A process (100, 400) of assaying biological activity in a micro-fluidic device, said process comprising: culturing (104, 402) one or more biological cells in a holding pen of a micro-fluidic device, wherein said one or more cells produce a biological material of interest, wherein said micro-fluidic device comprises a housing, and wherein said housing comprises: a base; a micro-fluidic structure disposed on said base; and a plurality of holding pens, wherein said micro-fluidic structure and said base define a flow region comprising a channel and said plurality of holding pens, and wherein each said holding pen of said plurality of holding pens is disposed on said base and comprises an isolation region having a single opening, and a connection region, said connection region having a proximal opening to said channel and a distal opening to said isolation region, and wherein a length of said connection region is greater than a penetration depth of a flow flowing in said channel outside of said holding pen, whereby said connection region enables substantially only diffusive fluidic communication between said channel and said isolation region, such that said isolation region of said holding pen is an unswept region of said micro-fluidic device; introducing (102, 404) one or more capture micro-objects into said holding pen of said plurality of holding pens, wherein each said capture object comprises a binding substance that specifically binds said biological material of interest; allowing said biological material of interest produced by said one or more biological cells to bind to said one or more capture micro-objects in said holding pen; and detecting (110, 412) whether said biological material of interest is bound to said capture micro-objects. culturing (104, 402) one or more biological cells in a holding pen of a micro-fluidic device, wherein said one or more cells produce a biological material of interest, wherein said micro-fluidic device comprises a housing, and wherein said housing comprises: a base; a micro-fluidic structure disposed on said base; and a plurality of holding pens, wherein said micro-fluidic structure and said base define a flow region comprising a channel and said plurality of holding pens, and wherein each said holding pen of said plurality of holding pens is disposed on said base and comprises an isolation region having a single opening, and a connection region, said connection region having a proximal opening to said channel and a distal opening to said isolation region, and wherein a length of said connection region is greater than a penetration depth of a flow flowing in said channel outside of said holding pen, whereby said connection region enables substantially only diffusive fluidic communication between said channel and said isolation region, such that said isolation region of said holding pen is an unswept region of said micro-fluidic device; a base; a micro-fluidic structure disposed on said base; and a plurality of holding pens, wherein said micro-fluidic structure and said base define a flow region comprising a channel and said plurality of holding pens, and wherein each said holding pen of said plurality of holding pens is disposed on said base and comprises an isolation region having a single opening, and a connection region, said connection region having a proximal opening to said channel and a distal opening to said isolation region, and wherein a length of said connection region is greater than a penetration depth of a flow flowing in said channel outside of said holding pen, whereby said connection region enables substantially only diffusive fluidic communication between said channel and said isolation region, such that said isolation region of said holding pen is an unswept region of said micro-fluidic device; introducing (102, 404) one or more capture micro-objects into said holding pen of said plurality of holding pens, wherein each said capture object comprises a binding substance that specifically binds said biological material of interest; allowing said biological material of interest produced by said one or more biological cells to bind to said one or more capture micro-objects in said holding pen; and detecting (110, 412) whether said biological material of interest is bound to said capture micro-objects.