From a fundamental perspective, transdermal drug delivery (TDD) has been limited by the skin's tough and lipid rich outer layer known as the stratum corneum (SC) thus rendering the skin impermeable to most biopharmaceuticals and small molecules (Erdő, Hashimoto, Karvaly, Nakamichi, & Kato, 2016). The anatomical structure of the skin is shown in Figure 1a (Erdő et al., 2016). The hydrophobic nature of the skin's outer layer limits the formulations for percutaneous delivery to be creams, gels, ointments and non-invasive transdermal patches (Pastore, Kalia, Horstmann, & Roberts, 2015). Due to the skin being a versatile and effective biological barrier possessing a heterogeneous anatomy and complex physiology (Figure 1a), there is a high variability in the pharmacokinetic properties of substances applied topically (Erdő et al., 2016). The SC lends itself as the rate-limiting step in the course of cutaneous penetration or transdermal absorption, and several systemic physiologically based models exist for assessing cutaneous absorption and transdermal delivery of xenobiotics from the pharmacokinetic and toxicokinetic aspects (Erdő et al., 2016). Molecules pass through the SC via three paths: transcellular, intercellular or appendageal routes, with most products reaching the viable epidermis via passive diffusion phenomena involved with intercellular absorption as shown in Figure 1b (Erdő et al., 2016). TDD systems are considered to be patient-friendly as they are non-invasive, do not need to be administered by professionals, decrease gastrointestinal (GI) adverse effects and increase patient adherence (Economidou, Lamprou, & Douroumis, 2018). Further, since they bypass the metabolic processes that are exhibited by oral administration, bioavailability, efficacy and translocation are improved. This also eliminates the use of invasive, irritating needles that generate medical waste, pose the risk of infection and need to be administered by medically trained professionals (Economidou et al., 2018). Some of the disadvantages associated with transdermal drug delivery include potential skin sensitization or irritation, discomfort from adhesives, imperfect skin adhesion, cost and selectivity for specific physicochemical drug properties

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The US Department of Defense (DoD) realizes the many uses of additive manufacturing (AM) as it has become a common fabrication technique for an extensive range of engineering components in several industrial sectors. 3D Printed (3DP) sensor technology offers high-performance features as a way to track individual warfighters on the battlefield, offering protection from threats such as weaponized toxins, bacteria or virus, with real-time monitoring of physiological events, advanced diagnostics, and connected feedback. Maximum protection of the warfighter gives a distinct advantage over adversaries by providing an enhanced awareness of situational threats on the battle field. There is a need to further explore aspects of AM such as higher printing resolution and efficiency, with faster print times and higher performance, sensitivity and optimized fabrication to ensure that soldiers are more safe and lethal to win our nation’s wars and come home safely. A review and comparison of various 3DP techniques for sensor fabrication is presented.