You could do a loop over all cells, then extract the contents and use fprintf(). Here is a start
fid = fopen('output.txt', 'wt');
for k = 1 : length(ca)
thisCell = ca{k}; % Get contents of this one cell. Returns a structure
% Get the character string
fprintf(fid, '%s\n', thisCell.stringVariable); % Or whatever you called your string variable.
% Get the sub-cell
subCell = thisCell.ca; % Or whatever the cell array is called.
subCellContents = subCell{1};
% Now use fprintf to write out subCellContents into the file.
% How you do that depends on what's in that cell.
end
fclose(fid);
