## 1 Introduction  
Whereas the burden of brain diseases is ever increasing, the treatment of these disorders often falls short. In search of new treatments, animal models were instrumental in unravelling disease mechanisms and yielded potential drug targets, yet up to 96% of human trials fail to reproduce drug effects that are eminent in animal models [1]. As animal models often partly mimic human pathophysiology, they show limited external validity [2]. To replace animal-based modelling, there is a growing interest in the development of alternative, clinically relevant disease models [3, 4]: for example, in epilepsy, which is a devastating disease affecting nearly 50 million people worldwide [5] with about one in three patients whom suffer from drug-refractory seizures [6]. As a last drastic resort, 10-15% of these drug-refractory patients may undergo resective brain surgery, a procedure whereby the epileptic focus is surgically removed. In addition to health gain for the patient, the hereby obtained biopsy is a unique source for research on the disease pathology. Traditionally, the neuronal activity of brain biopsies is evaluated in organotypic slices. Although this technique is commonly used in rodent studies [7], it is less often applied on human brain biopsies. Interestingly, organotypic slice cultures of human brain biopsies received more attention since just 10 years ago. In these slices, some studies reported localized (single-electrode) epileptiform activity [8; 9; 10], yet the culture duration of the slices was limited up to 20 h. To evaluate the activity of an epileptic network, it is necessary to record a number of neurons over a longer period. This may be obtained from a multi-electrode array (MEA), yet there are challenges, such as loss of contact between electrode and biopsy and sufficient perfusion require attention. For example, [PERSON] et al. applied a MEA to a temporal lobe slice from an epilepsy patient [11]. Although they reported artefacts due to a slice shift on the MEA, they were able to record 4-aminopyridine-evoked epileptiform activity for up to 11 h. In addition, [PERSON] et al. evaluated MEA recordings from human epileptic cortical tissue and reported repetitive epileptic activity over a period of 24 h [12]. The most comprehensive study on long-term recordings of epileptic activity in organotypic cultures was achieved by [PERSON] et al. (2018) [13]. Yet, these recordings were obtained by patch clamp, and consequently, hardly represent network activity, as cultures are on porous cell culture inserts. The latter are not compatible with MEA chips and setup. They successfully demonstrate single-neuron recordings by patch clamping for 6 weeks in culture. This can only be achieved by a highly specialized setup, including viral vector techniques with promoters for transgene expression, a customized culture medium, and sophisticated hardware. Indeed, it was shown that dedicated medium, such as human cerebrospinal fluid (hCSF), promotes longer-term neuronal viability and network function in human organotypic brain slice cultures, when compared with commercially available ones [14]. Yet, hCSF is not widely accessible, as it requires ethical approval, infrastructure, and only a limited amount can be collected per patient (making the experiments expensive and it is not readily available). The introduction of the human induced pluripotent stem cell (hiPSC) technology, pioneered by [PERSON] et al. [15], and advancements in 2D and 3D brain models [16; 17] greatly expanded the toolkit for human disease modelling and personalized treatment [18]. Regardless of the sophistication of these brain models (a.k.a. brain organoids), a major drawback of this approach is a lack of a complete set of cell types and the histoarchitecture of the brain. This is not trivial when discerning neurocircuitiries and their role in pathology. Still, the morphology and functionality of neurons derived from differentiated hiPSCs can meet that of neurons in the human brain and thus be studied as models for brain diseases [19; 20].