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Energy scavenging has increasingly become an interesting option for powering electronic devices because of the almost infinite lifetime and the non-dependence on fuels for energy generation. Moreover, the rise of wireless technologies promises new applications in medical monitoring systems, but these still face limitations due to battery lifetime and size. A trade-off of these two factors has typically governed the size, useful life and capabilities of an autonomous system. Energy generation from sources such as motion, light and temperature gradients has been established as commercially viable alternatives to batteries for human-powered flashlights, solar calculators, radio receivers and thermal-powered wristwatches, among others. Research on energy harvesting from human activities has also addressed the feasibility of powering wearable or implantable systems. Biomedical sensors can take advantage of human-based activities as the energy source for energy scavengers. This review describes the state of the art of energy scavenging technologies for powering sensors and instrumentation of physiological variables. After a short description of the human power and the energy generation limits, the different transduction mechanisms, recent developments and challenges faced are reviewed and discussed.

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A new physiologic monitor for use in the home has been developed and used for the Collaborative Home Infant Monitor Evaluation (CHIME). This monitor measures infant breathing by respiratory inductance plethysmography and transthoracic impedance; infant electrocardiogram, heart rate and R-R interval; haemoglobin O2 saturation of arterial blood at the periphery and sleep position. Monitor signals from a representative sample of 24 subjects from the CHIME database were of sufficient quality to be clinically interpreted 91.7% of the time for the respiratory inductance plethysmograph, 100% for the ECG, 99.7% for the heart rate and 87% for the 16 subjects of the 24 who used the pulse oximeter. The monitor detected breaths with a sensitivity of 96% and a specificity of 65% compared to human scorers. It detected all clinically significant bradycardias but identified an additional 737 events where a human scorer did not detect bradycardia. The monitor was considered to be superior to conventional monitors and, therefore, suitable for the successful conduct of the CHIME study.

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An electronic simulator of physiologic signals used in infant monitoring has been designed, constructed and applied in the Collaborative Home Infant Monitor Evaluation (CHIME). A unique feature of the simulator is that it contains actual physiologic waveforms recorded from infants rather than artificial, idealized signals. The simulator stores breathing waveforms that can be used to test transthoracic-impedance- and inductance-plethysmography-based monitors, and heart rate channels are tested by playing a neonatal QRS complex at preset fixed rates or a variable rate as determined from infant recordings. The transfer characteristics of the simulator are constant over frequencies ranging from 0.5 to 8 Hz for the respiration channels. Data stored in memory are divided into 60 second epochs that can be presented to the monitor being tested in a programmable sequence. A group of 66 CHIME monitors was tested using a simulator programmed with 17 apnoea and bradycardia waveforms. The agreement between monitors as to the duration of detected apnoea decreases as the amount of artefact in the signal increases. Discrepancies between monitors in detecting apnoea duration were found to be similar to inconsistencies between CHIME investigators manually scoring similar waveforms.

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CONTEXT: Home monitors designed to identify cardiorespiratory events are frequently used in infants at increased risk for sudden infant death syndrome (SIDS), but the efficacy of such devices for this use is unproven. OBJECTIVE: To test the hypothesis that preterm infants, siblings of infants who died of SIDS, and infants who have experienced an idiopathic, apparent life-threatening event have a greater risk of cardiorespiratory events than healthy term infants. DESIGN: Longitudinal cohort study conducted from May 1994 through February 1998. SETTING: Five metropolitan medical centers in the United States. PARTICIPANTS: A total of 1079 infants (classified as healthy term infants and 6 groups of those at risk for SIDS) who, during the first 6 months after birth, were observed with home cardiorespiratory monitors using respiratory inductance plethysmography to detect apnea and obstructed breathing. MAIN OUTCOME MEASURES: Occurrence of cardiorespiratory events that exceeded predefined conventional and extreme thresholds as recorded by the monitors. RESULTS: During 718 358 hours of home monitoring, 6993 events exceeding conventional alarm thresholds occurred in 445 infants (41%). Of these, 653 were extreme events in 116 infants (10%), and of those events with apnea, 70% included at least 3 obstructed breaths. The frequency of at least 1 extreme event was similar in term infants in all groups, but preterm infants were at increased risk of extreme events until 43 weeks' postconceptional age. CONCLUSIONS: In this study, conventional events are quite common, even in healthy term infants. Extreme events were common only in preterm infants, and their timing suggests that they are not likely to be immediate precursors to SIDS. The high frequency of obstructed breathing in study participants would likely preclude detection of many events by conventional techniques. These data should be important for designing future monitors and determining if an infant is likely to be at risk for a cardiorespiratory event.

