Control method for bio-actuators based on muscle contraction model.

For the practical application of bio-actuators, it is desired to develop a precise control method for skeletal muscles. In this study, a novel model-based control method has been proposed. The control method enables skeletal muscles to exert an arbitrary magnitude of contraction force, rather than just the conventional on/off control. First, we proposed a control system to obtain the optimized electrical stimulation voltage, that can reproduce reference force. In order to actually apply the method, a bio-actuator consisting of a skeletal muscle of a frog was manufactured for a verification experiment. First, muscle contraction model parameters that could reproduce the contractile response of the gastrocnemius muscle were identified from the experimental data. Next, based on this model, the proposed control method was used to calculate the stimulation voltage to exert the reference force. The voltage was applied to the bio-actuator to control the contraction force. As a results, the output of the actuator was able to follow the stepwise reference force. Our proposed control method demonstrates the feasibility of precise control of bio-actuators.Clinical Relevance- If skeletal muscles could be used as actuators for power-assistive suits, it would be possible to develop power-assistive suits that are more compatible with people and reduce the burden on caregivers in the field of nursing care. The method proposed in this study will enable the control of skeletal muscle contraction force by electrical stimulation, bringing skeletal muscle actuators closer to practical application.

Spiral groove bearing design for improving plasma skimming in rotary blood pumps.

High-efficiency plasma skimming is hopeful to prevent hemolysis inside spiral groove bearings (SGBs) because it can exclude red blood cells from the ridge gap with a high shear force. However, no study reveals the shape design of SGBs to improve plasma skimming. Therefore, this study proposed and applied a groove design strategy to designing an optimal SGB for enhancing plasma skimming in a rotary blood pump (RBP). Initially, we proposed the design strategy that the shape of the groove for enhancing plasma skimming corresponds to the direction of blood flow in the ridge gap. Second, we visualized the cell flow in a specially designed experimental RBP to determine the direction of blood flow, which was helpful in the subsequent SGB design. Then, we created an SGB to provide superior plasma skimming and applied it to the experimental RBP. We evaluated the plasma skimming effect of SGB at rotational speeds ranging from 2400 to 3000 rpm and hematocrit conditions between 1% and 40%. At a 1% hematocrit, the plasma skimming efficiency for the entire SGB was greater than 95%. In all hematocrit conditions, the efficiency at the inner ridges of the SGB was greater than 80%. The results showed the designed SGB successfully induced excellent plasma skimming within ridge gaps. This study is the first to propose and apply a shape design strategy to generate excellent plasma skimming within an SGB. This study may contribute to the prevention of SGB hemolysis inside SGB for use in RBPs.

Prevention of thrombus formation in blood pump by mechanical circular orbital excitation of impeller in magnetically levitated centrifugal pump.

BACKGROUND: Mechanical circulatory support devices, such as left ventricular assist devices, have recently been used in patients with heart failure as destination therapy but the formation of thrombus in blood pumps remains a critical problem. In this study, we propose a mechanical antithrombogenic method by impeller excitation using a magnetically levitated (Maglev) centrifugal pump. Previous studies have shown that one-directional excitation prevents thrombus; however, it is effective in only one direction. In this study, we aimed to obtain a better effect by vibrating it in a circular orbit to induce uniform changes in the shear-rate field entirely around the impeller. METHODS: The blood coagulation time was compared using porcine blood. (1) The flow rate was set to 1 L/min, and applied excitation was at a frequency of 280 Hz and amplitude of 3 µm. (2) Moreover, the effect was compared by varying the frequency, amplitude, and direction of the excitation. In this experiment, the flow rate was set to 0.3 L/min. RESULTS: (1) The thrombus formation time was 77 min without excitation and 133 min with excitation, which was 1.7 times longer. (2) The results showed no difference between (280 Hz, 3 µm) and (50 Hz, 16 µm) circular orbital excitations, and no directional difference, with thrombus formation of 2.5 times longer under all conditions than that without excitation. CONCLUSION: In the case of simple reciprocating excitation, the time was approximately 1.2 times longer. This indicated that the circular orbital excitation is more effective.

