# In-Plane Material Handling: A Systematic Review
## 3 Results
### Answer to RQ2  
This subsection focuses on the answer to the second research question (RQ2), i.e., the resulting categorization of these manipulative systems. Considering what has already been said in the previous subsection (Section 3.2), the classification of these devices can be realized based on the following features:  
* Employed technology employed;
* Transportable material (geometry, size, weight, and contact sensitivity);
* Field of application. The first term of distinction is one of the most evident outcomes from RQ1 analysis, i.e., the employed technology or, in other words, the physical working principle applied in each implementation. The possible variations identified for this technology cluster are MEMS, vibrations, cilia, pneumatic, variable morphology, the use of rotors, and mobile platforms. The second and third differentiation features are strictly related to the first one and to each other. The transportable material is affected by geometry, size, weight, and contact sensitivity, depending on the limitations of the technology. Specifically, the geometry differs between bodies with at least one large, flat face (\"planar\") and bodies with a more curved shape (\"curved\") when in contact with the device. The sizes of spaces between micro-, meso-, and macroscopic dimensions are referenced as follows: micro \(<10\) mm, \(10\) mm \(<\) meso \(<10\) cm, and macro \(>10\) cm [73]. The weight is distinguished as heavy (\(>1\) kg) or light (\(<1\) kg), while contact sensitivity refers to contactless transport capability (\"yes\" means contactless transport allowed, and \"no\" means contactless transport is not allowed). Similarly, the field of application is based on the previous two characteristics and differences between the intralogistics, biomedical, manufacturing, and research fields. With regard to this topic, the illustrations presented in Figure 6 schematize the different technologies and working principles. Under each device's representation, we summarize the main features of the transportable material and the field of application. As a result, the following paragraphs present the features and the corresponding research for each technology, one by one. The first devices illustrated in Figure 6 are MEMS devices, which, given their intrinsic dimensions, are suitable for micro and lightweight applications [74; 75] on planar objects, like for the micro-assembly of electronic components [6; 27]. Transport is achieved using contact friction, while the main implementations, in addition to micro-assembly, are micro-manufacturing and research. Among the retrieved publications on the subject, the majority propose a repeated pattern of four cantilevers made of polyimide [25; 38; 76] or silicon [6; 18; 19; 27] to manipulate parts, mimicking the ciliary motion of some living organ isms [25, 38]. On the other hand, a few researchers [74, 77] did not specify the actuators' layout, as they focused on the module's novel self-assembly [74] manipulation control [77]. The vibrating surfaces presented in Figure 6 appear to span a wider range of transportable dimensions according to the specific solution employed. However, planarity and lightness remain essential for the correct manipulation of the object. Additionally, contact is fundamental for motion, and the main fields of application are manufacturing and research, as as for MEMS. Other key distinctions between the analyzed solutions include the types of vibrations, i.e., longitudinal or transversal to the plane, and modularity. Referring to these attributes, many studies have presented a single plate (not a modular solution) driven by rotary [51, 78] or linear [19, 28, 29, 79] actuators to move mesoscopic and macroscopic objects through longitudinal [29, 51, 78, 79] or transversal [19, 28] waves. In [20, 80], two modular solutions using transversal waves were presented.