Steffen Leonhardt

Steffen Leonhardt
RWTH Aachen University · Helmholtz Institute of Biomedical Engineering

Prof. Dr. med. Dr.-Ing.

About

749
Publications
184,367
Reads
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10,205
Citations
Citations since 2017
230 Research Items
7141 Citations
201720182019202020212022202302004006008001,0001,2001,400
201720182019202020212022202302004006008001,0001,2001,400
201720182019202020212022202302004006008001,0001,2001,400
201720182019202020212022202302004006008001,0001,2001,400
Additional affiliations
August 2003 - present
RWTH Aachen University
Position
  • Director, Full Professor
December 1989 - November 1998
Technische Universität Darmstadt
Position
  • PhD Student

Publications

Publications (749)
Article
Full-text available
In today’s neonatal intensive care units, monitoring vital signs such as heart rate and respiration is fundamental for neonatal care. However, the attached sensors and electrodes restrict movement and can cause medical-adhesive-related skin injuries due to the immature skin of preterm infants, which may lead to serious complications. Thus, unobtrus...
Article
Full-text available
Electrical impedance tomography (EIT) is an imaging modality that can estimate a visualization of the conductivity distribution inside the human body. However, the spatial resolution of EIT is limited because measurements are sensitive to noise. We investigate a technique to incorporate a priori information into the EIT reconstructions of the D-Bar...
Article
This paper presents an integration of two significant targets, i.e., reducing the output impedance and the vibration at the trajectory tracking, into one variable stiffness actuator (VSA) system. The VSA is operated within the cascade impedance control framework, in which the inner torque control loop is designed with the unknown input observer (UI...
Article
Full-text available
Introduction Aging is accompanied by changes in muscle mass, strength and loss of sensory, visual and auditive functions. However, these changes do not occur linearly, most spatiotemporal gait parameters change with aging. Age simulation suits have been invented to give young people an impression of the implications of being older and may be a usef...
Article
Full-text available
For many elderly, wheeled rollators play an essential role in everyday life’s mobility. In addition, exoskeletons are increasingly used in daily mobility as well as in rehabilitation therapy. We present a motorized rollator to combine the advantages of both support devices. Based on the developed hardware, we design two model-based control concepts...
Article
Full-text available
Active lower limb exoskeletons are increasingly used in rehabilitation therapy. An important aspect and ongoing research topic in developing such assistive devices is safe human–machine interaction. Another major challenge in the control of exoskeletons is determining the subject's intention to move for providing cooperative assistance. To address...
Article
Full-text available
Lyapunov exponent is a promising parameter to ascertain the stability of the human gait. In this work, we use a time-series model based on a second-order delay-system with inertial measurement units placed on the foot and wrist. Stability is analyzed in a localized sense, with the Lyapunov exponent computed in the temporal region between two heel-s...
Article
Full-text available
Walking gait correction is fundamental to restoring body motion function for patients. Functional Electrical Stimulation (FES) is regarded as a promising method and essential to Neurotherapy, and the exoskeleton is fast becoming a key instrument in rehabilitation. Applying FES or exoskeleton alone, however, has its inherent disadvantages. Therefore...
Article
Full-text available
Background Measuring the respiratory rate is usually associated with discomfort for the patient due to contact sensors or a high time demand for healthcare personnel manually counting it. Methods In this paper, two methods for the continuous extraction of the respiratory rate from unobtrusive ballistocardiography signals are introduced. The Hilber...
Preprint
Full-text available
We study the statistical properties of facial behaviour altered by the regulation of brain arousal in the clinical domain of psychiatry. The underlying mechanism is linked to the empirical interpretation of the vigilance continuum as behavioral surrogate measurement for certain states of mind. Referring to the classical scalp-based obtrusive measur...
Preprint
Preterm babies in the Neonatal Intensive Care Unit (NICU) have to undergo continuous monitoring of their cardiac health. Conventional monitoring approaches are contact-based, making the neonates prone to various nosocomial infections. Video-based monitoring approaches have opened up potential avenues for contactless measurement. This work presents...
