**2. Microneedles array fabrication**

Technologies, skills and facilities for Micro and Nano Electro Mechanical Systems (MEMS and NEMS) fabrication are the key elements for the development of new biomedical devices [1–3, 10]. Fabrication methods for Microneedles (MNs) strongly depend on the MNs shape, tip model, length, density of the MNs matrix, and the material of which they are made of.

Moreover, structural characteristics of the MNs matrix in turn depend on the specific application considered [11]. In fact, MNDs are exploited in fluid extraction [12] and *in-situ* diagnosis of diseases [13], in drug and gene delivery strategies [1, 11], in cell therapy [3] and so on.

At first, Silicon and silicon-based nanostructured materials, such as porous silicon, were largely employed in MNDs fabrication due to the well-established functionalization chemistry protocols and fabrication techniques, extenseively used in microelectronics, which simplified the integration into more complex systems [14]. However, silicon revealed to be a non-biocompatible material, due to its fragility and to the local inflammations (silicosis) it could provoke; for this reason its use has been limited in cell applications [15].

To overcome limitations on the use of silicon, polymers have been extensively proposed as alternative materials in many applications. Poly Dimethyl Siloxane

(PDMS) is one of the most used materials in microfluidics to design biomedical devices, due to its well-known biological compatibility [16]. Usually, PDMS is employed as mold to fabricate MNDs by replica molding (see **Figure 2**). In case of PDMS molding, the fabrication involves the following steps: female PDMS mold fabrication by means of standard photolithography or laser drilling; patterned MNs in PDMS mold filling with liquid polymers in vacuum conditions; curing of the polymers by temperature and/or UV exposure; mold removal; eventually, an additional curing step [16]. Biodegradable polymers have been largely employed in MNDs for drugs delivery application [20–24], but the biodegradability is not required for biosensing.

A direct method for MNDs fabrication is the so-called drawing lithography [19]. Drawing lithography is a fabrication method, which does not need light irradiation and a mask, since it is based on the use of a thermosetting polymer directly drawn from a 2D solid surface (see **Figure 2**). In drawing lithography, commercial photoresist is usually spin coated or drop casted onto the substrate and cooled down. Drills are fixed in an array on a PDMS frame and used as pillars contacted with the photoresist. Conical-shaped bridges between the substrate and the pillars appear when their relative distance is increased by drawing (elongation). The bridges are cured to generate a rigid structure. Finally, the separation of the bridges produces the desired MND.

However, drawing method lacks in flexibility and the curing at high temperature of the polymers encapsulating biopharmaceutical molecules can cause their denaturation or inactivation. In fact, MNDs encapsulating drugs or bioprobes must be fabricated in a controlled environment to preserve the biological activity.

The increasing demand for simple methods that preserve the biological activity by utilizing the natural properties of polymers has conducted to the idea of centrifugal lithography [17]. In [17], centrifugal lithography was used for the fabrication of MNDs in a single centrifugation, by exploiting the self-shaping properties

#### **Figure 2.**

*Main fabrication strategies for MNs fabrication. Replica molding [16] centrifugal lithography [17] photolithography [18] drawing lithography [19].*

*Theranostic Microneedle Devices: Innovative Biosensing and Transdermal Drugs Administration DOI: http://dx.doi.org/10.5772/intechopen.95050*

of hyaluronic acid (HA). Briefly, fabrication involves the following steps: HA drops encapsulating drugs molecules are casted onto the substrate; centrifugal force is applied under refrigerated conditions (4°C) to the droplets in order to shape in hourglass microstructures; finally, the mirroring shapes are separated to form MNs. Also in the case of HA, drug delivery is successfully obtained, but biosensing is unavailable due to its biodegradability.

On the other hand, hydrogel polymers are very attractive materials for MNDs and, generally, for biomedical devices, since a hydrated gel provides near physiological conditions. These gels are excellent encapsulation matrices for biological probes, such as enzymes and peptides [18, 20, 25, 26]. Moreover, the standard photolithographic processes can be employed to fabricate micrometric devices based on polymeric hydrogels [materials] (**Figure 2**). In [6, 18, 27, 28], authors proposed procedures of standard direct photolithography, where a mixture of Poly(ethylene glycol) diacrylate (PEGDA) and a commercial photoinitiator were used as an ordinary photoresist, without any etching step being required. In fact, PEGDA is a biocompatible polymer that solidifies at room temperature in presence of a photoinitiator after exposure to ultraviolet (UV) light for few seconds. In case of photolithographic process, the fabrication involves the following steps: the liquid photosensitive polymeric mixture is casted onto a UV-transparent substrate and exposed to ultraviolet radiation, in order to fabricate the MNDs base; a vessel is fulfilled with a second quantity of liquid mixture and the MNDs base is put on; a second exposure through a mask, whose pattern is an array of holes, is applied; finally the structure is developed by simply washing in deionized water. The PEGDA mixture can be customized to encapsulate a variety of drugs or sensing probes as biological molecules or inorganic nanoparticles [29–31].

Comparing the fabrication methods, all produced MNs have demonstrated high quality in indentation proof and a good grade of reproducibility, with some critical issues during the mold removal step in replica molding method.

#### **Figure 3.**

*The photolithographic methods offer a wide range of solutions for MNDs. Changing time exposure and/or photolithographic mask several configurations and arrays of MNs for both therapeutics and biosensing can be fabricated. From above: Mask type 1 (simple circle) enables MNs with several heights depending on time exposure; mask type 2 (ring) enables hollow MNs with height and closure depending on time exposure; mask type 3 (mismatched concentric ring) enables in one only exposure hollow MNs with a lateral oblique aperture, which is smaller as the exposure time increases.*

Finally, we highlight that the photolithographic approach allows the fabrication of MNDs for a wide range of applications. In fact, this process allows the design of a wide range of MN types with different shape, length and tip, simply by adjusting the exposure parameters or shape photolithographic masks [18, 28]. In **Figure 3**, the whole range of possibilities enabled by photolithographic method are summarized: mask type 1 (simple circle) enables MNs with several heights depending on time exposure; mask type 2 (ring) enables hollow MNs with height and closure depending on time exposure; mask type 3 (mismatched concentric ring) enables in a single exposure the fabrication of hollow MNs with a lateral oblique aperture as in hypodermic syringes. Also in this case, lateral aperture is smaller as the exposure time increases.
