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J Molenaar

Publications and source records attributed to J Molenaar.

At least 19 recordsLinked to original sources
PubMed2005

Flow-induced correlation effects within a linear chain in a polymer melt.

A framework for a consistent description of the flow-induced correlation effects within a linear polymer chain in a melt is proposed. The formalism shows how correlations between chain segments in the flow can be incorporated into a hierarchy of distribution functions for tangent vectors. The present model allows one to take into account all the major relaxation mechanisms. Special cases of the derived set of equations are shown to yield existing models and shed some light on the connection between them. Consequences of several assumptions widely used in the literature are analyzed within the developed framework.

Journal Article↗
PubMed2005

A molecular model for cohesive slip at polymer melt/solid interfaces.

A molecular model is proposed which predicts wall slip by disentanglement of polymer chains adsorbed on a wall from those in the polymer bulk. The dynamics of the near-wall boundary layer is found to be governed by a nonlinear equation of motion, which accounts for such mechanisms on surface chains as convection, retraction, constraint release, and thermal fluctuations. This equation is valid over a wide range of grafting regimes, including those in which interactions between neighboring adsorbed molecules become essential. It is not closed since the dynamics of adsorbed chains is shown to be coupled to that of polymer chains in the bulk via constraint release. The constitutive equations for the layer and bulk, together with continuity of stress and velocity, are found to form a closed system of equations which governs the dynamics of the whole "bulk+boundary layer" ensemble. Its solution provides a stick-slip law in terms of the molecular parameters and extruder geometry. The model is quantitative and contains only those parameters that can be measured directly, or extracted from independent rheological measurements. The model predictions show a good agreement with available experimental data.

Journal Article↗
PubMed2001

Prediction of survival in patients with head and neck cancer.

BACKGROUND: In patients with head and neck squamous cell carcinoma (HNSCC) the estimated prognosis is usually based on the TNM classification. The relative weight of the three contributing parameters is often not completely clear. Moreover, the impact of other important clinical variables such as age, gender, prior malignancies, etc is very difficult to substantiate in daily clinical practice. The Cox-regression model allows us to estimate the effect of different variables simultaneously. The purpose of this study was to design a model for application in new HNSCC patients. In our historical data-base of patients with HNSCC, patient, treatment, and follow-up data are stored by trained oncological data managers. With these hospital-based data, we developed a statistical model for risk assessment and prediction of overall survival. This model serves in clinical decision making and appropriate counseling of patients with HNSCC. PATIENTS AND METHODS: All patients with HNSCC of the oral cavity, the pharynx, and the larynx diagnosed in our hospital between 1981 and 1998 were included. In these 1396 patients, the prognostic value of site of the primary tumor, age at diagnosis, gender, T-, N-, and M-stage, and prior malignancies were studied univariately by Kaplan-Meier curves and the log-rank test. The Cox-regression model was used to investigate the effect of these variables simultaneously on overall survival and to develop a prediction model for individual patients. RESULTS: In the univariate analyses, all variables except gender contributed significantly to overall survival. Their contribution remained significant in the multivariate Cox model. Based on the relative risks and the baseline survival curve, the expected survival for a new HNSCC patient can be calculated. CONCLUSIONS: It is possible to predict survival probabilities in a new patient with HNSCC based on historical results from a data-set analyzed with the Cox-regression model. The model is supplied with hospital-based data. Our model can be extended by other prognostic factors such as co-morbidity, histological data, molecular biology markers, etc. The results of the Cox-regression may be used in patient counseling, clinical decision making, and quality maintenance.

