**6.1. Pathogenesis of cognitive impairment**

The mechanisms by which chemotherapy-induced cognitive impairment are unclear [96].

Chemotherapy may reduce the number of neural stem/progenitor cells, which have role in memory and learning ability [97], and neural precursor cells are chemo-sensitivity, neural stem, and line age-restricted progenitor cells that form, among other cell types, the myelinat‐ ing oligodendrocytes in the frontal white matter [98].

Primary pathological lesions including demyelination, inflammation, and microvascular injury [99].

Multiple chemotherapeutic agents affect hippocampal neurogenesis causing decrease in cell proliferation within the germinal region of the hippocampus and development of cognitive deficit [101].

Genetic role in chemotherapy-induced cognitive decline may be impilicated, and there is increased risk of cognitive impairment after RT with the apolipoprotein E4 alleles [102].

#### **6.2. Cognitive impairment of chemotherapeutic agents**

Chemotherapy added to slow the tumor progression especially in children to postpone radiation therapy or to reduce the dose of radiation therapy to decrease the neurocognitive sequelae of increasing doses of RT. Neurotoxic side-effects of chemotherapy alone can be difficult, because most patients of brain tumor have been treated with RT and chemotherapy [103].

The evidence that chemotherapy alone causes neurocognitive effects is not consistent. Studies have concluded that chemotherapy effects are negligible and not clinically significant compared to craniospinal irradiation (CSI) [104].

Neurotoxicity of chemotherapy arises during, or shortly after, chemoterapy. RT causes disturbance in BBB, so the toxicity of chemotherapy increased when was given during or after RT. In these cases, the chemotherapeutic drugs reach higher concentrations in brain tissue because of leakage of the blood–brain barrier due to RT. Intrathecal chemotherapy has higher CNS toxicity compared to systemic chemotherapy [105].

Chemotherapeutic agents, such as BCNU, CDDP, cytosine arabinoside, and intrathecal or intravenous methotrexate, have toxic effect to the CNS. Chemotherapy-related cognitive impairment in primary CNS lymphoma was observed in one or more domains: (attention, executive function, memory, psychomotor speed, and language). Other studies have shown that cognitive stability or cognitive improvement during chemotherapy provided that the tumor was responsive to chemotherapy treatment [106, 107]. Uses of high-dose IV methotrex‐ ate or interathecal methotrexate with radiation therapy result in dementia particularly when the radiation is given prior to the methotrexate. Leukoencephalopathy more commonly occurs. MRI shows bilateral periventricular white matter changes. The radiation therapy disrupts the BBB and results in increased permeability of the white matter to the methotrexate [108].

Copeland et al. [109] concluded that chemotherapy had only a slight effect on neurocogni‐ tive status and was confined to perceptual motor skills with observed age effect on perform‐ ance IQ.

Chemotherapeutic agents, such as BCNU, cisplatin, and cytarabine, have proved to be more toxic to neural precursor cells than cancer cells [110]. Carmustine, methotrexate, and cytara‐ bine have been found to induce central neurotoxicity to neural stem cell populations located in the subventricular zone and dentate gyrus [99].

Prabhu et al. [111] conducted a study on LGG patients and concluded that the addition of chemotherapy procarbazine, lomustine, and vincristine (PCV) to RT for LGGs did not result in significant MMSE score decline when compared to RT alone.

Regarding the HRQOL, there is a short-lasting negative impact of PCV chemotherapy on HRQOL during and shortly after treatment, but no long-term effects on HRQOL have been established [112].

Patients with previously untreated anaplastic astrocytoma, OA, or oligodendroglioma were evaluated for the long-term efficacy and safety of accelerated fractionated RT combined with intravenous carboplatin. In a phase II study conducted by Levin et al. [113], they found that after RT, patients received procarbazine, lomustine (CCNU), and vincristine (PCV) for 1 year or until tumor progression, 10% of those patients developed serious clinical neurologic deterioration and/or dementia requiring full-time caregiver attention.

Hilverda et al. [114] reported that glioblastoma patients undergoing RT with concomitant and adjuvant temozolomide treatment did not develop cognitive deterioration.

In LGG patients, temozolomide is not only successful in terms of extending the survival duration but also has been proven to maintain or even improve HRQOL while patients are on treatment [115].

Patients with recurrent high-grade glioma (HGG), successfully treated with temozolomide, achieved significant improvement in the HRQOL domains, whereas patients with disease progression had significant deterioration in most HRQOL domains [116].


*4.3. Electrochemical Reactors*

The coenzyme nicotinamide adenine dinucleotide is required as a co-substrate for over 300 dehydrogenases [141]. A number of studies have been carried out to use electrochemical methods to regenerate the cofactor. Yoon et al. prepared an electrochemical laminar flow microreactor to regenerate NADH in the synthesis of chiral L-lactate from the achiral substrate pyruvate (Figure 4). For this purpose, gold electrodes were deposited on the inside of a Y-shaped reactor, with two separate streams, one with buffer and the second with reagents (FAD, NAD+, enzymes and substrate), with the flow directed to the cathode. The reduced cofactor, FADH2 was produced at the cathode and used for the reduction of NAD+ to NADH, resulting in a 41% yield of L-lactate (theoretical yield of 50%) [142].