Innovative experimental animal models for real-time comparison of antithrombogenicity between two oxygenators using dual extracorporeal circulation circuits and indocyanine green fluorescence imaging.

BACKGROUND: Antithrombogenicity of extracorporeal membrane oxygenation (ECMO) devices, particularly oxygenators, is a current problem, with numerous studies and developments underway. However, there has been limited progress in developing methods to accurately compare the antithrombogenicity of oxygenators. Animal experiments are commonly conducted to evaluate the antithrombogenicity of devices; however, it is challenging to maintain a steady experimental environment. We propose an innovative experimental animal model to evaluate different devices in a constant experimental environment in real-time. METHODS: This model uses two venous-arterial ECMO circuits attached to one animal (one by jugular vein and carotid artery, one by femoral vein and artery) and real-time assessment of thrombus formation in the oxygenator by indocyanine green (ICG) fluorescence imaging. Comparison studies were conducted using three pigs: one to compare different oxygenators (MERA vs. CAPIOX) (Case 1), and two to compare antithrombotic properties of the oxygenator (QUADROX) when used under different hydrodynamic conditions (continuous flow vs. pulsatile flow) (Cases 2 and 3). RESULTS: Thrombi, visualized using ICG imaging, appeared as black dots on a white background in each oxygenator. In Case 1, differences in the site of thrombus formation and rate of thrombus growth were observed in real-time in two oxygenators. In Case 2 and 3, the thrombus region was smaller in pulsatile than in continuous conditions. CONCLUSIONS: We devised an innovative experimental animal model for comparison of antithrombogenicity in ECMO circuits. This model enabled simultaneous evaluation of two different ECMO circuits under the same biological conditions and reduced the number of sacrificed experimental animals.

Impact of gap size and groove design of hydrodynamic bearing on plasma skimming effect for use in rotary blood pump.

Plasma skimming can exclude red blood cells from high shear regions in the gaps formed by hydrodynamic bearings in rotary blood pumps. We investigated the effect of the gap size and groove design on the plasma skimming efficiency. Spiral groove bearings (SGBs) were installed into a specially designed test rig for in vitro experiments performed using human blood. The measured gap between the ridges of the bearing and the rotor surface was 17-26 µm at a flow rate of 150 ml/min and a rotor speed of 2400 rpm. Three different patterns of SGBs were designed (SGB-0, SGB-30, and SGB-60) with various degrees of the circumferential component. The hematocrit measured by a high-speed camera was compared with the hematocrit in the circuit, and the plasma skimming efficiency for the three bearing patterns was evaluated at hematocrits of 20%, 25%, and 30%. SGB-60, which had the strongest circumferential component, provided the best plasma skimming efficiency. When the gap size was less than 20 µm, the red blood cells in the gaps between the ridges of the bearing and rotor surface reduced significantly and the efficiency became higher than 90%. The gap size had the strongest effect on producing a significant plasma skimming. The plasma skimming efficiency can be significantly improved by optimizing the bearing gap size and groove design, which facilitates the further development of SGBs for use in applications such as rotary blood pumps.

Individual Variability in von Willebrand Factor Fragility in Response to Shear Stress: A Possible Clue for Predicting Bleeding Risk.

Acquired von Willebrand syndrome (AVWS), characterized by reduced von Willebrand factor (VWF) large multimers, has recently been implicated as the principal mechanism underlying bleeding in patients implanted with left ventricular assist devices (LVADs). Hematological severity of AVWS varies among patients, even if an identical device is implanted. We investigated whether this diversity in hematological severity is due to individual variability in VWF fragility, according to responses to incremental shear stress. Whole-blood samples were sheared at 20,000-40,000 s -1 shear rate, an index of shear stress, using a custom-made shear stressor that could generate shear stress compatible with that produced by an LVAD. The degree of VWF large multimers degradation was evaluated using the VWF large multimer index. A significant inverse correlation was observed between the VWF large multimer index and LVAD-compatible magnitudes of shear stress: the VWF large multimer indices were 68.5 ± 18.3, 48.0 ± 13.9, 33.9 ± 12.1, 23.7 ± 7.9, and 18.7% ± 8.7% at 20,000, 25,000, 30,000, 35,000, and 40,000 s -1 of shear rates, respectively ( P < 0.0001). Furthermore, experimental VWF large multimer index values were compatible with those derived from patients with implanted LVADs (median; 28.9%). Finally, reduction in the VWF large multimer index corresponding to shear stress showed individual variation. We demonstrated that the combined use of a novel high shear stress loading device and quantitative evaluation of VWF large multimers may predict risk of bleeding before LVAD implantation.