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The analytical performance of amperometric microcells with different electrode geometries is compared for enzyme activity measurements. The microcells were fabricated with thin film photolithography or thick film screen-printing in four different designs. The cells made with the thin film process used flexible substrate with microelectrode array or a circular, disk-shaped working electrode. The screen-printed working electrodes had semicircle or disk shape on ceramic chips. Putrescine oxidase (PUO) activity measurement was used as a model. The determination of PUO activity is important in the clinical diagnosis of premature rupture of the amniotic membrane. An electropolymerized m-phenylenediamine size-exclusion layer was used to eliminate common interferences. The size exclusion layer revealed also to be advantageous in protecting the electrodes from fouling by putrescine (enzyme substrate). The electrode fouling of bare electrodes was insignificant for screen-printed electrodes, but very severe for electroplated platinum working electrodes. The microelectrode array electrodes demonstrated smaller RSD and higher normalized sensitivities for hydrogen peroxide and PUO activity. All the other electrodes were demonstrating comparable analytical performances.

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As part of the Collaborative Home Infant Monitoring Evaluation (CHIME) we compared apnea identified by a customized home monitor using respiratory inductance plethysmography (RIP) with simultaneously recorded polysomnography-acquired nasal end-tidal CO(2) (PET(CO(2))) and nasal/oral thermistor in 422 infants during overnight laboratory recordings to determine concordance between techniques, sources of disagreement, and capacity of RIP to detect obstructed breaths within an apnea. Among 233 episodes of apnea identified by at least one method as >/= 16 s, 120 were observed by the CHIME monitor, 219 by PET(CO(2)), and 163 by thermistor. The positive predictive value of the CHIME-identified apnea was 89.2% (95% CI 83, 95) and 73% (95% CI 65, 81) for PET(CO(2)) and thermistor, respectively. However, the sensitivity of the CHIME monitor in identifying events detected by the other methods was only approximately 50%. Among 87 apnea events identified by all three techniques, no two methods showed high agreement in measurement of apnea duration: RIP and PET(CO(2)) (ICC = 0.54), RIP and thermistor (ICC = 0.13), PET(CO(2)) and nasal thermistor (ICC = 0.41). Among the 179 breaths identified by RIP as obstructed, 79.9% were judged to be obstructed on the PET(CO(2)) and 80.4% were judged to be obstructed on the thermistor channel. Among 238 breaths identified on PET(CO(2)) as obstructed, 54.2% were determined to be obstructed by RIP. Among 204 breaths identified on thermistor as obstructed, 55. 4% were determined to be obstructed by RIP. Reasons for discrepancies in apnea detection among channels included body movement, partial airway obstruction, and obstructed breaths. Despite these limitations the CHIME monitor provides an opportunity to record physiological data previously unavailable in the home.

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A microfabricated amperometric microcell was designed and used for the determination of proline iminopeptidase (PIP) enzyme activity in 2-10-microl samples. The measurements were made in the range of 10.3-841.5 mU/ml enzyme activities. The sensitivity of the determinations was between - 0.0195 and - 0.0203 microA ml/mU per min. The coefficient of variation of the determined values ranged between 2.8 (at 561.2 mU/ml) and 24.1% (at 10.3 mU/ml). The microcell was manufactured on an alumina substrate using screen-printed graphite working and Ag/AgCl reference electrodes. Elevated PIP activity in the vaginal fluid is a biochemical indicator of bacterial vaginosis. The method is appropriate to differentiate between normal (66+/-145 mU/ml) and elevated, diseased (704+/-145 mU/ml), values.

