Polymeric implants matrix-type

In a matrix-type implant the drag is distributed throughout a polymeric matrix (Figure 4.6). 

Figure 4.7 A matrix-type implant in which a drug is dissolved. The initial diffusion of drug molecules leaves a drug-depleted polymeric zone with a length h, which increases with time. This event leads to an increase in diffusional distance over time...

Figure 4.10 Hybrid-type polymeric implants (a) Syncro-Mate-C matrix containing microreservoirs of drug, (b) Implanon membrane coating a drug containing matrix...

Reservoir/matrix hybrid-type non-degradable polymeric implants are also available. Such systems are designed in an attempt to improve the M t1/2" release kinetics of a matrix system, so that release approximates the zero-order release rate of a reservoir device. Examples of these types of systems include ... 

Reservoir/matrix hybrid-type polymeric implants... 

The release rate of a dmg from conventional non-degradable matrix-type polymeric implant usually decreases over time. Describe a technique that can be used to overcome this problem. 

What is the main reason for developing a reservoir/matrix hybrid-type polymeric implant ... 

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