Comma between subcategory list

In my single.php file I am trying to list all the child categories under a specific parent category. I’m using the following code:

if(!function_exists('subcat')){
	function subcat(){
		global $post;
foreach((get_the_category()) as $category)
{
    if ($category->category_parent == "177")
    {
        echo '<a href="' . get_category_link($category->cat_ID) . '">' . $category->cat_name . '</a>';
    }
}
	}
}

The code works, but in instances where there are multiple subcategories I would like to separate those subcategories with a comma. How do I do that? Anyone know?