (7)

   (9) (11)

   (b)

   0

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   S11 (dB) for all steps

    

   4

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   Wsub

    

   (a)

   Lsub

   x

    

   Wpatch

    

   Lpatch

   (2)

   (4)

   (6)

   (8) (10) (12)

   12

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   Step 2

   Step 3

   (b)

   (c)

   Starch Nanoparticle

   1.0 1.5 2.0 2.5 3.0 3.5

   y Frequency (GHz)

   Fig. 6: (a) Design evolution and (b) corresponding return loss plots

   Coaxial Feed

    

   Lground

    

   x In the next step (Step 2), two cuts (cut no. 1 and 2) are incorporated in patch to make one meander structure. Corresponding return loss plot is plotted in Step 2 of Fig.

   Wground

    

   6(b). The S11 of this antenna is -18.32 dB at 2.77 GHz. The y addition of more meanders on the patch can increase the effective current path length and effective wavelength which

   1. leads to decrease the operating frequency of the antenna. In this context, 12 cuts are incorporated to design 6 meanders on patchEncapsulationAntenna 

      as depicted in Fig. 6 (a). The return loss plot of this structure symbolized as Step 3 in Fig. 6 (b) is shown that this antenna is operating at 2.44 GHz with 15.87 dB return loss value. This frequency is within 2.45 GHz ISM band (2.4-2.48 GHz).

   2. Therefore, this St-NP encapsulated antenna is finalized to propose as novel St-NP encapsulated implantable antenna.

   x The encapsulated final antenna has fractional bandwidth of

   Fig. 5: (a) Top view, (b) Bottom view and (c) side view of the antenna

   6.75 % with -23.5 dBi realized gain. Based on std. guidelines

   [26-27], Specific Absorption Rate (SAR), safety measure of implantable antenna, should be <1.6 and <2 Watt/Kg respectively for 1g and 10g cubical body model. 345 Watt/Kg and 62.58 Watt/Kg simulated peak SARs are obtained for 1W RF input power respectively for 1g and 10g. The simulated Co- and Cross- Polar radiation plots are shown in Fig. 7. The

   simulated 3-D SAR distributions of implantable antenna are illustrated in Fig. 8.

   Co Polar Cross Polar

Co Polar Cross Polar

(a) (b)

Fig. 7: Co- and Cross-polar simulated radiation configurations of St-NP encapsulated antenna at 2.44 GHz for (a) E and (b) H-planes

1. FABRICATION AND MEASUREMENTFinalized implantable antenna has been practically fabricated and encapsulated within gelatinized St-NP. The simulated architecture is practically fabricated using photo-lithography technology. Fabricated prototype of the antenna is shown in Fig. 9 (a-b). The fabricated antenna is then placed within gelatinized St-NP solution (refer to Fig. 9 (c)) prepared in Section II (B). Then the antenna engulfed St-NP gelatinized model, is kept at15ºC for 3 days to make it solid. The different views of encapsulated antenna are shown in Fig. 9 (d).For practical in-vitro measurement of simulated antenna responses, homogeneous body equivalent model is fabricated by mixing 71% DI water, 29% DGME and 0.58 g NaCl. The measured respective relative permittivity and conductivity values are observed as 52.48 and 1.92 S/m respectively. Final antenna is placed within this equivalent body model as shown in Fig. 10 (a). The measured return loss plot is presented and compared with simulated response in Fig. 10 (b). From this plot, it is shown that the measured antenna 14.53 dB at 2.43 GHz which is almos similar to the simulated return loss plot. The measured realized gain of the antenna is -25.69 dBi at 2.43 GHz.Fig. 8: Simulated SAR distribution for (a) 1g and (b) 10g model at 2.45 GHz for 1W applied powerFig. 9: (a) Top view and (b) bottom view of the fabricated antenna, (c) gelatinized St-NP and (d) Top and bottom views of St-NP encapsulated Implantable antenna

   Simulated Measured

    

   Antenna inside homogeneous phantom

    

   0

   2

   4

   S11 (dB)

    

   6

   8

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   12

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   1.0 1.5 2.0 2.5 3.0 3.5

   Frequency (GHz)

   (a) (b)

   Fig. 10: (a) In-Vitro measurement of St-NP encapsulated implantable antenna within homogeneous equivalent human body model, (b) return loss

   plots from simulation and measurement

2. NOVELTY OF THIS WORKBiocompatibility is one of the main design considerations for implantable antenna for ensuring patients safety, avoiding radiation to nearby tissues, isolating the human tissues from conductors of the antenna to overcome short circuit possibility between them. Several biocompatible materials were used previously [9, 13-14, 28-29] such as Teflon, zirconica, MACOR, alumina etc. There is no reported work where nanomaterial based encapsulation is used having facility of drug loading for controlled drug delivery along with the facility of sensing and transfer of sensed health parameters of the patient for smart patient monitoring. The designed St-NP based implantable antenna provides many advantages over previously reported antennas such as – (i) biocompatible encapsulation, (ii) controlled drug delivery, (iii) patients health parameters sensing and wirelessly transfer the sensed data to external monitoring devices etc. The designed antenna also has novelty over some reported works which is discussed in Table II.TABLE II. Comparison with Previously Reported Works
   

   Ref. of work	Realized Gain (dBi)	1g 1W Max SAR (W/Kg)	10g 1W Max SAR (W/Kg)
   [30]	-22.8	807.34	102.04
   [31]	-22.37	759.72	87.24
   [32]	-46	338	–
   This design	-23.5	345	62.58

3. CONCLUSION

A meanderline implantable antenna encapsulated by corn starch based nanomaterial layer is designed in this work to operate at 2.45 GHz ISM band. Starch is a well-known biocompatible material. The St-NP is synthesized here by using Corn starch powder, DI water and 90% Ethanol. Several processes like ultrasonication and centrifugation are performed to develop solution containing St-NP; DLS and TEM are used

to characterize the solution of St-NP in terms of size, PDI, distribution of particles etc. According to DLS and TEM, it is observed that the particle size of this solution is below 100 nm which concludes it as nanoparticles. The dielectric properties of St-NP are measured in terms of relative permittivity and conductivity using 85070E dielectric measurement kit. A meander-line implantable antenna is designed and fabricated. It is placed within gelatinized St-NP solution and kept in refrigerator for solidification. Finally, St-NP encapsulated antenna is measured within a homogeneous human body phantom produced by mixing DI water, DGME and NaCl. The simulated antenna has S11 and realized gain of -15.87 dB and –

23.5 dBi respectively at 2.44 GHz. The measured respective S11 and realized gain of biocompatible St-NP encapsulated antenna within developed phantom are -14.53 dB and -25.69 dBi at 2.43 GHz (measured operating frequency). The use of Starch nanoparticles as biocompatible encapsulation of implantable antenna is the novel approach of this work for Electronic Drug Delivery by sensing the target place to deliver through implantable antenna.

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