Conference Paper
In this contribution, the development and validation of a digital twin of a decentralized source of breathing gas used for mechanical ventilation is described. A hardware setup is built in analogy to a Simscape ® model with almost identical architecture and geometry. An experimental trial of volume controlled ventilation was conducted and the obtai...
Conference Paper
In this work, we evaluated the possibility to use synthesized IMU data for training a deep neural network to generate a more complex, full-body description of the human gait in terms of joint angle trajectories from a sparse sensor setup. In this context, a sparse sensor setup consists of a few sensors attached to human body segments in an unobtrus...
Conference Paper
Preterm babies in the Neonatal Intensive Care Unit (NICU) have to undergo continuous monitoring of their cardiac health. Conventional monitoring approaches are contact-based, making the neonates prone to various nosocomial infections. Video-based monitoring approaches have opened up potential avenues for contactless measurement. This work presents...
Conference Paper
Gait behavior is considered an important indicator for the assessment of the general health status and provides a diagnostic observation for neuro-degenerative and musculo-skeletal diseases. Individual changes in gait behavior often reflect a deterioration of the current health status in a general sense and therefore provide significant information...
Conference Paper
Step Length is an important metric that can be used for the analysis and assessment of the gait. Proper dynamical models are not available in current literature associated with the wrist that can adequately determine the step length using recursive estimation techniques. This study presents a method to estimate the step length using angular velocit...
Conference Paper
While mechanical ventilation (MV) can lead to ventilator-induced diaphragmatic atrophy due to diaphragm inactivity, electrical phrenic nerve stimulation (PNS) can keep the diaphragm active and therefore prevent diaphragmatic weakness. To quantify the effectivity of PNS, an identification experiment during PNS is presented, and its data is used in G...
Conference Paper
Supervision of mechanical ventilation is currently still performed by clinical staff. With the increasing level of automation in the intensive care unit, automatic supervision is becoming necessary. We present a fuzzy-based expert supervision system applicable to automatic feedback control of oxygenation. An adaptive fuzzy limit checking and trend...
Article
Full-text available
Background Models of hypoxemic lung injury caused by lavage-induced pulmonary surfactant depletion are prone to prompt recovery of blood oxygenation following recruitment maneuvers and have limited translational validity. We hypothesized that addition of injurious ventilation following surfactant-depletion creates a model of the acute respiratory d...
Conference Paper
Full-text available
Electrical Impedance Myography (EIM) is a painless and non-invasive approach for quantitative assessment of muscle-condition of the limbs, which is based on consideration of dielectric tissue properties. The purpose of this work is to analyse the impact of isometric (constant muscle length) and isotonic (constant muscle force) contraction on the re...
Article
Full-text available
Background: Hemocompatibility of left ventricular assist devices is essential for preventing adverse events. In this study, we compared the hemocompatibility of an axial-flow (Sputnik) to a centrifugal-flow (HeartMate 3) pump. Methods: Both pumps were integrated into identical in vitro test circuits, each filled with 75 mL heparinized human bloo...
Article
The determination of step length, an important gait parameter, has been a challenging task. Although unobtrusive sensors (inertial measurement units) have been developed recently, they cannot facilitate the automatic estimation of step length. In this article, we use a model-based technique to determine the step length using the Unscented Kalman Fi...
Chapter
In this work, we present an approach for non-contact automatic extraction of respiration in infants using infrared thermography video sequences, which were recorded in a neonatal intensive care unit. The respiratory signal was extracted in real-time on low-cost embedded GPUs by analyzing breathing-related temperature fluctuations in the nasal regio...
Article
Full-text available
The continuous monitoring of vital signs is a crucial aspect of medical care in neonatal intensive care units. Since cable-based sensors pose a potential risk for the immature skin of preterm infants, unobtrusive monitoring techniques using camera systems are increasingly investigated. The combination of deep learning–based algorithms and camera mo...
