Could anyone please tell me if the state channels are unidirectional or bidirectional? Also, if a node goes off-line with an open channel, is it still able to receive transactions? I mean, not to sign but to receive something. I'm really curious about what would happen when there are short-term disconnections.
State channels are a general concept without a single implementation. State channels can be unidirectional or bidirectional (or even involve more than two participants). One participant going offline can be handled a number of ways, but generally after some amount of non-responsiveness, the online party resorts to an on-chain transaction.
State channels don't really need or have a "direction" because they are a generalization and more powerful version of payment channels.
For example, the lack of "full" state is what may have led to the unidirectional and bidirectional terms. From this perspective, consider a unidirectional channel as one whose state is simply one value: "Alice's payment to Bob". Or consider a bidirectional channel as one that has 2 values: "Alice's balance" and "Bob's balance".
"Direction" is also different from how many parties can be participate in a state channel. You can have multiple parties in a state channel by setting it (usually a smart contract) to be updatable by multiple parties.
When a node goes offline, it can still receive "transactions" the same way that you can receive a token without needing to be online. If you mean will the node still be able to route transactions on the network, then it would not be able to. When there are disconnections, nodes need to find other peers along the channel path.