Peter C. Y. Chen

Spiral microchannel with rectangular and trapezoidal cross-sections for size based particle separation

The paper reports a new method for three-dimensional observation of the location of focused particle streams along both the depth and width of the channel cross-section in spiral inertial microfluidic systems. The results confirm that particles are focused near the top and bottom walls of the microchannel cross-section, revealing clear insights on the focusing and separation mechanism. Based on this detailed understanding of the force balance, we introduce a novel spiral microchannel with a trapezoidal cross-section that generates stronger Dean vortices at the outer half of the channel. Experiments show that particles focusing in such device are sensitive to particle size and flow rate, and exhibits a sharp transition from the inner half to the outer half equilibrium positions at a size-dependent critical flow rate. As particle equilibration positions are well segregated based on different focusing mechanisms, a higher separation resolution is achieved over conventional spiral microchannels with rectangular cross-section.

A micromanipulation system with dynamic force-feedback for automatic batch microinjection

In this paper, we report the development of a prototype micromanipulation system for automatic batch microinjection of zebrafish embryos. Such automatic batch processing is made possible by (i) the development of a machine vision algorithm to identify the number of embryos in a batch and to locate the centerline of each embryo, (ii) the integration of a piezoresistive micro-force sensor with a micropipette to measure the penetration force of the embryo in real time and (iii) the synthesis of a position control with dynamic force feedback by exploiting the characteristics of the force profile associated with the microinjection process. The effectiveness of this prototype micromanipulation system has been demonstrated in an experiment. The experimental results demonstrate that the technique of position control with dynamic penetration-force feedback is practicable for automatic batch microinjection applications.

Force Sensing and Control in Micromanipulation

In micromanipulation, the size of the manipulated object is usually much less than 1 mm in a single dimension, in which case gravitational and inertial forces are no longer dominant. This leads to problems (for manipulation through force) that are not evident in the macroworld, and for which the macroworld techniques alone may not be adequate to provide solutions. This paper surveys critical issues and their available solutions related to force control in micromanipulation. It focuses on: 1) techniques for dealing with adhesion forces and 2) methods for force sensing and control

Focal length calibration from two views: method and analysis of singular cases

We consider the problem of estimating the focal length of a camera from two views while the focal length is not varied during the motion of the camera. An approach based on Kruppas equations is proposed. Specifically, we derive two linear and one quadratic equations to solve the problem. Although the three equations are interdependent in general, each one may be singular for different configurations. We study in detail the generic singularities of the problem and the actual singularities of the individual calibration equations. Results of our experiments using synthetic and real data underline the effect that singular configurations may have on self-calibration. However, these results are stable once the singularities are avoided.

New extensometer to measure in vivo uniaxial mechanical properties of human skin

Biomechanical properties of skin are important for clinical decision making as well as clinical intervention. Measuring these properties in vivo is critical for estimating dimensional behaviour of skin flap or graft after harvest. However, existing methodologies and devices often suffer from lack of standardisation and unwanted peripheral force contribution due to the deformation of surrounding tissues during measurement. This naturally leads to measurement inaccuracies and lack of reproducibility. In order to improve the measurement accuracy, a new portable extensometer, which measures the non-invasive in vivo biomechanical properties of skin, has been designed and constructed. This design incorporates three pads that attach to the skin, including a C-shaped pad to shield the force sensor from peripheral forces. Such design produces data that are significantly closer to in vitro measurements. The results have been verified by finite element analysis, and experiments on rubber sheets and pig skins. This device can be used to obtain biomechanical properties of skin that will aid doctors in measuring skin elasticity and surgical planning, especially in skin flap surgery.

Real-Time Supervisory Control of a Processor for Non-Preemptive Execution of Periodic Tasks

In this article, a method for scheduling a processor for non-preemptive execution of periodic tasks is presented. This method is based on the formal framework of supervisory control of timed discrete-event systems. It is shown that, with this method, the problem of determining schedulability and the problem of finding a scheduling algorithm are dual since a solution to the former necessarily implies a solution to the latter and vice versa. Furthermore, the solution to the latter thus obtained is complete in the sense that it contains all “safe” sequences of task execution with the guarantee that no deadline is missed. Examples are described to illustrate this method. Implication of the results and computational complexity associated with this method are discussed.

A Markovian approach to the control of genetic regulatory networks.

This paper presents an approach for controlling gene networks based on a Markov chain model, where the state of a gene network is represented as a probability distribution, while state transitions are considered to be probabilistic. An algorithm is proposed to determine a sequence of control actions that drives (without state feedback) the state of a given network to within a desired state set with a prescribed minimum or maximum probability. A heuristic is proposed and shown to improve the efficiency of the algorithm for a class of genetic networks.

A force-feedback control system for micro-assembly

In this paper, we report the development of an explicit force-feedback control system for micro-assembly, focusing on the key issues of force transmission and control. The force-feedback system is incorporated with a compound flexure stage, which is driven by a voice-coil actuator and designed to provide frictionless translation motion along one axis. A force sensor measures the interaction force between the micromanipulator and its environment, while an explicit force controller controls the interaction force to follow a desired force trajectory. The effectiveness of this prototype force-control system has been demonstrated in an experimental application, where parts (with dimensions in microns) were picked up and assembled under explicit force-feedback control.

The feasibility of using thermal strain imaging to regulate energy delivery during intracardiac radio-frequency ablation

A method is introduced to monitor cardiac ablative therapy by examining slope changes in the thermal strain curve caused by speed of sound variations with temperature. The sound speed of water-bearing tissue such as cardiac muscle increases with temperature. However, at temperatures above about 50°C, there is no further increase in the sound speed and the temperature coefficient may become slightly negative. For ablation therapy, an irreversible injury to tissue and a complete heart block occurs in the range of 48 to 50°C for a short period in accordance with the well-known Arrhenius equation. Using these two properties, we propose a potential tool to detect the moment when tissue damage occurs by using the reduced slope in the thermal strain curve as a function of heating time. We have illustrated the feasibility of this method initially using porcine myocardium in vitro. The method was further demonstrated in vivo, using a specially equipped ablation tip and an 11-MHz microlinear intracardiac echocardiography (ICE) array mounted on the tip of a catheter. The thermal strain curves showed a plateau, strongly suggesting that the temperature reached at least 50°C.

Models of maximum stress and strain of zebrafish embryos under indentation

Micro-injection of zebrafish embryo is widely applied in biology for the analysis of early developmental processes. The success of a micro-injection to a large extent depends on the mechanical interaction between the micro-pipette and the membrane of the zebrafish embryo. In this paper, we present the development of (i) a maximum stress model of the deformed membrane with respect to the depth of indentation, (ii) a family-of-conics elongation model to determine the length of the deformed membrane for the estimation of the maximum strain at a given indentation depth, and (iii) an experimental system to generate the required data for these two models. The significance of these results is that the estimated maximum stress provides a performance target for the penetration process, while the estimated corresponding maximum strain serves as an indicator of the extent of deformation sustained by the embryo prior to penetration. Implications of these modeling and experimental results are discussed in the context of optimizing the process of micro-injection.