Article
Objective: For driver state estimation, physiological features might be promising input parameters. As cable-bound sensing of these parameters is impractical for ubiquitous monitoring, the measurement certainly has to be based on unobtrusive and contact-free technologies. In this work, unobtrusive methods for heart rate (HR) and respiration rate (R...
Article
Background In vitro modelling of the intracranial pressure (ICP) profile is an important tool to test potential prototypes for the diagnosis and treatment of hydrocepahlus. Objective Here, a novel feedback-controlled mechatronic test-bench, which replicates the pulsatile ICP wave, is presented. Methods The design is based on a MATLAB Simscape sim...
Article
Full-text available
Background: Electrical Impedance Tomography (EIT) is a radiation-free technique for image reconstruction. However, as the inverse problem of EIT is non-linear and ill-posed, the reconstruction of sharp conductivity images poses a major problem. With the emergence of artificial neural networks (ANN), their application in EIT has recently gained inte...
Article
Full-text available
The detection of muscle contraction and the estimation of muscle force are essential tasks in robot-assisted rehabilitation systems. The most commonly used method to investigate muscle contraction is surface electromyography (EMG), which, however, shows considerable disadvantages in predicting the muscle force, since unpredictable factors may influ...
Article
Full-text available
Premature infants are among the most vulnerable patients in a hospital. Due to numerous complications associated with immaturity, a continuous monitoring of vital signs with a high sensitivity and accuracy is required. Today, wired sensors are attached to the patient’s skin. However, adhesive electrodes can be potentially harmful as they can damage...
Article
Full-text available
Volume measurement is beneficial in left ventricular assist device (LVAD) therapy to quantify patient demand. In principle, an LVAD could provide a platform that allows bioimpedance measurements inside the ventricle without requiring additional implants. Conductance measured by the LVAD can then be used to estimate the ventricular radius, which can...
Article
Full-text available
Creating highly functional prosthetic, orthotic, and rehabilitation devices is a socially relevant scientific and engineering task. Currently, certain constraints hamper the development of such devices. The primary constraint is the lack of an intuitive and reliable control interface working between the organism and the actuator. The critical point...
Article
Full-text available
The electrical impedance myography method is widely used in solving bionic control problems and consists of assessing the change in the electrical impedance magnitude during muscle contraction in real time. However, the choice of electrode systems sizes is not always properly considered when using the electrical impedance myography method in the ex...
Article
The objective of this paper is to identify dynamic parameters of the human-exoskeleton system (HES) with the motor torque data considering modeling the passive elastic joint torque of the user. Based on a lower limb exoskeleton prototype we recently developed, the dynamics of the joint actuators and two degrees of freedom link-based swing leg are f...
Article
Full-text available
The step length is an important parameter in gait analysis. Long-term monitoring applications for gait analysis are often based on inertial measurement units (IMUs) due to their low-cost and unobtrusive nature. Spatial gait parameters, such as step or stride length, are therefore not directly accessible. In this contribution, we focus on model-base...
Article
Full-text available
In this work, electric machine of implantable rotary blood pumps (RBPs) of axial-flow type was theoretically investigated. Electromagnetic coupling of rotor and stator in axial-flow RBPs was described with the Maxwell’s equations of the classical electrodynamics given quasi-magnetostatic approximation of electromagnetic field in the simplified mode...
Article
Full-text available
Bioimpedance measurements use current or voltage sources to inject an excitation signal into the body. These sources require a high bandwidth, typically from 1 kHz to 1 MHz. Besides a low common mode, current limitation is necessary for patient safety. In this paper, we compare a symmetric enhanced Howland current source (EHCS) and a symmetric volt...
Article
A driving study was performed to induce stress with 24 participants performing different inducement tasks (n-back task, Sing-a-Song Stress Test and noise exposure). Both performance-based measures (Tactile Detection Response Task) as well as subjective measures were recorded to assess the driver state. Subjective ratings indicate that stress was mo...
Article
Full-text available
Nowadays, rotary blood pumps for the treatment of end-stage heart failure patients are usually fixed-speed controlled. Therefore, without sufficient heart activity, patients may reveal reduced blood flow pulsatility. However, there is evidence that pulsatile flow reduces adverse events. Furthermore, optimized pump flow profiles could be used to ach...