**Figure 4.** Schematic diagram of a laminar Y-shaped microreactor used for the electrochemical regeneration of NAD+ using formate dehydrogenase (FDH) [142].

A filter-press microreactor with semi-cylindrical channels on the electrode surface resulting in high surface areas was used to electrochemically regenerate FADH2 (Figure 5) [143].

**Figure 5.** A schematic of a filter-press microreactor. Reprinted from [143] Copyright (2020), with permission from John Wiley and Sons.

− − − As electrochemical cofactor regeneration has, by definition, to occur at the surface of the electrode, scaling up reactions is likely to cause diffusion limitations resulting in reduced rates of reaction [23]. This challenge can be addressed by using three-dimensional electrodes in continuous flow reactors. Kochius et al. designed a system for the efficient electrochemical regeneration of NAD+, based on 3D electrodes with a high working surface area 24 m2 . The anode, comprised of a packed bed of glassy carbon particles, was surrounded by two cathodes (titanium net) (Figure 6). The oxidation of NADH was carried out using ABTS as a mediator, with turnover numbers of 1860 and 93 h−1 for the mediator and the cofactor, respectively, rates that were significantly higher than previously reported. A space time yield 1.4 g L−1 h −1 was achieved for the three-dimensional electrochemical reactor, a value higher than that at a two-dimensional cell [48]. − − −

**Figure 6.** Schematic diagram of the three-dimensional electrode [48].

− Ruinatscha et al. fabricated a reactor with three-dimensional porous carbon electrodes with a surface area to volume ratio of 19,685 m2m−3 (Figure 7).

− The reactor was used to produce FADH2, which was then coupled with styrene monooxygenases for the synthesis of styrene oxide. The rate of mass transfer increased in this system and FAD was reduced at a rate of 93 mM h−1 producing styrene oxide at a rate of 1.3 mM h−1 [144]. Due to the benefits of using microreactors for cofactor regeneration, the direct regeneration of NAD+ was examined. Rodríguez-Hinestroza et al. designed an electrochemical filter press microreactor for the direct anodic regeneration of NAD+, via the horse-liver alcohol dehydrogenase-catalysed synthesis of β-alanine (Figure 8). The platinum and gold electrodes used had 150 microchannels, with a high surface area

−

of 250 cm−2 . NADH was directly oxidized at the gold electrode, with a 92% conversion of NADH to NAD+. The produced NAD+ was used for the enzymatic oxidation of carboxybenzyl-β-amino propanol [145]. −

− −

**Figure 7.** Schematic diagram of system for a reactor for the regeneration of FAD. Reprinted from [144] Copyright (2020), with permission from Elsevier.

β **Figure 8.** Schematic diagram of an electrochemical microreactor system and the electroenzymatic oxidation of carboxybenzyl-β-amino propanol. Reprinted from [145]. Copyright (2020), with permission from Elsevier.

Fisher et al. prepared a multichannel segmented flow bioreactor in which the oxidoreductase pentaerythritol tetranitrate reductase (PETNR) was regenerated electrochemically with the use of methyl viologen as a mediator (Figure 9), replacing NADPH as cofactor. The use of methyl viologen resulted in rates of substrate reduction that were 15–70% of those observed with NADPH [146].

Mazurenko et al. carried out mediated regeneration NAD+ via poly(methylene green) in a flow reactor for the synthesis of D-fructose from D-sorbitol using D-sorbitol dehydrogenase [142]. Electrochemical methods can also be used to immobilise enzymes on the surface of electrodes for subsequent use in a flow reactor. For example; Xiao et al. designed a continuous flow bioreactor based on lipase immobilised by the electrochemical generation of a silica film nanoporous gold-modified glassy carbon electrodes (Figure 10a). In addition, −1.1 V vs. Ag/AgCl was applied on the electrode surface immersed in tetraethoxysilane and lipase solution. This triggered hydroxyl ion production that caused tetraethoxysilane condensation on the electrode surface and lipase was entrapped into silica structure.

β

−

− −

β

β

**Figure 9.** (**a**) Schematic diagram of a segmented flow bioreactor system, (**b**) 3D representation of the bioreactor. Reprinted from [146] Copyright (2020), with permission from The Royal Society of Chemistry.

**Figure 10.** (**a**) Schematic diagram of the electrodeposition of a silica layer with entrapped enzyme on an electrode and (**b**) an image of the continuous flow reactor [107].

The modified electrodes were inserted into the flow reactor (Figure 10b) and used to prepare p-nitrophenol from 4-nitrophenyl butyrate (4-NPB), with the full conversion of the substrate (0.075 mM in 2 mL) after eight cycles [107].


*2.5. Validation*

To ensure the stability of the numerical solution, the sphere tests of Charters and Thomas [26] were chosen for validation. Figure 5 shows a comparison of the drag coefficient obtained from our simulations and the work of Charters and Thomas, in which experiments were conducted with small spheres at various velocities. A 2D axisymmetric sphere was simulated with an unsteady moving mesh. The numerical method used for the validation was the same as those mentioned in Section 2.4. Figure 5 shows a similar trend in the drag coefficient between the two studies, indicating a good agreement. Therefore, our selected numerical method is appropriate to simulate the moving pod–tube configuration.

**Figure 5.** Comparison of drag coefficients between the 2D axisymmetric simulation and the experimental data for a 14.28-mm sphere reported by Charters and Thomas [26].