**1. Introduction**

Liver, kidney, brain, thyroid scans, imaging of bone lesions, and localization of myocardial infarctions are just some of the basic diagnostic tests performed daily with technetium-99m (99mTc), a gamma ray emitting radioisotope (Eγ = 140 keV, t1/2 = 6 h) which covers over 85% of diagnostic applications in Nuclear Medicine. Technetium-99m is an everlasting radionuclide which has seen the birth of Nuclear Medicine, and the recent advances in technetium chemistry and detector technologies make it still modern and competitive with trendy PET radionuclides [1]. In the late 2000s, a recurrent lack of availability of 99mTc in the hospitals due to a global shortage of molybdenum-99 (99Mo) because of frequent shutdowns of the ageing reactor-based <sup>235</sup>U fission production chain, revealed the fragility of the traditional supply chain and, consequently, prompted the research community to look for alternative production routes.

Strong commitment has been devoted to the cyclotron-based direct production of 99mTc, through the <sup>100</sup>Mo(p, 2n)99mTc nuclear reaction, as a valuable alternative [2–12]. Accurate studies on the cross section of this reaction and of collateral nuclear reactions have determined the optimal energy range (15–24 MeV) to maximize the production of 99mTc, ensuring a radionuclidic purity level suitable for clinical applications [13]. This energy range is covered by most of the conventional medical cyclotrons already in operation in many hospital radiopharmacies, which can therefore be used for the production of 99mTc on site ensuring a constant and on demand supply, without the aid of nuclear reactors.

**Citation:** Martini, P.; Uccelli, L.; Duatti, A.; Marvelli, L.; Esposito, J.; Boschi, A. Highly Efficient Micro-Scale Liquid-Liquid In-Flow Extraction of 99mTc from Molybdenum. *Molecules* **2021**, *26*, 5699. https://doi.org/ 10.3390/molecules26185699

Academic Editor: Makoto Tsunoda

Received: 7 September 2021 Accepted: 17 September 2021 Published: 21 September 2021

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Other reactor-based routes have been investigated based on the indirect production of 99mTc induced by thermal or fast neutron beams, respectively through the <sup>98</sup>Mo(n,γ) <sup>99</sup>Mo and <sup>100</sup>Mo(n,2n)99Mo nuclear reactions, or gamma-ray beam <sup>100</sup>Mo(γ,n)99Mo [11,14,15]. These routes are all affected by low specific activity molybdenum, being products and targets made of the same element material, thus complicating the generator-like extraction and separation process required for the isolation of high purity 99mTc, since it would require large columns to adsorb molybdenum, decreasing the radioactive concentration of 99mTc obtained to unacceptably low levels [16–18].

The extraction and purification of 99mTc from the irradiated metal target is a key step in the production cycle, necessary to make the radioisotope suitable for further radiopharmaceutical processing and patient injection. This process must be simple, reproducible, and efficient, in order to rapidly supply a radioisotope with high purity. The automation of this process is essential to ensure these requirements and to minimize operators' radiation exposure.

In this regard and in the framework of the LARAMED project of the Legnaro National Laboratories of the INFN, we developed an automatic module for the extraction, separation, and purification of cyclotron-produced 99mTc from the molybdenum metal target, based on the solvent extraction technique [8,9,19]. This system exploits a helium-bubbling into a separation column to boost solvent extraction of technetium into a biphasic system, composed of an alkaline aqueous solution (containing Tc, Mo and contaminants) and an organic phase of methyl ethyl ketone (MEK). The developed system, although very efficient, may however be improved in terms of processing times and costs. Indeed, both dissolution and extraction-separation are the time-consuming steps of all procedures (about 20 and 30 min respectively, over 60 min total). Moreover, the developed module consists of an assembly of commercially-available modular units which are overall quite expensive (i.e., in the range 50–80 k€).

The current trend in technology aiming to achieve even more compact systems is leading to the development of micro-scale reactors (lab-on-chip) in the field of radiochemical separation and radiopharmaceutical production, in order to improve performance and minimize chemical and radiological risks [20–31]. In this view, a latest generation device, the membrane-based Liquid–Liquid separator, 10 September (Figure 1), patented and produced by ZAIPUT Flow Technologies company (Cambridge, MA, USA), has been recently used for the radiochemical separation of radioisotopes of nuclear medical interest [23,29,30,32], for the miniaturization of liquid–liquid extraction processes in an in-flow chemistry regime. This device allows two immiscible phases to be effectively separated by exploiting interfacial tension and the affinity of one of the two phases for a microporous hydrophilic or hydrophobic polytetrafluoroethylene membrane (PTFE). Finally, thanks to a self-regulating differential pressure applied inside the device by a diaphragm, the separation can take place continuously [32].

**Figure 1.** Picture (**a**) and scheme (**b**) of the ZAIPUT separation device.

The in-flow extraction process carried out at the micro-scale level creates some advantages when compared to similar processes performed on a macro-scale, including: shorter extraction times, an increase in the mass transfer coefficient, a better surface-volume interface ratio (S/V), etc. [31,33–36].

The aim of this work is to test the efficiency of the solvent extraction and separation process of technetium from molybdenum in an in-flow chemistry regime with the aid of the Zaiput separator. A versatile separation system that would allow for minimization of costs, times, dimensions, and volumes involved in the process, as well as being applicable also to 99mTc indirect production methods, such as from low specific activity reactor-produced <sup>99</sup>Mo, where the solvent extraction remains the best extraction-separation method [16,37].