Aged↗
PubMed2000

Grazing impact oscillations

An impact oscillator is a periodically driven system that hits a wall when its amplitude exceeds a critical value. We study impact oscillations where collisions with the wall are with near-zero velocity (grazing impacts). A characteristic feature of grazing impact dynamics is a geometrically converging series of transitions from a nonimpacting period-1 orbit to period-M orbits that impact once per period with M=1,2,ellipsis. In an experiment we explore the dynamics in the vicinity of these period-adding transitions. The experiment is a mechanical impact oscillator with a precisely controlled driving strength. Although the excitation of many high-order harmonics in the experiment appeared unavoidable, we characterize it with only three parameters. Despite the simplicity of this description, good agreement with numerical simulations of an impacting harmonic oscillator was found. Grazing impact dynamics can be described by mappings that have a square-root singularity. We evaluate several mappings, both for instantaneous impacts and for impacts that involve soft collisions with a yielding wall. As the square-root singularity appears persistent in the reduction of the dynamics to mappings, and because impact dynamics appears insensitive to experimental nonidealities, the characteristic bifurcation scenario should be observed in a wide class of experimental systems.

Journal Article↗
PubMed1993

Surgical treatment of congenital bronchopulmonary disease in children.

From 1987 to 1992, 22 children (age 0 days-14 years) were operated for congenital bronchopulmonary disease. One patient had a hamartoma. Four children had a bronchogenic cyst, in 1 patient combined with an esophageal duplication. Intrapulmonary sequestration was diagnosed in 3 children; one of whom had an esophageal duplication as well. Four children had localized emphysema. In 1 patient histology showed rhabdomyosarcoma. A lobectomy was performed following chemotherapy but recurrence was not resectable at a third thoracotomy. Ten patients had cystic adenomatoid malformation, 6 were dependent on artificial ventilation before surgery. Three patients with cystic adenomatoid malformation died in hospital: 2 after pneumonectomy who were shown to have bilateral disease and 1 died after bilobectomy and was shown to have a hypoplastic contralateral lung. Hospital morbidity involved 1 patient with recurrent pneumothorax, 2 with atelectasis of the operated lung and 3 with prolonged artificial ventilation. Late mortality involved 1 patient due to pulmonary rhabdomyosarcoma. After 1-26 months of follow-up there were no late complications. Adequate multidisciplinary treatment allows acceptable mortality and low morbidity in surgery for congenital bronchopulmonary disease in children.

Adolescent↗
PubMed1992

Increased risk of second cancers in managing Hodgkins disease: the 20-year Leiden experience.

Between January 1969 and December 1988, 482 patients were treated for Hodgkin's disease at the Leiden University Hospital. All cases were routinely recorded in the Hospital Information System, which has an active annual follow-up. Of all patients, 57% remained relapse free. According to the kinds of treatment they received, the following major categories were established: radiotherapy only (28.2%), chemotherapy only (20.1%), only initial combination of radiotherapy and chemotherapy (34.2%), all other combinations of radio- and chemotherapy (15.4%), or not registered (2.1%). Twenty-seven second cancers were observed; six leukemias, five non-Hodgkin lymphomas, and 16 solid tumors. Of all solid tumors only nine occurred in relapse-free patients. The overall relative risk of second cancers increased with the duration of follow-up. Using general population incidence rates to calculate expected numbers, the risk for developing leukemia, non-Hodgkin lymphoma, and solid tumors was increased 36-fold, 31-fold, and 2.4-fold, respectively. The cumulative risk of developing a second cancer 10 years after diagnosis of Hodgkin's disease was 7% for both the radiotherapy-only and the initial combination of radio- and chemotherapy group. It was 16% and 17% for the chemotherapy-only and the other combinations of radio- and chemotherapy group, respectively. Multivariate analysis (using the Cox regression model) show an increased risk of second cancers (RR = 0.7) when a relapse of Hodgkin's disease resulting in increasing cumulative therapy occurred. Age at diagnosis of Hodgkin's disease was an important determinant for the risk of non-Hodgkin lymphoma and solid tumors. Cumulative chemotherapy intensity was an important factor in increasing leukemic risk in a dose-response fashion. Apart from this, the stage of Hodgkin's disease, although closely related to the kind of therapy, seemed to have an independent effect on leukemic risk.

Adult↗
PubMed1989

The spike generating mechanism of cat retinal ganglion cells.