Development of muscle connection components for implantable power generation system.

We have been developing an implantable power generation system that uses muscle contraction following electrical stimulation as a permanent power source for small implantable medical devices. However, if the muscle tissue is overloaded for power generation, the tissue may rupture or blood flow may be impaired. In this study, we developed a new muscle-connecting component that solves these problems. The new connection device has three rods attached to the muscle fibers, and the force exerted on the muscle fibers is converted from horizontal to vertical when the muscle contracts. We conducted simulations with a three-dimensional (3D) model, as well as pulse wave muscle measurements and in vivo tests using the actual muscle. The pulse wave in the connecting part and its downstream were optically measured from the muscle surface, and the blood flow was not obstructed. The 3D model simulations revealed that the distribution of stress was preferable compared with the case in which a rod was stuck vertically in the muscle. In the in vivo muscle tests, the metal rod and resin parts were attached to the muscle, and a load of up to approximately 9 N was applied to the connecting part. Consequently, the connecting part was stable and integrated with the muscle, and there was no damage in the muscle. Although no long-term or histological evaluations were conducted, the device may be useful because of the intramuscular power generation owing to the minimal load applied on the part connected with the muscle.

Quantitative investigation of platelet aggregation under high shear force for anti-platelet aggregation in vitro tests.

Blood pumps are often used for hemofiltration in patients with renal failure. To design effective centrifugal blood pumps for hemofiltration, it is important to suppress clogging caused by platelet aggregation. However, the optimal conditions for conducting anti-platelet aggregation tests in vitro have not yet been established. This study aimed to quantify the effect of the shear loading value and shear loading time on platelet aggregation and determine the optimal conditions for anti-platelet aggregation testing in vitro. To quantitatively evaluate platelet aggregation in terms of the negative logarithm-platelet aggregation threshold index (NL-PATI), which reflects the propensity of residual platelets to aggregate after shear loading, the following parameters were examined: blood collection method (collected from porcine vein using a syringe or collected from a slaughterhouse), type of anticoagulant (sodium citrate or heparin), shear rate, and shear time. The results showed that platelet aggregation in porcine blood increased under a high shear load applied at shear rates of approximately 20,000 s-1 or higher for 30 s. Platelet aggregation propensity was 2-3 times higher in heparin-anticoagulated blood than in sodium citrate-anticoagulated blood. Moreover, platelet aggregation was 1.5-2 times more in blood collected from the slaughterhouse than in syringe-collected blood. Testing with an integrated shear time of 30 s or less in relation to the total blood volume may be effective for conducting in vitro circulation experiments using hemofiltration blood pumps. The conditions established in this study may be useful for hemocompatibility testing of cardiovascular devices based on NL-PATI.

Novel application of indocyanine green fluorescence imaging for real-time detection of thrombus in a membrane oxygenator.

