Anders E. Wallin

Stiffer optical tweezers through real-time feedback control

Using real-time re-programmable signal processing we connect acousto-optic steering and back-focal-plane interferometric position detection in optical tweezers to create a fast feedback controlled instrument. When trapping 3μm latex beads in water we find that proportional-gain position-clamping increases the effective lateral trap stiffness ∼13-fold. A theoretical power spectrum for bead fluctuations during position-clamped trapping is derived and agrees with the experimental data. The loop delay, ∼19μs in our experiment, limits the maximum achievable effective trap stiffness.

Daytime changes in postural stability and repeatability of posturographic measurements

Objective: Daytime sleepiness correlates with sleep-related accidents, but convenient tests for occupational sleepiness monitoring are scarce. The effect of daytime on balance, on posturographic measurements, and on their repeatability was investigated in 30 healthy volunteers as part of our work to develop such a test. Methods: The daytime effect was assessed by measuring balance at 8:30 am, 10:30 am, and 1:30 pm. The repeatability was assessed with morning trials once a week for 1 month. The posturographic test was performed on a static force platform, and the balance was evaluated from a fractal dimension of sway, most common sway amplitude, and time interval for open-loop stance control. Results: The balance worsened during the day, and it was possible to determine whether the measurement was performed in the morning or in the afternoon. The morning balance remained unchanged during the month-long test. Conclusions: Posturographic measurements are repeatable and have a circadian effect, which may be influenced by sleepiness.

Evaluating sleepiness using force platform posturography

We have investigated the feasibility to use posturography as a method to estimate sleep deprivation. This manuscript presents a proof-of-concept of this idea. Twenty-one healthy subjects aged 20-37 years participated in the study. The subjects were deprived of sleep for up to 36 h. Their postural stability was measured as a function of sleep deprivation time. As a reference the critical fusion frequency method for measuring sleepiness was used. The 163 posturographic parameters used for analyzing the posturographic data were found from the literature. Of these parameters, the fractal dimension of the sway path, the most common frequency of the sway, the time-interval for open-loop control of stance, and the most common amplitude of the sway showed the highest linear correlations with sleep deprivation time. Using these four parameters we were able to estimate the sleep deprivation time with an accuracy better than 5 h for 80% of the subjects

Posturographic sleepiness monitoring

Although reduced sleep often underlies traffic and occupational accidents, convenient sleepiness testing is lacking. We show that posturographic balance testing addresses this issue, because balance testing predicts hours of wakefulness, which could facilitate sleepiness testing. Here, we equate balance scores from separate trials, blinded to the experimenter, with those recorded as a function of known and increasing time awake (i.e. during sustained wakefulness); we show, that the time awake in separate trials is posturographically measurable: positive predictive value 69%, sensitivity 56%, and specificity 96%. These results encourage further work developing posturographic sleepiness monitoring.

Optical position clamping with predictive control

We increase the effective stiffness of optical tweezers by position clamping a polystyrene bead with a predictive feedback control algorithm. This algorithm mitigates the effect of feedback loop delay. Hence, higher gain than with proportional control can be employed, which results in higher effective trap stiffness, without trap instability. In experiments (initial trap stiffness 0.056 pN/nm with a 1.78u2002μm diameter polystyrene bead), predictive control increased the effective trap stiffness by 55% relative to proportional control. We also derive theoretical expressions for the power spectra of the bead position controlled by our algorithm.

Principal component analysis detects sleepiness-related changes in balance control

Computerized posturography exploits balance scores that quantify the size, dynamics, or structure of the recorded sway. Since people employ different balance strategies, one single balance score will not detect balance changes in all subjects. Principal component analysis (PCA) can combine balance scores that quantify different features into one new balance score. We tested the score with 20 subjects by measuring their balance every 2 h during 28 h of sustained waking. The new balance score was more sensitive than its components (p<0.001 vs. p≥0.051) to the small sleepiness-related balance decrements that occurred during the short 28-h period. PCA provided a more sensitive balance score that applied to all of the subjects.

