Microfluidic separation technology has garnered significant attention over the past decade where particles are being separated at a micro/nanoscale in a rapid, low-cost, and simple manner. Amongst a myriad of separation technologies that have emerged thus far, acoustic microfluidic separation techniques are extremely apt to applications involving biological samples attributed to various advantages, including high controllability, biocompatibility, and non-invasive, label-free features. With that being said, downsides such as low throughput and dependence on external equipment still impede successful commercialization from laboratory-based prototypes. Here, we present a comprehensive review of recent advances in acoustic microfluidic separation techniques, along with exemplary applications. Specifically, an inclusive overview of fundamental theory and background is presented, then two sets of mechanisms underlying acoustic separation, bulk acoustic wave and surface acoustic wave, are introduced and discussed. Upon these summaries, we present a variety of applications based on acoustic separation. The primary focus is given to those associated with biological samples such as blood cells, cancer cells, proteins, bacteria, viruses, and DNA/RNA. Finally, we highlight the benefits and challenges behind burgeoning developments in the field and discuss the future perspectives and an outlook towards robust, integrated, and commercialized devices based on acoustic microfluidic separation.

Purpose: Recently, the constrained surface projection stereolithography (SL) technology is gaining wider attention and has been widely used in the 3D printing industry. In constrained surface projection SL systems, the separation of a newly cured layer from the constrained surface is a historical technical barrier. It greatly limits printable size, process reliability and print speed. Moreover, over-large separation force leads to adhesion failures in manufacturing processes, causing broken constrained surface and part defects. Against this background, this paper investigates the formation of separation forces and various factors that affect the separation process in constrained surface projection SL systems. Design/methodology/approach: A bottom-up projection SL testbed, integrated with an in-situ separation force measurement unit, is developed for experimental study. Separation forces under various manufacturing process settings and constrained surface conditions are measured in situ. Additionally, physical models are constructed by considering the liquid resin filling process. Experiments are conducted to investigate influences of manufacturing process settings, constrained surface condition and print geometry on separation forces. Findings: Separation forces increase linearly with the separation speed. The deformation and the oxygen inhibition layer near the constrained surface greatly reduce separation forces. The printing area, area/perimeter ratio and the degree of porousness of print geometries have a combined effect on determining separation forces. Originality/value: This paper studied factors that influence separation force in constrained surface SL processes. Constrained surface conditions including oxygen inhibition layer thickness, deformation and oxygen permeation capability were investigated, and their influences on separation forces were revealed. Moreover, geometric factors of printing layers that are significant on determining separation forces have been identified and quantified. This study on separation forces provides a solid base for future work on adaptive control of constrained surface projection SL processes.