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Limitations in home monitoring technology have precluded longitudinal studies of hemoglobin oxygen saturation during unperturbed sleep. The memory monitor used in the Collaborative Home Infant Monitoring Evaluation addresses these limitations. We studied 64 healthy term infants at 2 to 25 weeks of age. We analyzed hemoglobin oxygen saturation by pulse oximetry (SpO(2)), respiratory inductance plethysmography, heart rate, and sleep position during 35, 127 epochs automatically recorded during the first 3 minutes of each hour. For each epoch baseline SpO(2) was determined during >/=10 s of quiet breathing. Acute decreases of at least 10 saturation points and <90% for >/=5 s were identified, and the lowest SpO(2) was noted. The median baseline SpO(2) was 97.9% and did not change with age or sleep position. The baseline SpO(2) was <90% in at least 1 epoch in 59% of infants and in 0.51% of all epochs. Acute decreases in SpO(2) occurred in 59% of infants; among these, the median number of episodes was 4. The median lowest SpO(2) during an acute decrease was 83% (10th, 90th percentiles 78%, 87%); 79% of acute decreases were associated with periodic breathing, and >/=16% were associated with isolated apnea. With the use of multivariate analyses, the odds of having an acute decrease increased as the number of epochs with periodic breathing increased, and they lessened significantly with age. We conclude that healthy infants generally have baseline SpO(2) levels >95%. The transient acute decreases are correlated with younger age, periodic breathing, and apnea and appear to be part of normal breathing and oxygenation behavior.

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In looking at fetal and neonatal measurement technology as covered in this issue and as reported in areas not covered in this issue, we need to ask the question: Has this technology made a difference? Unfortunately, the answer is not clear. There are cases where the answer is a solid "yes" and other cases where "maybe" is the best we can say at the present time. There are also a few examples where even though noble attempts have been made, the answer must be "no." What does this mean in terms of what needs to be done? We certainly need to continue the development of new measurement technology and to carry out this development based on sound physiologic and engineering principles. We need to understand better the physical and biologic basis of the measurements that we make and to perform carefully controlled clinical trials of technology before bringing it to the marketplace. Data presentation and archiving is an important issue that, although it is being addressed today, still has future implications as the amount of data increase. Finally, we must not forget the importance of our ultimate goal of helping families to have healthy, happy offspring.

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There are numerous reports of cardiorespiratory patterns in infants on home monitors, but no data to determine whether "experts" agree on the description of these patterns. Therefore, we evaluated agreement among four experienced investigators and five trained technicians who assessed independently the same sample of physiologic waveforms recorded from infants enrolled in a multicenter study. The monitor used respiratory inductance plethysmography and recorded waveforms for apnea > or = 16 s or a heart rate < 80 beats/min for > or = 5 s. The investigators and technicians initially assessed 88 waveforms. After additional training, the technicians assessed another 113 additional waveforms. In categorizing waveforms as apnea present or absent, agreement among technicians improved considerably with additional training (kappa 0.65 to 0.85). For categorizing waveforms as having bradycardia present versus absent, the trends were the same. Agreement in measurement of apnea duration also improved considerably with additional training (intraclass correlation 0.33-0.83). Agreement in measurement of bradycardia duration was consistently excellent (intraclass correlation 0.86-0.99). Total agreement was achieved among technicians with additional training for measurement of the lowest heart rate during a bradycardia. When classifying apnea as including > or = 1, > or = 2, > or = 3, or > or = 4 out-of-phase breaths, agreement was initially low, but after additional training it improved, especially in categorization of apneas with > or = 3 or > or = 4 out-of-phase breaths (kappa 0.67 and 0.94, respectively). Although researchers and clinicians commonly describe events based on cardiorespiratory recordings, agreement amongst experienced individuals may be poor, which can confound interpretation. With clear guidelines and sufficient training raters can attain a high level of agreement in describing cardiorespiratory events.

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We used in vivo magnetic resonance (MR) microscopy to follow the growth of fibrous capsule as a foreign body reaction to silicone implants in rats. Anesthetized rats were imaged 1, 7, 14, and 28 days after silicone-coated MR imaging coils were sutured to their neck muscles. On the twenty-eighth day, rats were sacrificed and coils and adjacent tissues were removed en bloc and fixed in formalin, reimaged with MR, and sectioned for conventional histology. Three-dimensional (3-D) spin-echo [3DFT] acquisition gave in-plane resolution of 32 x 32 microns in vivo and 16 x 16 microns ex vivo. All MR images showed a diffuse band of elevated signal intensity between the silicone of the coil and adjacent tissue. The border of the hyperintense band was thin and not well defined at seven days post-implantation. From 7-28 days, the band showed relatively homogeneous signal intensity and its thickness increased 44% on the rectus muscle side and 78% on the subcutaneous side. The capsule thickness determined either by MR in vivo and ex vivo microscopy or conventional histology was not significantly different, and there was a significant correlation between thickness measurements among those methods. MR in vivo microscopy provides sufficient resolution and spatial information to serially evaluate the growth of the foreign body fibrous capsule over time, thus achieving greater accuracy and consistency in measurements.

