The solutions to this crossword are all in some way or another connected with the "cross-boundary" theme, although in some cases the connection might be seen as a bit tenuous! Thus part of the enjoyment, if that's the word, of solving some of the clues might come from making one or more such links. Some clues have been provided by Bob MacKenzie, the rest by me.
I constructed the puzzle using Crossword Weaver. This works tolerably well but I am not very happy with the way many responses have little interaction with each other - this is particularly true in the bottom left-hand corner. Nor is it feasible to produce a traditional symmetrical grid using one's own words.
Also it's not possible to reproduce the grid interactively - you have to download and print it I'm afraid. For a future version I would use the software, then type out all the spaces in a Word table, and then "manually" add some additional words and change the square reference numbers.
For every clue I have included a web-link. The hyperlink is embedded in the clue number and also listed in full in the table following the clues. Some of these act as additional clues, most others give further information about the answer, although not necessarily from the "boundary" perspective. Hopefully they may lead to some semi-random voyages of exploration, surprise, pleasure discovery or disturbance! In principle I have tried not to include the solution as such in the url - you will need to visit the site, although there are one or two exceptions to this.
Some clues are basically straightforward "facts" or pieces of information; others are rather or indeed very cryptic. At least three are connected with this publication. One contains a symbol amongst its letters.
After the number of certain clues, you’ll find the key ‘A’, which indicates ‘Across’, or ‘D’, which indicates ‘Down’.
The prize for solving these puzzles is a sense of satisfaction and discovery. I hope you enjoy playing with it.
4. Movement of a solvent (eg water) through a semi-permeable membrane (eg covering of a living cell) into a solution of higher solute concentration (7)
5. Grassy edge (5)
8a+24d. 28's lasted for ages - some of it still there - this one came down in 1989 (6, 4)
9. Often beaten annually, if you step over 13d you might be out of them! (3, 6)
10. As covered in this edition, two of these don't make a 25d! (8)
14. Metal added to 40d to extend it from Stettin to Trieste (4)
15. Where we might find the limits of knowledge or the most advanced achievements in a field (8)
16. Found on a violin, it connects two sides (6)
18. Race formed by scrambled definite article, reversed in large conurbation (9)
21. An indefinite, ambiguous or contested area where guidelines and authority are not clear but a famous Christmas day football match was played (2, 4, 4)
22. Are you an un-made member of this association? Or in Bali? (4)
23. Male or female, renegade loses first two vowels and becomes confused (6)
27. Partially symbolic and virtual rather than entirely literal, here we are all in this together! (4)
28. Well before referenda, he allegedly built the Vallum Aelium to keep the barbarians out (7)
29 & 33. These need to be integrated in the sense of working together effectively if we are to be well looked after (6,3,6,4)
33. See 29a
35. At the edge, the outskirts or margins - a vision that's sometimes hard to see! (10)
38. Often associated with which 23a - if any - you fancy (9)
39. A distinct part of a society or economy; a zone or section of a city or country (6)
40. Circular 13d (13)
41. Stop at the end of the line (8)
42. Antipodean coral barrier (4)
43 & 4d. 1494 agreement between Portugal and Spain creating a line of demarcation dividing lands newly discovered outside Europe (6, 2, 11)
44. Confused route before restrictions or restraints will take you to the furthest extent - you cannot go past these, not even in a 1960s black-and-white TV series! (5,6)
45. The referee may be making a disclosure in the public interest
1 & 6. Absence of war, together with re-establishment of relationship (5,3,14)
2. Restrictions that sounds like criminal monetary penalties? (8)
3 & 11. Two cultures separated by wintry precipitation (10, 7)
4. See 43a
6. See 1d
7. Suffix used to derive words in philosophy, politics, religion or other areas pertaining to an ideology of some sort, sometimes with a derogatory or hostile sense, thus forming a boundary (3)
11. See 3d
12. Male, sandwiched between the edge and a vessel, creates deliberate act of pushing matters to their limits (13)
13. Line bounding or marking off an enclosed area or measuring the distance all the way round (9)
17. Rather than reconnecting us it quite often is the cause of major barriers and conflicts (8)
19. Armed unit intent on preventing uncontrolled migration or smuggling (5, 6)
20. Pam is upset and seeking information about the terrain (3)
24. See 8a (4)
25. See 10a (8)
26. Vegetation added to 40a to indicate an unstable political demarcation between some communist and non-communist states in East Asia (6)
30. Basis of much discrimination and misunderstanding, referred to in The Passionate Pilgrim, allegedly by Shakespeare, as crabbed (3)
31. Often seen at 24 per second, though sometimes 48 or 60 - when still they set pictures etc in context; (6)
32. Person in past times given to plundering raids between Southern Scotland and Northern England (6)
34. Your guest editors of this edition of 27, 22's journal (5,3,3)
36 & 37. Taking a non-passive role in the community (6, 11)
37. See 36
40. Not Hitchcock's torn one, but may be metallic as in 21, or vegetative as in 26d (7)