Byong-Ho Park

Scalable rotary actuators with embedded Shape Memory Alloys

Scalable rotary actuators are being developed with the help of Shape Memory Alloy (SMA) wires. Subjected to elevated temperatures SMA wires deliver large strains and high stresses. Yet their bandwidth is limited because of the time required for cooling. Bandwidth can be enhanced by miniaturizing the actuating system, which results in higher cooling rates. Using SMA wires with diameter of about 100 micron, several miniature actuators have been designed and fabricated. For example, a rotary joint with bi-directional actuation, which is about 4mm wide, 9mm long and 3mm high, can achieve angular deflection of around +/- 60 deg. Based on this bi-directional actuator, SMA ratchet mechanisms with minimum feature sizes of about 200 micron have been developed. The ratchet mechanisms can transform angular motion to continuously rotating motion. SMA wires are embedded into polyurethane structures using Shape Deposition Manufacturing (SDM). SDM is a layered manufacturing process capable of building complex 3D parts through the combination of material addition and subtraction. A key advantage of this process is the ease by which process interruption may occur to embed sensors and actuators into components. SDM can also fabricate functional mechanisms in an assembled configuration eliminating the need for assembly. The present article will describe design and manufacturing processes which enable the fabrication of the scalable rotary actuators. System performance and behavior of the SMA actuators will be also discussed in this contribution.

Laser-Machined Shape Memory Alloy Actuators for Active Catheters

Medicine is undergoing a transformation toward minimally invasive surgery, and with it comes an increasing need for more precise miniature instruments to accurately execute complex procedures. In catheter-based surgeries, the physician must control the tip of the catheter from a distal point outside the body, which makes it difficult to achieve precise manipulative control. Providing actuation local to the desired point of manipulation will greatly improve the physicians ability to intervene on diseases in a minimally invasive fashion. We present a new actuator made from laser machining shape memory alloy (SMA) tubes for use in an active steerable catheter. Using finite-element analysis and experimental verification, we have designed a 1.5-mm-long SMA actuator cut from 1.27-mm-diameter NiTi tubing that exhibits good fatigue properties, and can produce forces of 12 N at 20% elongation. We use an iterative rapid design process, so that many actuator geometries can be tested quickly to achieve the desired mechanical properties without complex material models. In this paper, we describe the design and testing of the actuator, and verify its force, elongation, and fatigue properties.

Design and Fabrication of Tubular Shape Memory Alloy Actuators for Active Catheters

There is a growing trend in medicine toward minimally invasive surgery, and with it comes an increasing need for precise miniature instruments to achieve accurate positioning for complex procedures. Catheter-based surgeries in particular suffer from a lack of active steering of the interventional device. We present a new actuator made by laser machining shape memory alloy (SMA) tubes for use in an active steerable catheter. Using finite element analysis and experimental verification, we have designed an SMA actuator cut from 1.27 mm diameter NiTi tubing that exhibits good fatigue properties and can produce forces up to 2 N at 20% elongation. In this paper, we describe the design and testing of the actuator, as well as its characterization to verify its mechanical properties

Part strength improvement in polymer shape deposition manufacturing

Shape deposition manufacturing (SDM) is a layered manufacturing process which iteratively combines material addition and removal to create artifacts in a variety of materials. Castable thermoset resins have been used to build a variety of parts via polymer SDM. The strength of these parts is determined by the bulk material properties of the part materials and by their interlayer adhesion. Early polyurethane materials had high bulk strength but poor interlayer adhesion, resulting in weak multilayer parts. Interlayer strength improvements were achieved through additional processing steps or the use of different polyurethane and epoxy part materials. These improvements allowed the fabrication of aerodynamic flap mechanisms used in wind‐tunnel testing. These parts are examples of the intricate, functional mechanisms to which the polymer SDM process is ideally suited.

A microfabricated intravascular ultrasound scanner for intravascular interventions

Minimally invasive medical therapy can reduce both healthcare costs and patient suffering. The development of submillimeter scale instruments falls in a gap of manufacturing technologies between traditional machining and microfabrication techniques. To address this need we have developed a fabrication technique based upon laser machining of tubular structures combined with shaped-memory alloy actuators to create compliant devices for minimally invasive interventions. The initial application of this approach has been to develop a forward viewing intravascular ultrasound scanner for use in guiding intravascular interventions in situations where traditional angiography and intravascular ultrasound are unable to provide adequate guidance. The ultrasound device is less than 1.5 mm in diameter and provides imaging at 20 frames per second. Imaging currently is performed with a 20 MHz 800 micron diameter transducer producing axial resolutions of approximately 150 microns. Device optimization has resulted in peak strains of less than 1% within the compliant structure resulting in device life greater than 200,000 cycles providing usable times greater than twice the anticipated procedure length. The design concepts embodied in this initial implementation will serve as a platform for a variety of self actuated minimally invasive tools.