Modeling balance control during sustained waking allows posturographic sleepiness testing.

We develop a method to quantify sleepiness. Sleepiness is a major risk factor in traffic and occupational accidents, but lack of convenient tests precludes monitoring impending sleepiness. Posturographic balance testing could address this need because sleepiness increases postural sway. It is, however, unclear how sleepiness influences balance control. Our results, for 12 subjects, show that balance control is more susceptible to increasing time awake (TA) compared to neuromuscular processes. This conclusion is reached since during sustained waking the control process slows down by 3.4% per hour of increased TA. This slowdown accounts for 65% of the variance in diurnal balance. We quantified balance control by modeling the body as an inverted pendulum and by expressing the control as the critical time interval for open-loop control (Deltat(c)) of the center-of-mass movements of this pendulum. To estimate the subjects TA, we regressed the Deltat(c) scores recorded during sustained waking against increasing TA, and equated separate Deltat(c) test scores with the diurnal Deltat(c) scores. We estimated TA with 68% positive predictive value. The results encourage implementing balance modeling into a device that performs clinical or industrial balance testing because the model-based Deltat(c) score responded to increasing TA.

Dual-trap optical tweezers with real-time force clamp control

Single molecule force clamp experiments are widely used to investigate how enzymes, molecular motors, and other molecular mechanisms work. We developed a dual-trap optical tweezers instrument with real-time (200 kHz update rate) force clamp control that can exert 0-100 pN forces on trapped beads. A model for force clamp experiments in the dumbbell-geometry is presented. We observe good agreement between predicted and observed power spectra of bead position and force fluctuations. The model can be used to predict and optimize the dynamics of real-time force clamp optical tweezers instruments. The results from a proof-of-principle experiment in which lambda exonuclease converts a double-stranded DNA tether, held at constant tension, into its single-stranded form, show that the developed instrument is suitable for experiments in single molecule biology.

Quantifying time awake posturographically

Although sleepiness is a major risk factor in traffic and occupational accidents, convenient, quantitative, and commercial sleepiness testing is lacking. The issue is relevant to policymakers concerned with legislation for, and surveillance of, traffic- and occupational safety. This work suggested and examined posturographic sleepiness testing for instrumentation purposes. In 63 subjects -for whom we tested balance with a force platform during sustained waking for maximum 36 h- sustained waking impaired the balance. The sustained waking explained 60% of the diurnal balance variations, whereas the time of day explained 40% of the balance variations. The first finding -that balance depends on the subjects time awake (TA)- allowed to posturographically estimate the subjects TA with 86% accuracy and 97% precision. Results also show that balance scores tested at 13:30 hours serve as a threshold to detect excessive sleepiness. This work provides guidelines for a posturographic sleepiness tester.

Optical tweezers reveal force plateau and internal friction in PEG-induced DNA condensation

The simplified artificial environments in which highly complex biological systems are studied do not represent the crowded, dense, salty, and dynamic environment inside the living cell. Consequently, it is important to investigate the effect of crowding agents on DNA. We used a dual-trap optical tweezers instrument to perform force spectroscopy experiments at pull speeds ranging from 0.3 to 270xa0μm/s on single dsDNA molecules in the presence of poly(ethylene glycol) (PEG) and monovalent salt. PEG of sizes 1,500 and 4,000xa0Da condensed DNA, and force–extension dataxa0contained a force plateau at approximately 1xa0pN. The level of the force plateau increased with increasing pull speed. During slow pulling the dissipated work increased linearly with pull speed. The calculated friction coefficient did not depend on amount of DNA incorporated in the condensate, indicating internal friction is independent of the condensate size. PEG300 had no effect on the dsDNA force–extension curve. The force plateau implies that condensation induced by crowding agents resembles condensation induced by multivalent cations.