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A microfabricated, flat form, amperometric microcell (microchip) is used in a simple, two-electrode arrangement for putrescine oxidase enzyme activity determinations. The cell contains a platinum microdisk working electrode and an Ag/AgCl reference electrode covered by a porous, hydrophilic membrane. An electrochemically-prepared size-exclusion layer is applied on the working electrode surface, to avoid the effect of electroactive interferences in the sample. The hydrophilic membrane, resting on the bottom of the cell, is soaked with a small volume of buffered substrate solution and a few mul enzyme containing sample solution is dispensed over the electrodes. During the enzyme activity measurement a catalytic reaction takes place in the membrane-supported liquid film over the working electrode surface. The hydrogen peroxide produced in the reaction is detected amperometrically. The amperometric current-time curves are used for evaluation. In our work putrescine was used as a substrate to determine the unknown putrescine oxidase enzyme activity of the sample. Elevated diamine oxidase enzyme activity in the vaginal milieu can indicate premature rupture of the amniotic membrane. Results with membrane discs, containing all the necessary chemicals in solid or lyophilized form, are very encouraging with respect of a single use, 'reagentless' biosensor for home care.

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Infant polysomnography (IPSG) is an increasingly important procedure for studying infants with sleep and breathing disorders. Since analyses of these IPSG data are subjective, an equally important issue is the reliability or strength of agreement among scorers (especially among experienced clinicians) of sleep parameters (SP) and sleep states (SS). One basic issue of this problem was examined by proposing and testing the hypothesis that infant SP and SS ratings can be reliably scored at substantial levels of agreement, that is, kappa (kappa) > or = 0.61. In light of the importance of IPSG reliability in the collaborative home infant monitoring evaluation (CHIME) study, a reliability training and evaluation process was developed and implemented. The bases for training on SP and SS scoring were CHIME criteria that were modifications and supplements to Anders, Emde, and Parmelee (10). The kappa statistic was adopted as the method for evaluating reliability between and among scorers. Scorers were three experienced investigators and four trainees. Inter- and intrarater reliabilities for SP codes and SSs were calculated for 408 randomly selected 30-second epochs of nocturnal IPSG recorded at five CHIME clinical sites from healthy full term (n = 5), preterm (n = 4), apnea of infancy (n = 2), and siblings of the sudden infant death syndrome (SIDS) (n = 4) enrolled subjects. Infant PSG data set 1 was scored by both experienced investigators and trained scorers and was used to assess initial interrater reliability. Infant PSG data set 2 was scored twice by the trained scorers and was used to reassess inter-rater reliability and to assess intrarater reliability. The kappa s for SS ranged from 0.45 to 0.58 for data set 1 and represented a moderate level of agreement. Therefore, rater disagreements were reviewed, and the scoring criteria were modified to clarify ambiguities. The kappa s and confidence intervals (CIs) computed for data set 2 yielded substantial inter-rater and intrarater agreements for the four trained scorers; for SS, the kappa = 0.68 and for SP the kappa s ranged from 0.62 to 0.76. Acceptance of the hypothesis supports the conclusion that the IPSG is a reliable source of clinical and research data when supported by significant kappa s and CIs. Reliability can be maximized with strictly detailed scoring guidelines and training.