Article
Full-text available
Reflective Photoplethysmography (PPG) sensors are less obtrusive than transmissive sensors, but they present patient-dependent variations in the so-called “Ratio of Modulation” (R). Thus, the conventionally employed calibration curves for determining peripheral oxygen saturation ( SpO2) may report inaccurate values. In this paper, we study the poss...
Article
Objective: Ventricular assist devices (VADs) are implanted in patients suffering from end-stage heart failure to sustain the blood circulation. Real-time volume measurement could be a valuable tool to monitor patients and enable physiological control strategies to provide individualized therapy. However, volume measurement using one sensor modalit...
Article
Introduction: Frailty is a central geriatric syndrome characterized by a state of increased physiological vulnerability. As the key components of frailty are difficult to capture in their entirety, easily measurable and reliable surrogate parameters are desirable. Since frailty influences heart rate variability (HRV), HRV may be such a surrogate p...
Article
Neonatal intensive care units provide vital medical support for premature infants. The key aspect in neonatal care is the continuous monitoring of vital signs measured using adhesive skin sensors. Since sensors can cause irritation of the skin and lead to infections, research focuses on contact-free, camera-based methods such as infrared thermograp...
Article
Objective: During launch and atmospheric re-entry in suborbital space flights, astronauts are exposed to high G-acceleration. These acceleration levels influence gas exchange inside the lung and can potentially lead to hypoxaemia. The distribution of air inside the lung can be monitored by Electrical Impedance Tomography (EIT). This imaging techni...
Conference Paper
Full-text available
The goal of this work is to design a Finite Element Method model enabling the simulation of the impact of muscle activity onto current density distribution, which is established by far-current electrode placement. Here assumed as an alteration in el. conductivity within specific muscle groups, coupled to increasing perfusion during repetitive tasks...
Article
Full-text available
Objective: Continuous volume estimation in Ventricular Assist Device (VAD) therapy would be an advantageous measure for identifying suitable pump settings and providing an individual therapy for patients suffering from heart failure. As the integration of electrodes on a catheter-based VAD could easily be realized, estimating the volume from bioimp...
Article
BACKGROUND: Members of the public will soon be taking commercial suborbital spaceflights with significant +Gx (chest-to-back) acceleration potentially reaching up to +6 Gx. Pulmonary physiology is gravity-dependent and is likely to be affected, which may have clinical implications for medically susceptible individuals. METHODS: During 2-min centrif...
Article
Full-text available
Rationale: Individualized positive end-expiratory pressure (PEEP) titration might be beneficial in preventing tidal recruitment. To detect tidal recruitment by electrical impedance tomography (EIT), the time disparity between the regional ventilation curves (regional ventilation delay inhomogeneity [RVDI]) can be measured during controlled mechani...
Article
Objective: Electrical impedance tomography (EIT) for lung perfusion imaging is attracting considerable interest in intensive care, as it might open up entirely new ways to adjust ventilation therapy. A promising technique is bolus injection of a conductive indicator to the central venous catheter, which yields the indicator-based signal (IBS). Lun...
Conference Paper
Capacitive electrocardiogram (cECG), the unobtrusive alternative to the monitoring of the electrical heart activity, is highly susceptible to the electrical noise and the motion artifacts, as the measurement takes place through the clothing of the subjects. Motion artifacts caused by the time-variant coupling impedance of the clothing have been inv...
Conference Paper
The control of the ankle joint induced by functional electrical stimulation (FES) is especially for paraplegic patients of high interest as it allows to partly re-gain the functionality of the lower limb. For this reason, we developed a Simulink model of the ankle joint which describes the dynamic behavior regarding the activation and force develop...
Article
Full-text available
Magnetic induction measurement (MIM) is a noninvasive method for the contactless registration of respiration in newborn piglets by using measurement coils positioned at the bottom of an incubator. Acute pulmonary problems may be determinants of poor neurological and psychomotor outcomes in preterm infants. The current study tested the detection of...