Extracorporeal membrane oxygenation (ECMO) plays an important role in the coronavirus disease 2019 (COVID-19) pandemic. Management of thrombi in ECMO is generally an important issue; especially in ECMO for COVID-19 patients who are prone to thrombus formation, the thrombus formation in oxygenators is an unresolved issue, and it is very difficult to deal with. To prevent thromboembolic complications, it is necessary to develop a method for early thrombus detection. We developed a novel method for detailed real-time observation of thrombi formed in oxygenators using indocyanine green (ICG) fluorescence imaging. The purpose of this study was to verify the efficacy of this novel method through animal experiments. The experiments were performed three times using three pigs equipped with veno-arterial ECMO comprising a centrifugal pump (CAPIOX SL) and an oxygenator (QUADROX). To create thrombogenic conditions, the pump flow rate was set at 1 L/min without anticoagulation. The diluted ICG (0.025 mg/mL) was intravenously administered at a dose of 10 mL once an hour. A single dose of ICG was 0.25mg. The oxygenator was observed with both an optical detector (PDE-neo) and the naked eye every hour after measurement initiation for a total of 8 hours. With this dose of ICG, we could observe it by fluorescence imaging for about 15 minutes. Under ICG imaging, the inside of the oxygenator was observed as a white area. A black dot suspected to be the thrombus appeared 6-8 hours after measurement initiation. The thrombus and the black dot on ICG imaging were finely matched in terms of morphology. Thus, we succeeded in real-time thrombus detection in an oxygenator using ICG imaging. The combined use of ICG imaging and conventional routine screening tests could compensate for each other's weaknesses and significantly improve the safety of ECMO.

Evaluation of real-time thrombus detection method in a magnetically levitated centrifugal blood pump using a porcine left ventricular assist circulation model.

Pump thrombosis induces significant complications and requires timely detection. We proposed real-time monitoring of pump thrombus in a magnetically levitated centrifugal blood pump (mag-lev pump) without using additional sensors, by focusing on the changes in the displacement of the pump impeller. The phase difference between the current and displacement of the impeller increases with pump thrombus. This thrombus detection method was previously evaluated through simulated circuit experiments using porcine blood. Evaluation of real-time thrombus detection in a mag-lev blood pump was performed using a porcine left ventricular assist circulation model in this study. Acute animal experiments were performed five times using five Japanese domestic pigs. To create thrombogenic conditions, fibrinogen coating that induces thrombus formation in a short time was applied to the inner surfaces of the pump. An inflow and an outflow cannula were inserted into the apex of the left ventricle and the carotid artery, respectively, by a minimally invasive surgical procedure that allowed minimal bleeding and hypothermia. Pump flow was maintained at 1 L/min without anticoagulation. The vibrational frequency of the impeller (70 Hz) and its vibrational amplitude (30 µm) were kept constant. The thrombus was detected based on the fact that the phase difference between the impeller displacement and input current to the magnetic bearing increases when a thrombus is formed inside a pump. The experiment was terminated when the phase difference increased by over 1° from the lowest value or when the phase difference was at the lowest value 12 hours after commencing measurements. The phase difference increased by over 1° in three cases. The pump was stopped after 12 hours in two cases. Pump thrombi were found in the pump in three cases in which the phase difference increased by over 1°. No pump thrombus was found in the other two cases in which the phase difference did not increase. We succeeded in real-time thrombus monitoring of a mag-lev pump in acute animal experiments.

Verification of a thrombus induction method at the target point inside the blood pump using a fibrinogen coating for a thrombus detection study.

Although the magnetically levitated centrifugal blood pump (mag-lev pump) is considered superior to other pumps in antithrombogenicity, thrombotic complications are still reported. Research into thrombus detection inside a mag-lev pump is very important for solving this problem. Our research group has already proposed a method to detect a thrombus inside a mag-lev pump in real time without an additional sensor, which is named the impeller vibration method. To efficiently advance our research with reproducibility, a preconditioning method to induce thrombus inside the pump was thought to be necessary. Therefore, this study aimed to develop a preconditioning method that induces thrombus formation. To verify this method, in vitro experiments for thrombus detection were performed. A mag-lev pump developed at Tokyo Institute of Technology was used. A fibrinogen solution was coated on the inner surfaces of the bottom housing to induce thrombus formation at the target point inside the pump. The thrombus is detected by utilizing the phenomenon that the phase difference between the impeller displacement and input current to the magnetic bearing increases when a thrombus is formed inside a pump. Five hundred mL of porcine blood anticoagulated with heparin sodium was circulated in the mock circuit, and protamine sulfate was administered. Flow rate (1 L/min), impeller vibrational frequency (70 Hz), and vibrational amplitude (30 µm) were set to constant. The experiment was terminated when the phase difference increased by over 2° from the minimum value. The experiments were performed in fibrinogen-coated (group F, n = 5) and non-coated pumps (group N, n = 5). In group F, thrombus formation was observed at the fibrinogen-coated point of the housing. In contrast, a relatively small thrombus was observed in varying locations such as the housing or the impeller in group N. Thrombus formation time (the time from when the phase difference takes the minimum value to when the experiment is terminated) was different between the two groups. The mean time was significantly shorter in group F (44 ± 29 minutes) than in group N (143 ± 38 minutes; p = 0.0019). Therefore, a preconditioning method that induced thrombus formation at the target point inside a blood pump was successfully developed.

