**3. MMs applications**

Besides all the aforementioned applications, MMs are promising candidates for micro-robotics and micro-electromechanical systems (MEMS). Nowadays, the CMMs researchers focus is on the microscale, where applying them in microrobotics and MEMS is spatially constrained. Moreover, literature reports endorsed those substantial applications of non-mechanical MM depend on the architecture microstructure.

Furthermore, MMs have been used in sensors and in micro-robotics as Negative permeability and permittivity of SRR arrays introduced them as promising sensors for molecule detection, deposit sensing, mechanical strain, temperature, gas detection and concentration [97].

The change in resonance frequency stimulates the detection, SRR is similar to IC-oscillator and the frequency is being calculated by:

$$\mathbf{f} = \mathbf{x} = \frac{1}{2\pi\sqrt{\mathbf{L}\mathbf{C}}}\tag{19}$$

Where L is inductance, C is capacitance of the narrow gap section.

Mechanical contortion changes the capacitance because of the geometrical change of the gap region. SRR was built with conductive polymer to sense the gas via the changes of the dielectric of the adsorption of the gas in the polymer [98]. (Publications of sensing SRR).

Also, CMMs are considered fundamental candidate for micro-robotics application due to the existence of anisotropy in the design which can be defined through the shape and the change in the material. A plenty of magnetic micro-robotics and chemical propulsion have been scrutinized [99–101]. These micro-robotics, have been applied in environmental cleaning devices [99, 100], drug delivery system and cargo transport [102, 103].

### **4. Composite metamaterials (CMMs) synthesis**

#### **4.1 Additives manufacturing**

Plethora of MM applications require microscale structure which complicates the manufacturing process. Additive manufacturing offers many benefits, such as decreasing the resource effort which develop sustainability, expedites design, prowess step from macroscale into nanoscale, and manufacturing on demand. This method is the most apt method in manufacturing CMMs, taking in consideration its simple batch manufacturing, modeling, research and development [104]. Thus, it is already applied in medical products, electronics, machining, aerospace and automobile who are possible users for the CMMs [105].

#### **4.2 Methods and size**

Additive and subtractive methods have been used in MM & CMMs production. Selective laser melting and sintering (SLM & SLS) are additive manufacturing methods that can be used in micromachining and established mesoscale. In

*Composite Metamaterials: Classification, Design, Laws and Future Applications DOI: http://dx.doi.org/10.5772/intechopen.100861*

microscale, subtractive methods with the necessary resolution have been scarce, besides the available methods like focused ion beam milling, has limited degree of freedom. Anyway, this method is critical to evolve a metamaterial with the desired characters. Whilst, negative Poisson ratio, mechanical linearity, zero thermal expansion and programmable mechanical MMs are size independent, while the other MM applications are size-dependent. Applications that depend on effect, such as artificial bandgap in acoustic and optics, SRR's resonance frequency require manufacturing in the appropriate size range. Moreover, microscale manufacturing is required in micro robotics and MEMS devices.
