Research Papers

Design of a Metered-Dose Inhaler Actuated by Shape Memory Alloy

[+] Author and Article Information
Brent Utter

Department of Mechanical Engineering,
Lafayette College,
Easton, PA 18042
e-mail: utterb@lafayette.edu

Contributed by the Applied Mechanics Division Technical Committee on Dynamics & Control of Structures & Systems (AMD-DCSS) of ASME for publication in the JOURNAL OF ENGINEERING AND SCIENCE IN MEDICAL DIAGNOSTICS AND THERAPY Manuscript received November 28, 2018; final manuscript received April 1, 2019; published online May 8, 2019. Assoc. Editor: David W. Kaczka.

ASME J of Medical Diagnostics 2(3), 031003 (May 08, 2019) (8 pages) Paper No: JESMDT-18-1061; doi: 10.1115/1.4043403 History: Received November 28, 2018; Revised April 01, 2019

The global prevalence of asthma and chronic obstructive pulmonary disease (COPD) is on the order of hundreds of millions of individuals. The most common treatment approach is to take a self-administered inhaled medication. This study focuses on pressurized metered-dose inhalers (MDIs) where, unfortunately, rates of mishandling and misuse are extremely high and lead to improper treatment. One significant challenge results from the timing miscoordination of the medicine dispersion and inhalation breath. To address this, this study demonstrates the feasibility of automating the timing of the medicine dispersion by integrating a shape memory alloy (SMA) actuator and a differential pressure sensor into the casing of a traditional MDI. The approach is to measure the vacuum pressure created by an inspiratory breath, evaluate criteria indicating an acceptable breath, and if those criteria are met, heat the SMA actuator to depress the cartridge and disperse medicine. To meet actuation requirements and reliably depress the inhaler cartridge, two concepts for configuring an SMA wire were designed and compared with respect to complexity, actuation timing, and energy consumption. The proposed concept was able to disperse medicine in 263 ms, averaged over 100 actuations on a single battery charge, facilitating the early dispersion of medicine during an inhalation breath. By describing the design process of an SMA-actuated MDI that does not result in a significant increase of its weight or size, this study provides a practical technological approach for reducing the improper treatment of asthma and COPD due to timing miscoordination.

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Fig. 1

Two architectures of the actuated inhaler. Two isometric views and a section view of the architectures show the main components of the actuated and instrumented MDIs. In both embodiments, the SMA wire is routed from the base of the inhaler housing, over the inhaler cap, and then back to the base of the other side of the inhaler housing. In the curved path embodiment, the SMA wire is also guided along a curved path by bearings affixed to both sides of the inhaler housing.

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Fig. 2

Diagram of electronic system. The electronic system measures the pressure within the inhaler mouthpiece and initiates the SMA to actuate when the criteria for a proper inhalation breath are met.

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Fig. 3

Graphical approach to specify SMA actuator length and diameter: (a) load and displacement of MDI cartridge, (b) stress–strain characterization of 0.38 mm (0.015 in) diameter wire prior to thermomechanical cycling, and (c) design space with 0.38 mm diameter wire

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Fig. 4

Effect of maximum wire stress and austenite free length on actuation stroke. Both embodiments can achieve the minimum required actuation stroke of 1.4 mm if the SMA wire is mounted such that the maximum wire stress is greater than 191 MPa for the straight path embodiment and greater than 187 MPa for the curved path embodiment. *The austenite free length of the wire is halved because the SMA wire actuator is configured such that it acts as two wires in parallel.

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Fig. 5

Effect of thermomechanical cycling on stress–strain curves of 0.38 mm diameter wire. A 0.38 mm diameter wire was cycled 1000 times between 1% and 4% strain at 210 MPa to determine if the manufacturer's recommended maximum stress of 172 MPa could be exceeded without leading to the wire's performance degradation (decreased actuation stroke).

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Fig. 6

Assembled prototype of curved path embodiment

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Fig. 7

Validation test of actuated and instrumented MDI, curved path embodiment. Pressure and cartridge displacement data were acquired for a typical actuation during a performance test of the curved path prototype. As programmed, a single pressure measurement below the threshold triggered the actuation of the SMA wire and the displacement of the inhaler cartridge. A pressure spike captured 620 ms after the threshold pressure was measured suggests the timing of medicine dispersion.

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Fig. 8

Actuation characteristics as function of actuation number: (a) time required to complete actuation, (b) power and energy drawn by SMA wire during actuation, and (c) per actuation electric charge depleted and total electric charge depleted



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