Detection of thrombosis in a magnetically levitated blood pump by vibrational excitation of the impeller.

The use of contactless support technology for the impeller has led to an increase in the durability of ventricular assist devices (VADs), and these have been in clinical use worldwide. However, pump thrombosis and stroke are still issues to be solved. We have developed a method for detecting the thrombosis in a magnetically levitated blood pump without the need for additional sensors or other equipment. In the proposed method, a sinusoidal current is applied to the electromagnets used for the magnetic bearing, resulting in vibration of the impeller. The phase difference between the current and displacement of the impeller increases with pump thrombosis. First, we describe the principle by which the pump thrombosis is detected. Pump thrombosis reduces the narrowest fluid gap in the pump and this gives rise to a change in the phase difference. Second, we report on experiments in which we changed the narrowest fluid gap using oriented polypropylene tape and showed that decreasing the narrowest fluid gap resulted in an increase in phase difference. For these experiments, the measurements were repeated three times for each condition. Third, we examine the relationship between the pump thrombosis and the phase difference evaluated by observations of the underside of the impeller when operating the pump with porcine blood. Since light was unable to penetrate the blood layer, the erythrocytes were removed for this observation. Only one observation was made. The results showed the phase difference rapidly increased at the same moment when the pump thrombosis was observed. This implies the proposed method has the potential to detect the early stages of pump thrombosis. Finally, in vitro experiments to detect thrombosis when using whole porcine blood in the pump were conducted. The experiment was carried out five times. To intentionally form a thrombus inside the pump, the activated clotting time was controlled to be less than 200 s. In every case, the phase difference increased by more than one degree after tens of minutes. Then, the pump was disassembled and a small amount of pump thrombosis was observed. We conclude that real-time diagnosis of pump thrombosis may be realized by measuring the phase difference without the need for additional sensors.

Mechanical antithrombogenic properties by vibrational excitation of the impeller in a magnetically levitated centrifugal blood pump.

Mechanical circulatory support devices have been used clinically for patients with heart failure for over 10 years. However, thrombus formation inside blood pumps remains a risk to patient life, causing pump failure and contributing to neurological damage through embolization. In this article, we propose a method for preventing thrombus formation by applying vibrational excitation to the impeller. We evaluate the ability of this method to enhance the antithrombogenic properties of a magnetically levitated centrifugal blood pump and ensure that the impeller vibration does not cause undue hemolysis. First, 3 vibrational conditions were compared using an isolated pump without a mock circulation loop; the vibrational excitation frequencies and amplitudes for the impeller were set to (a) 0 Hz-0 µm, (b) 70 Hz-10 µm, and (c) 300 Hz-2.5 µm. The motor torque was measured to detect thrombus formation and obtain blood coagulation time by calculating the derivative of the torque. Upon thrombus detection, the pump was stopped and thrombi size were evaluated. The results showed an increase in the blood coagulation time and a decrease in the rate of thrombus formation in pumps with the impeller vibration. Second, an in vitro hemolysis test was performed for each vibrational condition to determine the effect of impeller vibration on hemolysis. The results revealed that there was no significant difference in hemolysis levels between each condition. Finally, the selected vibration based on the above test results and the non-vibration as control were compared to investigate antithrombogenic properties under the continuous flow condition. The blood coagulation time and thrombi size were investigated. As a result, vibrational excitation of the impeller at a frequency of 300 Hz and amplitude of 2.5 µm was found to significantly lengthen clotting time, decreasing the rate of pump thrombus compared to the non-vibration condition. We indicate the potential of impeller vibration as a novel mechanical antithrombogenic mechanism for rotary blood pumps.

