**1. Introduction**

A new type of biomedical devices was born when technologies and facilities for Micro and Nano Electro Mechanical Systems (MEMS and NEMS) fabrication have met medical and biological issues. These new kinds of biomedical devices are able to easily control physical and chemical parameters at a very small scale, down to nanomolar concentrations and nanometric sizes [1–3]. Moreover, the integration of such a device in wearable and/or mobile systems gave them popularity among commercial devices. In this technological frame, Microneedles based devices (MNDs) were born. Their height is sufficiently large to overcome the outer natural barrier of the human body, the stratum corneum of the skin, but not enough to reach the nerves, resulting in a lack of pain [4]. Usually, MN height is ranging from 10 to

1000 μm, depending on the application and how deep in the epidermis is the specific target analyte. Then, MNs based devices act as an interface between the body of the patient and a biomedical device, whose applications can range from fluid extraction for ex-situ analysis to drug and gene delivery, from *in situ* diagnostic tools to targeted cell therapy [1–4]. Material, length, shape of the body and the tip of the MNs drastically vary depending on the application [2, 3] and the fabrication technology, according to new needs and challenges. Biosensing systems based on MNs have to overcome the stratum corneum without pain and to detect the target

#### **Figure 1.**

*(A) Sketch of different MNDs for sensing purposes together with their working conditions into the human skin. Starting from the right, they are characterized by the locus of probe-analyte interaction: Swelling bulk MNs sensors (BMNDs), where probe-analyte interaction is inside the volume of MN; hollow MNs sensors (HMNDs), where a small material sampling of ISF is analyzed on or offline; coated MNs sensors (CMNDs), whose surface is the locus of the interaction between analytes and bioprobes; planar MNs sensors (PMNDs), where the probe–analyte interaction is on a specific zone of a flat MNs surface. (B) some configurations for MNs for drugs delivery: (from left) hollow MNs present a inner cavities to immediately administration of high dose and high MW drugs; soluble and hybrid MNs for fast administration with a high doses and medium MW; coated MNs for fast administration of low doses and any MW; swelling MNs for very slow administration of high doses of smaller molecules. Reproduced with permission of Ref. [8].*

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

analytes directly in the interstitial fluid. In those cases, more than a strategy could be used: the functionalization of the surface of MNs with a specific probe, realizing a coated MND [5], the trapping of probe molecules into a swelling material [6] or the extraction of fluids and the analysis into a microfluidic system [7] are only some examples. In **Figure 1A** a sketch of different MNDs for sensing purposes is shown.

Moreover, microneedle-based devices (MNDs) can combine diagnostic sensing and therapeutic administration of drugs in one single tool. From this point of view, more than a painless door to the human body, a MND represents the a perfect example of theranostic instrument, since a single device could quantify the real value of a relevant biomolecule, such as glucose, and accurately deliver a drug, the insulin, if needed. MNDs are particularly interesting as simple drug administration tools, too. In fact, the transdermal route for drug administration is a very fascinating way, not only for the very low invasiveness and the easiness of self-administration, but also for the absence of first pass metabolism. However, the intercellular lipid matrix of the epidermis consists of ceramides, free fatty acids, and cholesterol, a complex mixture of neutral lipids arranged as bilayers with hydrophobic chains facing each other (lipophilic bimolecular leaflet) [9]. Transdermal delivery works only for lipophilic uncharged drugs with low MW (<500 Da), which need low dose and continuous delivery. Moreover, components, formulations and drugs must be non-irritating and non-sensitizing. MNs can be used with both lipophilic and hydrophilic formulations, both charged and uncharged drugs, both small and oversized molecules.

For all these cases, MN configurations are illustrated in **Figure 1B**, where the possibility to use solving or hybrid soluble/insoluble MNs are considered.

MNDs could be integrated on printed circuit boards, flexible electronics and microfluidic channels, thus allowing a continuous monitoring of the physiological parameters with very low invasiveness, together with sustained and localized administration of drugs. MNDs can be designed for very specific applications, from the detection of skin cancer to the monitoring of metabolic pathways.