The spike generating mechanism (SGM) of sustained and transient-type ganglion cells has been investigated from intracellular recordings in the cat retina. The relationship between the generator potential and the impulse pattern, which are respectively the input and the output of the SGM, has been studied after separating one component from the other. Comparison of averaged generator potentials and the corresponding PSTHs showed that the spike generator is highly sensitive to changes of the generator potential. It proved to be relatively indifferent to the prevailing average levels of depolarization. During sustained parts of the responses the SGM exhibits a stochastic nature. At higher light flicker frequencies, during spike bursts, on the other hand spike generation is very regular and phase locked to the stimulus. The averaged generator potentials were also used to develop and test a set of minimal, computer simulated models of the spike generator. A slow threshold adaptation (time constant about 50 msec) is absolutely necessary in addition to the faster refractory recovery in order to produce spike patterns similar to those measured during the corresponding response periods. The developed model accounts completely for the observed characteristics of ganglion cell spike generation under a wide variety of light stimulus regimes and both for the sustained and for the transient type of ganglion cell.

Action Potentials↗
PubMed1989

Frequency transfer properties of the spike generating mechanism of cat retinal ganglion cells.

The dynamic properties of the spike generating (SG) mechanism of retinal ganglion cells have been studied from intracellular recordings in the cat eye. Intracellularly recorded light flicker responses were separated by computer into spike trains and corresponding generator potentials. Both the spike train and the generator potential responses to temporally modulated light spots were analysed in terms of amplitude and phase plots. The differences in dynamic properties between the two response measures reveal that the SG-mechanism affects the temporal frequency transfer properties of retinal ganglion cells to a considerable extent. With respect to the transfer of the amplitude of the first harmonic the SG-mechanism has differentiating (or high-pass) properties. This means that the responses to high temporal stimulus frequencies are amplified relatively much more than are the responses to lower frequencies. Furthermore, the SG-mechanism causes a phase lead of the spike train response relative to the generator potential by, on average, 37 degrees. The measured frequency responses, among other things, have been used to verify and to quantify the SG-model that we proposed in a previous paper (Lankheet, Molenaar & van de Grind, 1989). With this model it proved possible to reproduce the spike train responses as model output from the corresponding measured generator potentials as model input. A good qualitative and quantitative correspondence between model output and the measured spike trains was obtained for a wide range of stimulus frequencies and with fixed values of the model parameters. With parameter values that optimized this correspondence the model allowed us to investigate the dynamic behaviour of the SG-mechanism in more detail. It also provides a reliable and validated method to predict the shape of the generator potential from the spike train (the "inversion problem").

Action Potentials↗
PubMed1989

The modeling of skin expanders.

In this paper we present mathematical models of tissue expanders of several shapes. Within the restrictions of the models, relations are given between surface-area gain and volume of the expanders. It is shown that for the most frequently used expander shapes, the relevant formulas are simple and do not require computer facilities. It is discussed that the approach unavoidably leads to considerable overestimation of the area gain but may serve as a necessary starting point for further research. Furthermore, the rate of overestimation appears to follow such a regular pattern that it can quite easily be compensated for by means of a correcting factor.

Dermatologic Surgical Procedures↗
PubMed1988

Surface-area increase in tissue expansion.

In this paper, the subject of surface-area gain in tissue expansion is approached from three sides. With the help of a mathematical approach, the theoretical increase in surface area was calculated in relation to the specifications of the expanders. In an in vitro model, these mathematical data were confirmed to be reliable. To determine the real surface-area gain that takes place in vivo, measurements on pigs were performed. It appears that only about 35 percent of the mathematically expected increase in surface area takes place. Moreover, a clear difference was observed between the surface-area gain for the different shapes of tissue expanders.

Animals↗
PubMed1984

Clinical time-course and characteristics of islet cell cytoplasmatic antibodies in childhood diabetes.

