## 3 Results
### Electrophysiological Phenotype of the Hybrid Model  
Electrical activity recordings from the control construct, i.e., the biopsy mounted on the MEA without a neuronal cell layer as an interface, did not surpass the signal-to-noise ratio. Hence, these recordings were not further analyzed and are displayed in Supplementary Figure S1. The second control construct, i.e., DIV16 cortical neurons cultured on the MEA, did show electrophysiological activity above the signal-to-noise ratio. Consequently, its electrophysiological phenotype was used as baseline to compare with that of the construct with the subsequently added biopsy (Figure 2D). At DIV16, hiPSC-derived neuronal cultures exhibited a firing rate of \(\pm\)3 Hz and 5-10 burst events per minute with a burst duration of 0.1-0.3 s. In addition, these cells displayed isolated spikes in 60% of the recording time, 0-3 network bursts per minute with a duration 0.4-0.8 s per burst, an inter spike interval (ISI) up to 2 s, and a coefficient of variation between 2 and 4. Addition of the biopsy plug on the neuronal cell layer changed the baseline phenotype into hyperactive network bursting activity (example 1 h after the assembly of the biopsy plug shown in Figure 3A). The raw single-channel network burst signal that was recorded across the network with the threshold for spike detection is also shown (Figure 3B). The identified bursts and single spikes from the analysis are also presented (Figure 3C). This seizure-like activity lasted for 6 days (Figure 4A-C). Quantitative analysis of MEA recordings comparing the hybrid model with hiPSC alone revealed an increased firing rate, burst rate, burst duration, coefficient of variation in ISI and network burst durations, and a decreased number of isolated spikes and ISI (Figure 4D). Prior to the biopsy plug addition, electrophysiological MEA characterization at DIV16 demonstrated stable firing (around 3 Hz, Figure 2D), which is considered normal for such 2D hiPSC-derived neuronal cultures [29]. Some network bursts were already present in the neuronal culture (average of two network bursts detected, Figure 2D), prior to biopsy plug assembly, with shorter durations (up to 0.8 s). Figure 5 compares the electrophysiological phenotype of the hybrid model (orange, triangles) with the control hiPSC neuronal cultures (blue, squares). It shows that the firing rate increased considerably upon addition of the biopsy plug up to 25 Hz on DIV0 and then gradually decreased to 10 Hz at DIV6. Single-channel bursts increased on average from 3 to 10 bursts per minute up to 18 to 36 bursts per minute when the biopsy plug was assembled, with recorded network bursts duration increasing from 0.7 to 15 s (DIV0) and 28 s (DIV3) and dropping down again to 5 s on DIV6, indicating that the model was no longer capable of recording strong epileptic/seizurogenic signatures after this time point. In addition, isolated spikes in the hybrid model showed a considerable decrease, with typically 60 to 70% isolated spikes recorded from the hiPSC culture alone compared to 6 to 20% in the assembled biopsy-hiPSC co-culture for the first 3 days, and increasing up to 50% at day 6. Similarly, the inter-spike interval values were lower in the assembledbiopsy-hiPSC co-culture for the whole duration of the experiment (lower than 0.5 s), while the coefficient of variation in the inter-spike interval was considered elevated, especially during the first day of recording, when the value was above 5. Such types of firing patterns (Figure 5, Biopsy + hiPSC, orange) were not observed when we cultured the biopsy or the iPSC-derived neuronal layer separately on the MEA (Figure 5, hiPSC, blue).