Measuring real-time blood viscosity with a ventricular assist device.

Ventricular assist devices assist in blood circulation and form a crucial component of artificial hearts. While it is important to measure parameters such as the flow rate, pressure head and viscosity of the blood, implanting additional devices to do such measurements is inadvisable. To this end, we demonstrate the adaptation of a ventricular assist device for the purpose of measuring blood viscosity. Such an approach eliminates the need for additional dedicated viscometers in artificial hearts. In the proposed method, the blood viscosity is measured by applying radial vibrational excitation to the impeller in a ventricular assist device using its magnetic levitation system. During the measurement, blood is exposed to a combination of a low shear rate (≈100/s) generated by the radial vibration of the impeller and a high shear rate (>10,000/s) generated by the impeller's rotation. The apparent viscosity of blood depends on the shear rate, so we determined which shear rate was the dominant one in the proposed method. The measurement results showed that the viscosity measured by the proposed method was in good agreement with the reference viscosity measured with a high shear rate. The mean absolute deviation in the measurements using the proposed method and those obtained using a concentric cylindrical viscometer at a high shear rate was 0.12 mPa s for four samples of porcine blood, with viscosities ranging from 2.32 to 2.75 mPa s.

Development of an Electrostatic Oral Cavity Generator Driven by Occlusal Force.

Mouthguard type sensors have been developed to monitor the healthcare-related information, which is expected to be used for pre-illness and preventive medicine. However, a method for supplying the power to these sensors is still an issue to be solved. In this paper, we propose an electrostatic oral cavity generator driven by occlusal force to supply the power for mouthguard type sensors. The proposed generator is a sheet form and consists of the lamination of the electret, dielectric elastomer and copper electrode. In this paper, we estimated the power generation with a piezoelectric generator and the proposed electrostatic generator. Then, the generators were designed and fabricated. The generated power of the prototype electrostatic generator was 18.0 µW. It was about 560 times larger than that of a piezoelectric generator with the area equivalent to four pairs of molars. Although the generated power was still smaller than the required power to drive the mouthguard sensor, we considered they can be comparable with each other by optimizing the design parameters of the generator.

Development of an Intelligent Ventricular Assist Device with a Function of Sensorless Thrombus Detection.

Thrombus is one of the major problems in ventricular assist devices (VADs). However, method for detecting thrombus in early stage has not been established yet. In this study, we propose an intelligent function that the VAD itself can detect thrombus automatically and alert it to medical staffs. In the proposed method, thrombus formation inside a blood pump is detected by monitoring blood viscosity. This viscosity measurement is performed by using magnetic levitation system for the impeller. Hence, it can be implemented without any additional sensors or mechanisms in principle. For verification of the method, at first, we visualized inside of the pump during thrombus formation with measuring blood viscosity by using erythrocytes removed porcine blood. The result showed that the viscosity of the blood increased as blood coagulation progressed. Then, we conducted in vitro principle verification experiments with three different whole porcine blood. In all experiments, the measured blood viscosity increased and small thrombus was observed inside the pump. From these results, we confirmed that the proposed method has a possibility to detect and predict the thrombus in early stage.

Development of a suspension type sliding planar motion table using magnetic fluid lubrication.

A sliding planar motion table system that can be used for the lens driving actuator of a laser cutting machine was developed. The system uses magnetic fluid as the lubricant to avoid the leakage of lubricating oil under the table and reduce environmental pollution. The motion table is suspended from the guide surface by an attractive force generated by electromagnets to reduce the contact and frictional forces between the table and the guide surface. The table is capable of movement in one rotational and two translational directions over the guide surface using six electromagnets and three non-contact displacement sensors. Experimental results showed that the magnetic suspension of the table reduced the friction by 82.1% compared to the friction that would otherwise be generated by the dead weight of the table. Circular motion within a diameter of 2 mm was achieved with resolutions of 5 µm and 20 µrad in the translational and rotational directions, respectively. A bandwidth of higher than 100 Hz was also achieved in the three movement directions.