Article
Objective: Multi-channel Bioimpedance Spectroscopy (BIS) systems typically sample each channel's impedance sequentially using multiplexers and a single analog-to-digital converter (ADC). These systems may lose their real-time capability with an increasing number of channels, especially for low excitation frequencies. We propose a new method, calle...
Article
The distance that an electrically powered bicycle can cover depends on factors such as the route’s elevation profile, the motor support selected, and the fitness of the cyclist. This fact requires the cyclist to estimate which motor support may be chosen to reach the goal with the battery’s current state of charge. For this reason, we propose a bat...
Article
Full-text available
Optical heart rate monitoring (OHR) with reflective wrist photoplethysmography is a technique mainly used in the wellness application domain for monitoring heart rate levels during exercise. In the absence of motion, OHR technique is also able to estimate individual beat-to-beat intervals relatively well and can therefore also be used, for example,...
Article
Full-text available
The lower limb exoskeleton investigated in this work actively supports the knee and hip and is intended to provide full motion support during gait. Parallel elastic actuators are integrated into the hip joints to improve the energy efficiency in gait. The prototype was tested in sit-to-stand and gait trials, in which the actuators were cascade-cont...
Article
Full-text available
This study assesses the electric current parameters and reports on the analysis of the associated degree of myocardial function during left ventricular assist device (LVAD) support. An assumption is made that there is a correlation between cardiac output and the pulsatility index of the pump electric current. The experimental study is carried out u...
Article
Objective: Infrared thermography has the potential to complement the classification of chronic venous diseases (CVD), but lacks sophisticated insights on the association between recorded skin temperatures and the severity of CVD. This research aims to identify temperature patterns in the lower legs of patients that are distinct in specific forms o...
Article
Full-text available
Infrared thermography for camera-based skin temperature measurement is increasingly used in medical practice, e.g., to detect fevers and infections, such as recently in the COVID-19 pandemic. This contactless method is a promising technology to continuously monitor the vital signs of patients in clinical environments. In this study, we investigated...
Article
Full-text available
Background Transcellular fluid shifts during dialysis treatment could be related to the frequency and severity of intradialytic hypotension (IDH). We investigated that (i) in addition to ultrafiltration, extracellular fluid (ECF) is further depleted by transcellular fluid shifts and (ii) changes in intracellular fluid (ICF), which have been overloo...
Preprint
The human insulin-glucose metabolism is a time-varying process, which is partly caused by the changing insulin sensitivity of the body. This insulin sensitivity follows a circadian rhythm and its effects should be anticipated by any automated insulin delivery system. This paper presents an extension of our previous work on automated insulin deliver...
Article
We present a framework for achieving a robust stability test of the variable stiffness actuator (VSA) exoskeleton programmed with the gain scheduling-based variable impedance control (GSVIC). In this brief, the focused impedance control framework involves the cascaded position torque control loop, in which both control loops are closed, and the ind...
Article
Full-text available
Background Only a small fraction of the information available is generally used in the majority of camera-based sensing approaches for vital sign monitoring. Dedicated skin pixels, for example, fall into this category while other regions are often disregarded early in the processing chain. Methods We look at a simple processing chain for imaging w...
Chapter
Monitoring a patient's vital signs daily is of increasing interest to handle the economic challenges in our health-care systems. Comfort, ease of use, and reliability of these systems are necessary for the patient's compliance. Therefore, wearable or textile-integrated sensors are desirable. Unobtrusive measurement techniques are preferred by the p...
Book
This book talks about photoplethysmography (PPG) techniques based on computer-aided data processing. In particular, it presents the results of a co-operative Indo-German project on the topic between Indian Institute of Technology at Chennai and RWTH Aachen University. Measuring system design, experimental details and some preliminary results obtain...
Article
Optimizing the positive end-expiratory pressure remains challenging for any clinician treating a patient with acute respiratory distress syndrome. This paper presents an approach to automate a PEEP titration maneuver and identify the best PEEP according to maximal compliance. The respiratory system was modeled by a single-compartment model, and par...