## 1 Introduction  
The emergence of micro-electromechanical systems (MEMS) was regarded as the most significant milestone in the last century. By integrating electronic and mechanical elements such as sensors and actuators on a common substrate with advanced microfabrication capabilities, MEMS can provide us with ever smarter, smaller and cheaper devices [1]. As one type of MEMS, microfluidics refers to the fluids' manipulating technology based on channels with a dimension of tens to hundreds of micrometers [2]. Compared with traditional fluid handling technology, microfluidics can miniaturize the device sizes, reduce the reagent amounts, and achieve high sensitivity. Besides, the scale-dependence of fluid behaviors in microfluidics can bring numerous application opportunities [3]. Over the past decades, microfluidics has spread widely to chemical, biological, and biomedical research areas and related industries. However, effective and reliable interconnections between microfluidics and the macro world are still the technical challenges for its continuing development. To realize a stable world-to-chip interface, ideal fluidic interconnection should meet the following requirements: minimal dead volume, easiness of plugging/unplugging, reliability at high pressures, low-cost and straightforward techniques, and compatibility with commercial tubes and fittings [4]. Due to various materials and fabrication techniques for microfluidic chips, there are different interfacing solutions specific to the target applications [5]. The earliest materials introduced to microfluidic manufacturing are silicon and glass [6]. Benefiting from mature microfabrication technology, complex and precise microchannel networks could be easily defined. Deep reactive ion etching (DRIE) interlocking structures [7; 8] have been reported for the interconnections of silicon-based microfluidic devices. They are reversible, adhesive-free, geometric versatile, enabling high density but expensive. Silicon-based microfluidic devices provide well-characterized surface properties and high qualities. Nevertheless, they are not as attractive as polydimethylsiloxane (PDMS)-based and thermoplastic-based microfluidic devices because of the high cost [9]. With the emerging of PDMS in the early 2000s [10], the straightforward fluidic interconnects for PDMS-based microfluidic devices were developed. Commercial tubing or needle could be directly inserted into holes punched on PDMS by compression sealing [11; 12]. They are easy to plug/unplug, adhesive-free, reversible, and compatible with moderate pressures. PDMS is an excellent material for rapid prototyping and simplified world-to-chip interfaces. However, it still has drawbacks such as short-term stability after surface treatments, swelling in organic solvents, water permeability, and incompatibility with very high pressures [13]. Besides, PDMS is not the right choice for large scale production due to molding and curing processes. Thermoplastic and paper were preferred as low-cost, robust, and mass-producible alternatives in microfluidic industries [14; 15]. New approaches were proposed for thermoplastic-based microfluidic devices. For example, ultrasonically welded connectors were demonstrated in recent years [16; 17]. They are mechanically stable, capable of rapid implementation and large-scale production but face risks of clogging. Additionally, there are some packaging strategies compatible with different materials. One of the most common methods is adhesives-based gluing [18; 19]. It can enhance the robustness and reliability of fluidic connections. The main challenge of using adhesives is the clogging to the connecting tubing and microfluidic channels. Another type is the designed fixtures or frames for applying a significant compression force to ensure a leak-free connection. Press-fit interconnects [20; 21; 22], screw fittings [23], vacuum fixtures [24], and magnetic connectors [25; 26] have been developed.