Implantable power generation system utilizing muscle contractions excited by electrical stimulation.

An implantable power generation system driven by muscle contractions for supplying power to active implantable medical devices, such as pacemakers and neurostimulators, is proposed. In this system, a muscle is intentionally contracted by an electrical stimulation in accordance with the demands of the active implantable medical device for electrical power. The proposed system, which comprises a small electromagnetic induction generator, electrodes with an electrical circuit for stimulation and a transmission device to convert the linear motion of the muscle contractions into rotational motion for the magneto rotor, generates electrical energy. In an ex vivo demonstration using the gastrocnemius muscle of a toad, which was 28 mm in length and weighed 1.3 g, the electrical energy generated by the prototype exceeded the energy consumed for electrical stimulation, with the net power being 111 µW. It was demonstrated that the proposed implantable power generation system has the potential to replace implantable batteries for active implantable medical devices.

Sensorless Viscosity Measurement in a Magnetically-Levitated Rotary Blood Pump.

Controlling the flow rate in an implantable rotary blood pump based on the physiological demand made by the body is important. Even though various methods to estimate the flow rate without using a flow meter have been proposed, no adequate method for measuring the blood viscosity, which is necessary for an accurate estimate of the flow rate, without using additional sensors or mechanisms in a noninvasive way, has yet been realized. We have developed a sensorless method for measuring viscosity in magnetically levitated rotary blood pumps, which requires no additional sensors or mechanisms. By applying vibrational excitation to the impeller using a magnetic bearing, we measured the viscosity of the working fluid by measuring the phase difference between the current in the magnetic bearing and the displacement of the impeller. The measured viscosity showed a high correlation (R(2) > 0.992) with respect to a reference viscosity. The mean absolute deviation of the measured viscosity was 0.12 mPa·s for several working fluids with viscosities ranging from 1.18 to 5.12 mPa·s. The proposed sensorless measurement method has the possibility of being utilized for estimating flow rate.

Development of a disposable magnetically levitated centrifugal blood pump (MedTech Dispo) intended for bridge-to-bridge applications--two-week in vivo evaluation.

Last year, we reported in vitro pump performance, low hemolytic characteristics, and initial in vivo evaluation of a disposable, magnetically levitated centrifugal blood pump, MedTech Dispo. As the first phase of the two-stage in vivo studies, in this study we have carried out a 2-week in vivo evaluation in calves. Male Holstein calves with body weight of 62.4­92.2 kg were used. Under general anesthesia, a left heart bypass with a MedTech Dispo pump was instituted between the left atrium and the descending aorta via left thoracotomy. Blood-contacting surface of the pump was coated with a 2-methacryloyloxyethyl phosphorylcholine polymer. Post-operatively, with activated clotting time controlled at 180­220 s using heparin and bypass flow rate maintained at 50 mL/kg/min, plasma-free hemoglobin (Hb), coagulation, and major organ functions were analyzed for evaluation of biocompatibility. The animals were electively sacrificed at the completion of the 2-week study to evaluate presence of thrombus inside the pump,together with an examination of major organs. To date, we have done 13 MedTech Dispo implantations, of which three went successfully for a 2-week duration. In these three cases, the pump produced a fairly constant flow of 50 mL/Kg/min. Neurological disorders and any symptoms of thromboembolism were not seen. Levels of plasma-free Hb were maintained very low. Major organ functions remained within normal ranges. Autopsy results revealed no thrombus formation inside the pump. In the last six cases, calves suffered from severe pneumonia and they were excluded from the analysis. The MedTech Dispo pump demonstrated sufficient pump performance and biocompatibility to meet requirements for 1-week circulatory support. The second phase (2-month in vivo study) is under way to prove the safety and efficacy of MedTech Dispo for 1-month applications.