Circulating islet cell antibodies (ICA) were present in high frequency (80%) early after diagnosis and decreased in the time course of childhood diabetes mellitus. The complement fixing ability of islet cell antibodies (CF-ICA) in the course of the disease appeared to depend on the titre of ICA: the coefficient of correlation between ICA and CF-ICA titres was 0.79 and all ICA's with a titre over 16 were complement-fixing. Incubating fresh frozen human pancreatic sections thrice rather than once with the children's sera, increased the detectability of complement fixation by a factor 1.4 in all ICA-positive sera. Thus tested, the detection of complement fixation per se did not appear to have a separate pathogenic significance, as the fraction of complement fixing ICA's was almost constant throughout the clinical course. The presence of ICA-IgG subclasses also was dependent on the ICA titre: above a titre of 16 mostly all four subclasses could be detected. Incubating the pancreatic tissue thrice rather than once with ICA-positive sera resulted in enhanced detectability of ICA-IgG1. Early in the course of childhood diabetes, including two prediabetic children, most of the IgG subclasses could be detected in ICA, but after a duration of one year IgG1 alone was mainly seen. In two other children, having a family history of insulin-dependency, restriction to the IgG2 subclass was found.

Adolescent↗
PubMed1983

Dynamic properties of cat horizontal cell light responses.

The presented sinewave/squarewave light flicker responses of cat horizontal cells confirm the classification into Hn and Hm-units described by Foerster et al. (1977a,b) Expl Brain Res. 29, 347-385. However, we find somewhat higher order filter properties and develop a refinement of the model proposed by Foerster et al. to explain our new results. Using this model further analysis of Hm-unit responses suggests that cat retinal receptors have a smaller temporal bandwidth the smaller the spatial extent of the stimulus. Finally results from Gaussian white noise (GWN) stimulation are compared with those from classical flicker stimuli and Bode-plots. There are strong similarities in the results of these two types of analysis but some interesting differences are also found and discussed.

Action Potentials↗
PubMed1980

A mechanical oscilloscope for vision research.

A "mechanical oscilloscope" is described in which a lightbeam (Z-axis) is projected via a single front aluminized moving mirror. The mirror is mounted in a suspension frame allowing the simultaneous rotation about two mutually perpendicular axes. The linear displacements of two loudspeaker-coils are transformed mechanically into mirror rotations and an electronic position/velocity feedback control system is used to drive the loudspeakers. The bandwidth of the resulting X- and Y-axis deflection systems is of the order of 100-200 Hz. The flexibility of Z-axis control (colour, pattern, intensity, etc.) makes this instrument an attractive component of light stimulation systems for psychophysical or electrophysiological studies of vision.

Models, Neurological↗
PubMed1980

A stereotaxic method of recording from single neurons in the intact in vivo eye of the cat.

A method is described for recording stereotaxically from single retinal neurons in the optically intact in vivo eye of the cat. The method is implemented with the help of a new type of stereotaxic instrument and a specially developed stereotaxic atlas of the cat's eye and retina. The instrument is extremely stable and facilitates intracellular recording from retinal neurons. The microelectrode can be rotated about two mutually perpendicular axes, which intersect in the freely positionable pivot point of the electrode manipulation system. When the pivot point is made to coincide with a small electrode-entrance hole in the sclera of the eye, a large retinal region can be reached through this fixed hole in the immobilized eye. The stereotaxic method makes it possible to choose a target point on the presented eye atlas and predict the settings of the instrument necessary to reach this target. This method also includes the prediction of the corresponding light stimulus position on a tangent screen and the calculation of the projection of the recording electrode on this screen. The sources of error in the method were studied experimentally and a numerical perturbation analysis was carried out to study the influence of each of the sources of error on the final result. The overall accuracy of the method is of the order of 5 degrees of visual angle, which will be sufficient for most purposes.

Animals↗
PubMed1980

The complement system in hemolytic-uremic syndrome in childhood.

A dynamic estimation of the complement system was obtained by immunochemical estimation of C3, C4, C5, C1q, C3b + C3c, C3d, Ba in children with hemolytic-uremic syndrome. The presence of increased breakdown products of C3 (C3b + C3c, C3d) and of factor B (Ba) suggests an activation of the complement system possibly by the alternative pathway. No definite explanation for these complement abnormalities can be given.

Child↗