Baets, Roel

Bogaerts, Wim

Delbeke, Danaë

Hattasan, Nannicha

Morren, Geert

Ryckeboer, Eva

English

Ghent University Academic Bibliography

Power budget considerations for in vivo continuousglucose monitoring using absorption spectroscopyE. Ryckeboer,1 N. Hatassan,1 G. Morren,1 D. Delbeke,1 W. Bogaerts,1 and R. Baets11 IMEC-Ghent University, Photonics Research Group, Sint-Pietersnieuwstraat 41, 9000 Ghent,BelgiumDiabetes is a serious health condition that can be treated more efficiently when the bloodglucose level is monitored continuously. We study an implantable photonic chip thatdetermines the glucose concentration by means of near infrared spectroscopy in the firstovertone and combination band. In human tissue the glucose absorption spectrum is aweak variation on the water spectrum and light scattering is prevalent. Both conditionsnecessitate a high signal-to-noise ratio measurement. Power budget calculations of apost-dispersive integrated system elucidate how the low e´tendue of single mode opticalwaveguides in combination with strong water interference fail to realize such a high SNR.Unless adaptive optics is employed to compensate for wave front aberrations due to lightscattering, laser spectroscopy is the preferred alternative.IntroductionDiabetes mellitus is an auto-immune disease that deteriorates the insulin production ofthe pancreas. An elevation of the blood glucose level is a direct consequence (hypergly-caemia). The world health organization estimates that the number of diabetes patientsworldwide accounts to 220 million people [1]. As diabetes is a chronic disease the costfor health care is enormous. These medical care costs can be reduced substantially bycontinuous blood glucose level control. The ideal implementation is a closed-loop con-trol system with a glucose sensor and insulin pump to keep the blood glucose level in thehealthy range of 60-110 mg/dL. Huge investments are made in research towards this goal.For clinical usefulness the glucose concentration needs to be detected with an accuracyof 100 mg/L. Apart from the sensor’s accuracy, the sensor lifetime is a key parameter.Optical methods offer the largest chance on a long sensor lifetime as there is no need forreagentia whose effect diminishes over time. Still, methods using reagentia have a highspecificity to glucose. Assisted by multivariate calibration strategies, optical methods likenear-infrared and Raman spectroscopy also offer good specificity to glucose thanks to theuniqueness of the absorption and Raman spectrum. In the SBO-IWT GlucoSens projectwe aim at developing an implantable glucose sensor based on near-infrared absorptionspectroscopy.Glucose absorption featuresThe near-infrared (NIR) glucose absorption spectrum can be divided into two large re-gions. The first region is called the first overtone band (1550-1800 nm) and the secondrange is the combination band (2050-2400 nm). Figure 1 shows the different spectra.(a) First overtone band (b) Combination bandFigure 1: First overtone band(a) and combination band(b) absorption spectrum of glu-cose. The blue curve shows the glucose absorption during a hyperglycaemic state and thered curve during a hypoglycaemic state (overdose of insulin).In the human body the glucose absorption is 3 orders of magnitude smaller than the waterabsorption. It implies that in a spectroscopic measurement the glucose spectrum appearsas a weak variation on the water spectrum. The strong water absorption also limits thepath length of the measurement to 5 mm and 1 mm in the first overtone and combinationband respectively [2]. Apart from water absorption, the in vivo detection of glucose ishindered by the scattering nature of tissue. The resulting distribution of photon trajecto-ries and according path lengths make it difficult to relate the absorption spectrum to theglucose concentration.Layout of the implant:Our device will consist of an on-chip spectrometer embedded in a biocompatible materiallike PMMA or PDMS with a total size of about 1 cm2 x 0.3 mm. The spectrometer isintegrated on the silicon-on-insulator (SOI) platform. This platform is CMOS-compatibleallowing for future cheap mass fabrication. An electronic chip, 3D integrated with thephotonic SOI chip, will drive the source and detectors. An artist impression of such adevice is shown in figure 2.Figure 2: Possible device layoutThe device can work in two distinctive ways: either a post-dispersive approach is imple-mented in which broadband light is coupled to the tissue and afterwards the transmittedlight is demultiplexed by a planar concave grating (PCG) [3] or in the pre-dispersiveset-up we couple different wavelengths sequentially to the tissue and measure the trans-mission of 1 wavelength at a time. In the latter case we can use a broadband source whichwe demultiplex or we can use an array of laser with each a fixed emission wavelength.Power budget calculation:The choice between the different device layouts depends largely on the power budget. Wecompare the pre and post-dispersive set-up in the case we have a broadband light sourcelike a superluminescent light