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An amperometric diamine sensor is developed for clinical applications in diagnosis of bacterial vaginosis (BV). The sensor is based on crosslinked putrescine oxidase (PUO) which catalyzes the conversion of diamines (mainly putrescine and cadaverine) to products including hydrogen peroxide. The hydrogen peroxide is detected anodically at platinum electrode polarized at 0.5 V versus Ag/AgCl. Platinum-plated gold electrodes used as a substrate for the sensor construction, are batch-fabricated on a flexible polyimide foil (Kapton(R), DuPont). A three-electrode cell configuration is used in all amperometric measurements. The sensor construction is based on three layers: an inner layer to reject the interference effect of oxidizable molecules, an outer diffusion controlling layer, and in addition, an enzyme middle layer. The enzyme layer was immobilized by crosslinking PUO with bovine serum albumin (BSA) using glutaraldehyde (GA). An optimization study of the enzyme solution composition was carried out. With the optimized enzyme layer, the biosensor showed a very high sensitivity and fast response time of ca. 20 s. The sensor has a linear dynamic range from (0.5-300 muM) for putrescine that covers the expected biological levels of the analyte. Details on sensor fabrication and characterization are given in the present work.

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A dynamic model of oxygen transport through the outer skin layers and a polarographic sensor was developed for the analysis of transcutaneous oxygen tension (tcPO2). It provides a basis for quantifying the factors that determine the relationship between tcPO2 and arterial oxygen tension (PaO2). Model simulations show the importance of stratum papillare metabolic oxygen consumption; the oxygen permeability of the skin relative to that of the sensor membrane and electrolyte; and temperature and the oxyhemoglobin dissociation curve. These simulations were consistent with experimental data obtained by using microcathode transcutaneous oxygen sensors, which were placed on the skin of 10 healthy adults. Furthermore, the model indicates that accurate evaluation of arterial oxygen tension by using transcutaneous measurements requires continuous estimation of skin perfusion. On the basis of tcPO2 measurements made during arterial occlusion, simulations indicate that quantitative evaluation of the metabolic oxygen consumption of the viable skin tissues is possible only when the oxygen permeabilities of the skin and sensor are known.

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The potentiometric and impedance characteristics of polymeric membranes, based on aliphatic polyurethane (Tecoflex) as a matrix, are described and interpreted by theory and experiments for H(+) and alkali metal ion-sensitive sensors. Both dummy plasticized membranes and proton carrier-loaded membranes can show pH response. The pH response of dummy membranes is due to protonated natural negative sites in the polyurethane matrix. The electrodes with added proton carrier show improved rejection of Li(+), Na(+), and K(+) responses and give useful analytical responses. Optimal performance requires control of negative site concentration by addition of lipophilic salt (e.g. tetraphenylborate derivatives). Impedance analyses show surface-rate semicircles and, depending on the bathing electrolyte solution, appearance of a diffusional Warburg impedance. In addition to these time-dependence surface region effects, changes in the bulk membrane resistance with soaking time can be well correlated with equilibrium water content of plasticized membranes.

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Potassium ion-selective and pH membrane electrodes based on neutral carrier ionophores for K+ (valinomycin) and H+ (TDDA and ETH 5294), respectively, immobilized in carboxylated PVC (PVC-COOH) with normal (classical) and reduced amounts of plasticizer, were investigated with respect to their general analytical performances (linear range, slope, detection limit, selectivity, internal membrane resistance), their biocompatibility and cellular responses. The analytical performance of potassium selective electrodes was not affected by reducing the plasticizer content from 66% (m/m) to about 33% (m/m) while that of pH electrodes was significantly changed at the lower plasticizer concentration level. The adhesive properties of PVC-COOH membranes to an inert substrate such as polyimide-coated Kapton are greatly improved by reducing the plasticizer content of the membrane. In addition, as was reported earlier by this group, improved biocompatibility was observed with these membranes relative to those with increased plasticizer content. A ratio of 1:1 m/m for PVC-COOH to plasticizer is recommended for the construction of planar ISEs without massive use of internal solution.

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Potentiometric responses of microelectronically fabricated planar pH sensors based on both TDDA or ETH 5294 proton neutral carriers, in aminated PVC matrix, were evaluated with respect to main analytical parameters (linear ranges, slopes, reproducibility of potential measurements, potential drift and membrane resistance). In order to increase the electrode life time, increased amounts of membrane material were applied on the Ag/AgCl-poly-HEMA active spots of the polyimide substrate. The electrodes were implanted into an in situ porcine beating heart preparation at a midmyocardial depth in order to monitor H(+) concentration changes during the course of coronary artery occlusion.