**2.7. Acoustic devices**

Acoustic droplet ejection (ADE) is a recent touch-less technology that surges in popularity in recent years. It adopts acoustic energy to propel droplets from various types of solutions with good precision (Ellson et al. 2003; Harris et al. 2008; Rudnicki and Johnston 2009; Shieh et al. 2006). The source plate remains stationary as the transducer and destination plate shuf‐ fle to allow for solution transfer from any well in the source plate to any well in the destina‐ tion plate, the latter one lying in an inverted position (Olechno et al. 2006). This system does not require any additional consumable other than microplates (Olechno et al. 2006), and it speeds up the process by avoiding washing steps and having the capability to prepare as‐ say-ready plates (Turmel et al. 2010)

### **2.8. Microplate washers**

Microplate washers are laboratory instruments designed to automate and expedite assay ap‐ plications, where a washing step is essential. They play an important role in areas such as high-content screening and enzyme-linked immunosorbent assays (ELISA). In 1990, Stobbs developed the first multiple plate washer using readily available materials as a low cost al‐ ternative to the commercially available plate washers of the era (Stobbs 1990). Over the years, fully programmable plate washers have been developed with numerous features. The development of automated plate washers has decreased the time required for laborious washing steps involved in many screening assays and improved reproducibility through standardized plate handling across multiple wash cycles (defined as a single dispense and aspirate step per cycle).

The two most critical components of a plate washer are a plate carrier and a manifold con‐ taining a number of fixed stainless steel needle probes for solution dispensing (Fig.1A). This manifold (or a separate manifold depending on the design) aspirates the liquid from the wells after an optional soaking period, leaving a pre-defined residual volume in the wells. A third component is the vacuum/pump assembly, which supplies the necessary pressure dif‐ ferential to drive efficient aspiration. Sunghou Lee first developed an additional vacuum fil‐ tration system integrated with a conventional plate washer to speed up the wash process for applications involving filter plates (Lee 2006). Some plate washers have a built-in magnet or a vacuum filtration module for handling bead-based assays.

Microplate washers can be categorized into two types: strip washers, which wash a single column or row of a plate at a time, and full plate washers (Rudnicki and Johnston 2009). The availability of 8-/12-/16-channel manifolds for strip washers provides both single strip wash‐ ing and full-plate washing capability in the same device, but at the cost of increased wash time for full plates. On the other hand, full plate washers with either a 96- or 384-channel manifold may be preferred for time-efficient wash operations (from a few seconds to a few minutes), but lack the flexibility of the 8-/12-/16-channel units.

The combination of plate washing and bulk dispensing features within the same device may be favored for a space-efficient solution. They are designed to dispense reliably low volumes and reduce prime volume (Rudnicki and Johnston 2009). A major advantage of the washerdispenser combination comes into play with assay protocols that require the direct addition of fluid after or between the washing steps, such as cell fixation or microplate surface coat‐ ing reagents.