emitting diode (SLED).Figure 3: (a) Pre-dispersive layout with SLED and (b) a table showing the different lossfactors for a realistic implementation.For the pre-dispersive set-up as shown in figure 3 we can calculate the necessary totalinput power Pin [dBm] of the SLED using the following formula:Pin > 10log(3NEP)+dT +Lmedium +GCs +10log(Nchan)+Lpcg +LSLED (1)in which dT is the change in optical transmission due to 100 mg/L of glucose, Lmediumis the loss in the medium due to absorption and scattering, GCs is the loss of the gratingcoupler for narrowband light, Nchan is the number of channels, Lpcg is the loss of thedemultiplexer, LSLED is the loss for coupling the SLED light to a waveguide. The detectorsensitivity is characterized by the noise equivalent power (NEP) which is defined as theinput power Ps for which the signal-to-noise ratio (SNR) equals 1. A general expressionfor the SNR of a photodiode is [4]:SN=ηP2s2hνB[Ps + hνiDηe +2(hνe )(kTNe )(1ηRload )] (2)in which h is Planck’s constant, k is Boltzmann’s constant, ν is the light frequency, B = 12tis the measurement bandwidth with t the the integration time, TN is the effective noisetemperature, η is the quantum efficiency of the detector, e is the elemenatary charge, iDis the dark current and Rload is the load resistance.Similarly we can write down the needed power for a post-dispersive set-up:Pin > 10log(3NEP)+dT +Lmedium +F +GCb +10log(Nchan)+Lpcg +LSLED (3)in which GCb is the loss in the grating coupler for broadband light and F is a factorrepresenting the fraction of the transmitted light that couples to the fundamental modeof the input waveguide. This fraction is always smaller than 1 when scattering alters thewavefront. The inherent low e´tendue (characterized by F) of a post-dispersive systemfor highly scattering media like tissue suggest that the pre-dispersive set-up should bepreferred. We estimate F to be minimally 20 dB for a tissue-like environment, but furthermeasurements are necessary to set a definite value. For non-scattering media however, aspecial situation exist in which the post-dispersive set-up with a broadband light sourcecan compensate for the lower e´tendue by using very small integrated detectors (20 um x20 um). When the detector becomes dark-current-limited (Rload > 10MΩ) decreasing thedetector area results in a decrease of the NEP and the post-dispersive set-up can becomebeneficial compared to the pre-dispersive set-up with a large area detector.When we calculate the needed SLED power with realistic values of Rload = 1MΩ, F=20dB and the loss values as found in figure 3, we obtain 13.58 mW for the pre-dispersiveset-up and 461 mW for the post-dispersive set-up in the first overtone band. This showsthat the necessary SNR cannot be realized using an integrated SLED. Instead of using abroadband light source we can implement a laser spectroscopy system to boost the signal-to-noise ratio. We envisage a laser array heterogeneously integrated on SOI with 30 lasersspaced 3 nm apart. The efficient light generation in lasers in combination with the ease ofdirect modulation offer clear advantages. Using the formula Pin > 10log(3NEP)+dT +Lmedium +GCs we obtain an input power of 60 µW per laser for the first overtone band.Small microdisk lasers with a diameter of 7.5 um heterogeneously integrated on SOI canemit up to 120 µW [5] showing that this input power poses no problems. Furthermore thepower of the individual lasers can be adjusted in accordance with the water absorption.Conclusions:The stringent power budget requirements for a glucose detection scheme using absorptionspectroscopy show that a broadband source like a SLED is not suitable for integration ofa glucose sensor on SOI. An alternative measurement set-up using a heterogeneouslyintegrated laser array is proposed.Acknowledgements:The authors thank the SBO-IWT for the financial support of the GlucoSens project.References[1] World Health Organization, Diabetes Program on http://www.who.int/diabetes/en/ accessed on 30october 2010.[2] M.A. Arnold and G.W. Small, ”Noninvasive glucose sensing”, Analytical Chemistry, vol. 77, pp.5429-5439, 2005.[3] J. Brouckaert et al, ”Planar concave grating demultiplexer fabricated on a nanophotonic silicon-on-insulator platform”, J. of Lightwave Tech., vol. 25, pp. 1269, 2007.[4] R.H. Kingston, ”Noise and efficiency of semiconductor devices”, in Detection of optical and infraredradiation , Berlin and New York, Springer-Verlag (Springer Series in Optical Sciences. Volume 10),1978, pp. 70.[5] T. Spuesens, L. Liu, T. De Vries, P.R. Romeo, P. Regreny, D. Van Thourhout, ”Improved design of anInP-based microdisk laser heterogeneously integrated with SOI”, in Group IV Photonics, 6th IEEEInternational Conference on., 2009, pp. 202-204.

Power budget considerations for in vivo continuous glucose monitoring using absorption spectroscopy

https://biblio.ugent.be/publication/1155465/file/1155473.pdf

Power budget considerations for in vivo continuous glucose monitoring using absorption spectroscopy

Abstract

Similar works

Full text

Available Versions

Ghent University Academic Bibliography